About the Tutorial
Assembly language is a low-level programming language for a computer or other programmable device specific to a particular computer architecture in contrast to most high-level programming languages, which are generally portable across multiple systems. Assembly language is converted into executable machine code by a utility program referred to as an assembler like NASM, MASM, etc.

Audience
This tutorial has been designed for those who want to learn the basics of assembly programming from scratch. This tutorial will give you enough understanding on assembly programming from where you can take yourself to higher levels of expertise.

Prerequisites
Before proceeding with this tutorial, you should have a basic understanding of
Computer Programming terminologies. A basic understanding of any of the programming languages will help you in understanding the Assembly programming concepts and move fast on the learning track.

Copyright & Disclaimer
 Copyright 2014 by Tutorials Point (I) Pvt. Ltd.
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(I) Pvt. Ltd. The user of this e-book is prohibited to reuse, retain, copy, distribute or republish any contents or a part of contents of this e-book in any manner without written consent of the publisher.
We strive to update the contents of our website and tutorials as timely and as precisely as possible, however, the contents may contain inaccuracies or errors. Tutorials Point (I)
Pvt. Ltd. provides no guarantee regarding the accuracy, timeliness or completeness of our website or its contents including this tutorial. If you discover any errors on our website or in this tutorial, please notify us at contact@tutorialspoint.com

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Table of Contents
About the Tutorial ······································································································································
Audience ···················································································································································· i
Prerequisites ·············································································································································· i
Copyright & Disclaimer ······························································································································ i
Table of Contents ······································································································································ ii

1. INTRODUCTION ······················································································································· 1
What is Assembly Language? ···················································································································· 1
Advantages of Assembly Language ··········································································································· 1
Basic Features of PC Hardware·················································································································· 1
Binary Number System ····························································································································· 2
Hexadecimal Number System ··················································································································· 3
Binary Arithmetic ······································································································································ 4
Addressing Data in Memory······················································································································ 6

2. ENVIORNMENT SETUP ············································································································ 7
Try it Option Online ·································································································································· 7
Local Environment Setup ·························································································································· 7
Installing NASM ········································································································································ 8

3. BASIC SYNTAX ························································································································· 9
The data Section ······································································································································· 9
The bss Section ········································································································································· 9
The text section ········································································································································ 9
Comments ················································································································································ 9
Assembly Language Statements ·············································································································· 10
Syntax of Assembly Language Statements ······························································································ 10
The Hello World Program in Assembly ···································································································· 11

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Compiling and Linking an Assembly Program in NASM ··········································································· 11

4. MEMORY SEGMENTS ············································································································ 13
Memory Segments ·································································································································· 13

5. REGISTERS ····························································································································· 15
Processor Registers ································································································································· 15
Data Registers ········································································································································· 15
Pointer Registers ····································································································································· 16
Index Registers ······································································································································· 17
Control Registers····································································································································· 17
Segment Registers ·································································································································· 18

6. SYSTEM CALLS ······················································································································· 21
Linux System Calls ··································································································································· 21

7. ADDRESSING MODES ············································································································ 24
Register Addressing ································································································································ 24
Immediate Addressing ···························································································································· 24
Direct Memory Addressing ····················································································································· 25
Direct-Offset Addressing ························································································································· 25
Indirect Memory Addressing ··················································································································· 25
The MOV Instruction ······························································································································· 26

8. VARIABLES····························································································································· 29
Allocating Storage Space for Initialized Data ··························································································· 29
Allocating Storage Space for Uninitialized Data ······················································································ 30
Multiple Definitions ································································································································ 31
Multiple Initializations ···························································································································· 31

9. CONSTANTS ·························································································································· 33 iii The EQU Directive ··································································································································· 33
The %assign Directive ····························································································································· 35
The %define Directive ····························································································································· 35

10. ARITHMETIC INSTRUCTIONS ······························································································· 36
The INC Instruction ································································································································· 36
The DEC Instruction································································································································· 36
The ADD and SUB Instructions ················································································································ 37
The MUL/IMUL Instruction ····················································································································· 41
The DIV/IDIV Instructions ······················································································································· 43

11. LOGICAL INSTRUCTIONS······································································································ 47
The AND Instruction································································································································ 47
The OR Instruction ·································································································································· 49
The XOR Instruction ································································································································ 50
The TEST Instruction ······························································································································· 51
The NOT Instruction ································································································································ 51

12. CONDITIONS ······················································································································· 52
CMP Instruction ······································································································································ 52
Conditional Jump ···································································································································· 54

13. LOOPS ································································································································· 58
14. NUMBERS ··························································································································· 60
ASCII Representation ······························································································································ 61
BCD Representation ································································································································ 62

15. STRINGS ······························································································································ 65
String Instructions ··································································································································· 65
MOVS······················································································································································ 67

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LODS ······················································································································································· 68
STOS ······················································································································································· 69
CMPS ······················································································································································ 70
SCAS························································································································································ 71
Repetition Prefixes ································································································································· 73

16. ARRAYS ······························································································································· 74
17. PROCEDURES ······················································································································ 77
Stacks Data Structure ······························································································································ 78

18. RECURSION ························································································································· 81
19. MACROS······························································································································ 83
20. FILE MANAGEMENT ············································································································ 86
File Descriptor ········································································································································· 86
File Pointer ············································································································································· 86
File Handling System Calls ······················································································································· 86
Creating and Opening a File ···················································································································· 87
Opening an Existing File ·························································································································· 87
Reading from a File ································································································································· 87
Writing to a File ······································································································································ 88
Closing a File ··········································································································································· 88
Updating a File ········································································································································ 88

21. MEMORY MANAGEMENT ··································································································· 92

v

Assembly Programming

1. INTRODUCTION
What is Assembly Language?

Each personal computer has a microprocessor that manages the computer's arithmetical, logical, and control activities.
Each family of processors has its own set of instructions for handling various operations such as getting input from keyboard, displaying information on screen, and performing various other jobs. These set of instructions are called
'machine language instructions'.
A processor understands only machine language instructions, which are strings of 1's and 0's. However, machine language is too obscure and complex for using in software development. So, the low-level assembly language is designed for a specific family of processors that represents various instructions in symbolic code and a more understandable form.

Advantages of Assembly Language
Having an understanding of assembly language makes one aware of:


How programs interface with OS, processor, and BIOS;



How data is represented in memory and other external devices;



How the processor accesses and executes instruction;



How instructions access and process data;



How a program accesses external devices.

Other advantages of using assembly language are:


It requires less memory and execution time;



It allows hardware-specific complex jobs in an easier way;



It is suitable for time-critical jobs;



It is most suitable for writing interrupt service routines and other memory resident programs.

Basic Features of PC Hardware
The main internal hardware of a PC consists of processor, memory, and registers. Registers are processor components that hold data and address. To execute a program, the system copies it from the external device into the internal memory. The processor executes the program instructions.
1

Assembly Programming

The fundamental unit of computer storage is a bit; it could be ON (1) or OFF (0).
A group of nine related bits makes a byte, out of which eight bits are used for data and the last one is used for parity. According to the rule of parity, the number of bits that are ON (1) in each byte should always be odd.
So, the parity bit is used to make the number of bits in a byte odd. If the parity is even, the system assumes that there had been a parity error (though rare), which might have been caused due to hardware fault or electrical disturbance.
The processor supports the following data sizes:


Word: a 2-byte data item



Doubleword: a 4-byte (32 bit) data item



Quadword: an 8-byte (64 bit) data item



Paragraph: a 16-byte (128 bit) area



Kilobyte: 1024 bytes



Megabyte: 1,048,576 bytes

Binary Number System
Every number system uses positional notation, i.e., each position in which a digit is written has a different positional value. Each position is power of the base, which is 2 for binary number system, and these powers begin at 0 and increase by 1.
The following table shows the positional values for an 8-bit binary number, where all bits are set ON.
Bit value

1

1

1

1

1

1

1

1

Position value as a power of base 2

128

64

32

16

8

4

2

1

Bit number

7

6

5

4

3

2

1

0

The value of a binary number is based on the presence of 1 bits and their positional value. So, the value of a given binary number is:
1 + 2 + 4 + 8 +16 + 32 + 64 + 128 = 255 which is same as 28 - 1.

2

Assembly Programming

Hexadecimal Number System
Hexadecimal number system uses base 16. The digits in this system range from
0 to 15. By convention, the letters A through F is used to represent the hexadecimal digits corresponding to decimal values 10 through 15.
Hexadecimal numbers in computing is used for abbreviating lengthy binary representations. Basically, hexadecimal number system represents a binary data by dividing each byte in half and expressing the value of each half-byte. The following table provides the decimal, binary, and hexadecimal equivalents:
Decimal
number

Binary representation Hexadecimal representation 0

0

0

1

1

1

2

10

2

3

11

3

4

100

4

5

101

5

6

110

6

7

111

7

8

1000

8

9

1001

9

10

1010

A

11

1011

B

3

Assembly Programming

12

1100

C

13

1101

D

14

1110

E

15

1111

F

To convert a binary number to its hexadecimal equivalent, break it into groups of 4 consecutive groups each, starting from the right, and write those groups over the corresponding digits of the hexadecimal number.
Example: Binary number 1000 1100 1101 0001 is equivalent to hexadecimal 8CD1.
To convert a hexadecimal number to binary, just write each hexadecimal digit into its 4-digit binary equivalent.
Example: Hexadecimal number FAD8 is equivalent to binary - 1111 1010 1101
1000.

Binary Arithmetic
The following table illustrates four simple rules for binary addition:
(i)

(ii)

(iii)

(iv)
1

0

1

1

1

+0

+0

+1

+1

=0

=1

=10

=11

Rules (iii) and (iv) show a carry of a 1-bit into the next left position.

4

Assembly Programming

Example
Decimal

Binary

60

00111100

+42

00101010

102

01100110

A negative binary value is expressed in two's complement notation.
According to this rule, to convert a binary number to its negative value is to reverse its bit values and add 1.

Example
Number 53

00110101

Reverse the bits

11001010

Add 1

1

Number -53

11001011

To subtract one value from another, convert the number being subtracted to two's complement format and add the numbers.

Example
Subtract 42 from 53.
Number 53

00110101

Number 42

00101010

Reverse the bits of 42

11010101

Add 1

1

Number -42

11010110

5

Assembly Programming

53 - 42 = 11

00001011

Overflow of the last 1 bit is lost.

Addressing Data in Memory
The process through which the processor controls the execution of instructions is referred as the fetch-decode-execute cycle or the execution cycle. It consists of three continuous steps:


Fetching the instruction from memory



Decoding or identifying the instruction



Executing the instruction

The processor may access one or more bytes of memory at a time. Let us consider a hexadecimal number 0725H. This number will require two bytes of memory. The high-order byte or most significant byte is 07 and the low-order byte is 25.
The processor stores data in reverse-byte sequence, i.e., a low-order byte is stored in a low memory address and a high-order byte in high memory address.
So, if the processor brings the value 0725H from register to memory, it will transfer 25 first to the lower memory address and 07 to the next memory address. x: memory address
When the processor gets the numeric data from memory to register, it again reverses the bytes. There are two kinds of memory addresses:


Absolute address – a direct reference of specific location.



Segment address (or offset) – starting address of a memory segment with the offset value.

6

Assembly Programming

2. ENVIORNMENT SETUP
Try it Option Online

We already have set up NASM assembler to experiment with Assembly programming online, so that you can execute all the available examples online at the same time when you are doing your theory work. This gives you confidence in what you are reading and to check the result with different options. Feel free to modify any example and execute it online.
Try the following example using our http://www.compileonline.com/ section

online

compiler option

available

at

.text

global _start
_start:

;must be declared for linker (ld)
;tells linker entry point

mov

edx,len

;message length

mov

ecx,msg

;message to write

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

mov

eax,1

;system call number (sys_exit)

int

0x80

;call kernel

section

.data

msg db 'Hello, world!', 0xa len equ $ - msg

;our dear string

;length of our dear string

For most of the examples given in this tutorial, you will find a Try it option in our website code sections at the top right corner that will take you to the online compiler. So just make use of it and enjoy your learning.

Local Environment Setup
Assembly language is dependent upon the instruction set and the architecture of the processor. In this tutorial, we focus on Intel 32 processors like Pentium. To follow this tutorial, you will need:


An IBM PC or any equivalent compatible computer
7

Assembly Programming



A copy of Linux operating system



A copy of NASM assembler program

There are many good assembler programs such as:


Microsoft Assembler (MASM)



Borland Turbo Assembler (TASM)



The GNU assembler (GAS)

We will use the NASM assembler, as it is:


Free. You can download it from various web sources.



Well-documented and you will get lots of information on net.



Could be used on both Linux and Windows.

Installing NASM
If you select "Development Tools" while installing Linux, you may get NASM installed along with the Linux operating system and you do not need to download and install it separately. For checking whether you already have NASM installed, take the following steps:
1. Open a Linux terminal.
2. Type whereis nasm and press ENTER.
3. If it is already installed, then a line like, nasm: /usr/bin/nasm appears.
Otherwise, you will see just nasm:, then you need to install NASM.
To install NASM, take the following steps:
1. Check The netwide assembler (NASM) website for the latest version.
2. Download the Linux source archive nasm-X.XX.ta.gz, where X.XX is the
NASM version number in the archive.
3. Unpack the archive into a directory which creates a subdirectory nasm-X.
XX.
4. cd to nasm-X. XX and type ./configure . This shell script will find the best
C compiler to use and set up Makefiles accordingly.
5. Type make to build the nasm and ndisasm binaries.
6. Type make install to install nasm and ndisasm in /usr/local/bin and to install the man pages.
This should install NASM on your system. Alternatively, you can use an RPM distribution for the Fedora Linux. This version is simpler to install, just doubleclick the RPM file.

8

Assembly Programming

3. BASIC SYNTAX
An assembly program can be divided into three sections:


The data section,



The bss section, and



The text section.

The data Section
The data section is used for declaring initialized data or constants. This data does not change at runtime. You can declare various constant values, file names, or buffer size, etc., in this section.
The syntax for declaring data section is: section .data

The bss Section
The bss section is used for declaring variables. The syntax for declaring bss section is: section .bss

The text section
The text section is used for keeping the actual code. This section must begin with the declaration global _start, which tells the kernel where the program execution begins.
The syntax for declaring text section is: section .text global _start
_start:

Comments
Assembly language comment begins with a semicolon (;). It may contain any printable character including blank. It can appear on a line by itself, like:

9

Assembly Programming

; This program displays a message on screen or, on the same line along with an instruction, like: add eax ,ebx

; adds ebx to eax

Assembly Language Statements
Assembly language programs consist of three types of statements:


Executable instructions or instructions,



Assembler directives or pseudo-ops, and



Macros.

The executable instructions or simply instructions tell the processor what to do. Each instruction consists of an operation code (opcode). Each executable instruction generates one machine language instruction.
The assembler directives or pseudo-ops tell the assembler about the various aspects of the assembly process. These are non-executable and do not generate machine language instructions.
Macros are basically a text substitution mechanism.

Syntax of Assembly Language Statements
Assembly language statements are entered one statement per line. Each statement follows the following format:
[label]

mnemonic

[operands]

[;comment]

The fields in the square brackets are optional. A basic instruction has two parts, the first one is the name of the instruction (or the mnemonic), which is to be executed, and the second are the operands or the parameters of the command.
Following are some examples of typical assembly language statements:
INC COUNT

; Increment the memory variable COUNT

MOV TOTAL, 48

; Transfer the value 48 in the
; memory variable TOTAL

ADD AH, BH

; Add the content of the
; BH register into the AH register

AND MASK1, 128

; Perform AND operation on the
; variable MASK1 and 128

ADD MARKS, 10

; Add 10 to the variable MARKS
10

Assembly Programming

MOV AL, 10

; Transfer the value 10 to the AL register

The Hello World Program in Assembly
The following assembly language code displays the string 'Hello World' on the screen: section

.text

global _start

;must be declared for linker (ld)

_start:

;tells linker entry point

mov

edx,len

;message length

mov

ecx,msg

;message to write

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

mov

eax,1

;system call number (sys_exit)

int

0x80

;call kernel

section

.data

msg db 'Hello, world!', 0xa

;our dear string

len equ $ - msg

;length of our dear string

When the above code is compiled and executed, it produces the following result:
Hello, world!

Compiling and Linking an Assembly Program in NASM
Make sure you have set the path of nasm and ld binaries in your PATH environment variable. Now, take the following steps for compiling and linking the above program:
1. Type the above code using a text editor and save it as hello.asm.
2. Make

sure that saved hello.asm.

you

are

in

the

same

directory

as

where

you

3. To assemble the program, type nasm -f elf hello.asm
4. If there is any error, you will be prompted about that at this stage.

Otherwise, an object file of your program named hello.o will be created.
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Assembly Programming

5. To link the object file and create an executable file named hello, type ld -

m elf_i386 -s -o hello hello.o
6. Execute the program by typing ./hello

If you have done everything correctly, it will display ‘Hello, world!’ on the screen. 12

Assembly Programming

4. MEMORY SEGMENTS

We have already discussed the three sections of an assembly program. These sections represent various memory segments as well.
Interestingly, if you replace the section keyword with segment, you will get the same result. Try the following code: segment .text

;code segment

global _start
_start:

;must be declared for linker

;tell linker entry point

mov edx,len

;message length

mov ecx,msg

;message to write

mov ebx,1

;file descriptor (stdout)

mov eax,4

;system call number (sys_write)

int 0x80

;call kernel

mov eax,1

;system call number (sys_exit)

int 0x80

;call kernel

segment .data

;data segment

msg db 'Hello, world!',0xa

;our dear string

len equ

;length of our dear string

$ - msg

When the above code is compiled and executed, it produces the following result:
Hello, world!

Memory Segments
A segmented memory model divides the system memory into groups of independent segments referenced by pointers located in the segment registers.
Each segment is used to contain a specific type of data. One segment is used to contain instruction codes, another segment stores the data elements, and a third segment keeps the program stack.
In the light of the above discussion, we can specify various memory segments as: 13

Assembly Programming



Data segment - It is represented by .data section and the .bss. The
.data section is used to declare the memory region, where data elements are stored for the program. This section cannot be expanded after the data elements are declared, and it remains static throughout the program.
The .bss section is also a static memory section that contains buffers for data to be declared later in the program. This buffer memory is zerofilled.



Code segment - It is represented by .text section. This defines an area in memory that stores the instruction codes. This is also a fixed area.



Stack - This segment contains data values passed to functions and procedures within the program.

14

Assembly Programming

5. REGISTERS

Processor operations mostly involve processing data. This data can be stored in memory and accessed from thereon. However, reading data from and storing data into memory slows down the processor, as it involves complicated processes of sending the data request across the control bus and into the memory storage unit and getting the data through the same channel.
To speed up the processor operations, the processor includes some internal memory storage locations, called registers.
The registers store data elements for processing without having to access the memory. A limited number of registers are built into the processor chip.

Processor Registers
There are ten 32-bit and six 16-bit processor registers in IA-32 architecture. The registers are grouped into three categories:


General registers,



Control registers, and



Segment registers.

The general registers are further divided into the following groups:


Data registers,



Pointer registers, and



Index registers.

Data Registers
Four 32-bit data registers are used for arithmetic, logical, and other operations.
These 32-bit registers can be used in three ways:


As complete 32-bit data registers: EAX, EBX, ECX, EDX.



Lower halves of the 32-bit registers can be used as four 16-bit data registers: AX, BX, CX and DX.



Lower and higher halves of the above-mentioned four 16-bit registers can be used as eight 8-bit data registers: AH, AL, BH, BL, CH, CL, DH, and DL.

15

Assembly Programming

Some of these data registers have specific use in arithmetical operations.
AX is the primary accumulator; it is used in input/output and most arithmetic instructions. For example, in multiplication operation, one operand is stored in
EAX or AX or AL register according to the size of the operand.
BX is known as the base register, as it could be used in indexed addressing.
CX is known as the count register, as the ECX, CX registers store the loop count in iterative operations.
DX is known as the data register. It is also used in input/output operations.
It is also used with AX register along with DX for multiply and divide operations involving large values.

Pointer Registers
The pointer registers are 32-bit EIP, ESP, and EBP registers and corresponding
16-bit right portions IP, SP, and BP. There are three categories of pointer registers: 

Instruction Pointer (IP) - The 16-bit IP register stores the offset address of the next instruction to be executed. IP in association with the
CS register (as CS:IP) gives the complete address of the current instruction in the code segment.



Stack Pointer (SP) - The 16-bit SP register provides the offset value within the program stack. SP in association with the SS register (SS:SP) refers to be current position of data or address within the program stack.



Base Pointer (BP) - The 16-bit BP register mainly helps in referencing the parameter variables passed to a subroutine. The address in SS register is combined with the offset in BP to get the location of the parameter. BP can also be combined with DI and SI as base register for special addressing.

16

Assembly Programming

Index Registers
The 32-bit index registers, ESI and EDI, and their 16-bit rightmost portions, SI and DI, are used for indexed addressing and sometimes used in addition and subtraction. There are two sets of index pointers:


Source Index (SI) - It is used as source index for string operations.



Destination Index (DI) - It is used as destination index for string operations. Control Registers
The 32-bit instruction pointer register and the 32-bit flags register combined are considered as the control registers.
Many instructions involve comparisons and mathematical calculations and change the status of the flags and some other conditional instructions test the value of these status flags to take the control flow to other location.
The common flag bits are:


Overflow Flag (OF): It indicates the overflow of a high-order bit
(leftmost bit) of data after a signed arithmetic operation.



Direction Flag (DF): It determines left or right direction for moving or comparing string data. When the DF value is 0, the string operation takes left-to-right direction and when the value is set to 1, the string operation takes right-to-left direction.



Interrupt Flag (IF): It determines whether the external interrupts like keyboard entry, etc., are to be ignored or processed. It disables the external interrupt when the value is 0 and enables interrupts when set to
1.

17

Assembly Programming



Trap Flag (TF): It allows setting the operation of the processor in singlestep mode. The DEBUG program we used sets the trap flag, so we could step through the execution one instruction at a time.



Sign Flag (SF): It shows the sign of the result of an arithmetic operation.
This flag is set according to the sign of a data item following the arithmetic operation. The sign is indicated by the high-order of leftmost bit. A positive result clears the value of SF to 0 and negative result sets it to 1.



Zero Flag (ZF): It indicates the result of an arithmetic or comparison operation. A nonzero result clears the zero flag to 0, and a zero result sets it to 1.



Auxiliary Carry Flag (AF): It contains the carry from bit 3 to bit 4 following an arithmetic operation; used for specialized arithmetic. The AF is set when a 1-byte arithmetic operation causes a carry from bit 3 into bit
4.



Parity Flag (PF): It indicates the total number of 1-bits in the result obtained from an arithmetic operation. An even number of 1-bits clears the parity flag to 0 and an odd number of 1-bits sets the parity flag to 1.



Carry Flag (CF): It contains the carry of 0 or 1 from a high-order bit
(leftmost) after an arithmetic operation. It also stores the contents of last bit of a shift or rotate operation.

The following table indicates the position of flag bits in the 16-bit Flags register:
Flag:
Bit no:

O
15

14

13

12

D

I

T

S

Z

11

10

9

8

7

6

A
5

4

P
3

2

C
1

0

Segment Registers
Segments are specific areas defined in a program for containing data, code and stack. There are three main segments:


Code Segment: It contains all the instructions to be executed. A 16-bit
Code Segment register or CS register stores the starting address of the code segment.



Data Segment: It contains data, constants and work areas. A 16-bit
Data Segment register or DS register stores the starting address of the data segment.



Stack Segment: It contains data and return addresses of procedures or subroutines. It is implemented as a 'stack' data structure. The Stack
Segment register or SS register stores the starting address of the stack.
18

Assembly Programming

Apart from the DS, CS and SS registers, there are other extra segment registers
- ES (extra segment), FS and GS, which provide additional segments for storing data. In assembly programming, a program needs to access the memory locations. All memory locations within a segment are relative to the starting address of the segment. A segment begins in an address evenly divisible by 16 or hexadecimal
10. So, the rightmost hex digit in all such memory addresses is 0, which is not generally stored in the segment registers.
The segment registers stores the starting addresses of a segment. To get the exact location of data or instruction within a segment, an offset value (or displacement) is required. To reference any memory location in a segment, the processor combines the segment address in the segment register with the offset value of the location.
Example:
Look at the following simple program to understand the use of registers in assembly programming. This program displays 9 stars on the screen along with a simple message: section global

.text
_start

_start:

;must be declared for linker (gcc)
;tell linker entry point

mov

edx,len

;message length

mov

ecx,msg

;message to write

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

mov

edx,9

;message length

mov

ecx,s2

;message to write

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

mov

eax,1

;system call number (sys_exit)

int

0x80

;call kernel

section

.data

msg db 'Displaying 9 stars',0xa ;a message len equ $ - msg

;length of message
19

Assembly Programming

s2 times 9 db '*'
When the above code is compiled and executed, it produces the following result:
Displaying 9 stars
*********

20

Assembly Programming

6. SYSTEM CALLS

System calls are APIs for the interface between the user space and the kernel space. We have already used the system calls, sys_write and sys_exit, for writing into the screen and exiting from the program, respectively.

Linux System Calls
You can make use of Linux system calls in your assembly programs. You need to take the following steps for using Linux system calls in your program:


Put the system call number in the EAX register.



Store the arguments to the system call in the registers EBX, ECX, etc.



Call the relevant interrupt (80h).



The result is usually returned in the EAX register.

There are six registers that store the arguments of the system call used. These are the EBX, ECX, EDX, ESI, EDI, and EBP. These registers take the consecutive arguments, starting with the EBX register. If there are more than six arguments, then the memory location of the first argument is stored in the EBX register.
The following code snippet shows the use of the system call sys_exit: mov eax,1

; system call number (sys_exit)

int 0x80

; call kernel

The following code snippet shows the use of the system call sys_write: mov edx,4

; message length

mov ecx,msg

; message to write

mov ebx,1

; file descriptor (stdout)

mov eax,4

; system call number (sys_write)

int 0x80

; call kernel

All the syscalls are listed in /usr/include/asm/unistd.h, together with their numbers (the value to put in EAX before you call int 80h).
The following table shows some of the system calls used in this tutorial:
%eax

Name

%ebx

%ecx

%edx

%esx

%edi

1

sys_exit

int

-

-

-

21

Assembly Programming

2

sys_fork

struct pt_regs

-

-

-

-

3

sys_read

unsigned int

char *

size_t

-

-

4

sys_write

unsigned int

const char *

size_t

-

-

5

sys_open

const char *

int

int

-

-

6

sys_close

unsigned int

-

-

-

-

Example
The following example reads a number from the keyboard and displays it on the screen: section

.data ;Data segment

userMsg db 'Please enter a number: ' ;Ask the user to enter a number lenUserMsg equ $-userMsg

;The length of the message

dispMsg db 'You have entered: ' lenDispMsg equ $-dispMsg

section .bss

;Uninitialized data

num resb 5 section .text

;Code Segment

global _start
_start:
;User prompt mov eax, 4 mov ebx, 1 mov ecx, userMsg mov edx, lenUserMsg int 80h

;Read and store the user input mov eax, 3 mov ebx, 2
22

Assembly Programming

mov ecx, num mov edx, 5

;5 bytes (numeric, 1 for sign) of that information

int 80h
;Output the message 'The entered number is: ' mov eax, 4 mov ebx, 1 mov ecx, dispMsg mov edx, lenDispMsg int 80h

;Output the number entered mov eax, 4 mov ebx, 1 mov ecx, num mov edx, 5 int 80h
; Exit code mov eax, 1 mov ebx, 0 int 80h
When the above code is compiled and executed, it produces the following result:
Please enter a number:
1234
You have entered:1234

23

Assembly Programming

7. ADDRESSING MODES

Most assembly language instructions require operands to be processed. An operand address provides the location, where the data to be processed is stored.
Some instructions do not require an operand, whereas some other instructions may require one, two, or three operands.
When an instruction requires two operands, the first operand is generally the destination, which contains data in a register or memory location and the second operand is the source. Source contains either the data to be delivered
(immediate addressing) or the address (in register or memory) of the data.
Generally, the source data remains unaltered after the operation.
The three basic modes of addressing are:


Register addressing



Immediate addressing



Memory addressing

Register Addressing
In this addressing mode, a register contains the operand. Depending upon the instruction, the register may be the first operand, the second operand or both.
For example,
MOV DX, TAX_RATE

; Register in first operand

MOV COUNT, CX

; Register in second operand

MOV EAX, EBX

; Both the operands are in registers

As processing data between registers does not involve memory, it provides fastest processing of data.

Immediate Addressing
An immediate operand has a constant value or an expression. When an instruction with two operands uses immediate addressing, the first operand may be a register or memory location, and the second operand is an immediate constant. The first operand defines the length of the data.
For example,
BYTE_VALUE

DB

150

; A byte value is defined

WORD_VALUE

DW

300

; A word value is defined
24

Assembly Programming

ADD

BYTE_VALUE, 65

; An immediate operand 65 is added

MOV

AX, 45H

; Immediate constant 45H is transferred to AX

Direct Memory Addressing
When operands are specified in memory addressing mode, direct access to main memory, usually to the data segment, is required. This way of addressing results in slower processing of data. To locate the exact location of data in memory, we need the segment start address, which is typically found in the DS register and an offset value. This offset value is also called effective address.
In direct addressing mode, the offset value is specified directly as part of the instruction, usually indicated by the variable name. The assembler calculates the offset value and maintains a symbol table, which stores the offset values of all the variables used in the program.
In direct memory addressing, one of the operands refers to a memory location and the other operand references a register.
For example,
ADD BYTE_VALUE, DL

; Adds the register in the memory location

MOV BX, WORD_VALUE

; Operand from the memory is added to register

Direct-Offset Addressing
This addressing mode uses the arithmetic operators to modify an address. For example, look at the following definitions that define tables of data:
BYTE_TABLE DB

14, 15, 22, 45

; Tables of bytes

WORD_TABLE DW

134, 345, 564, 123

; Tables of words

The following operations access data from the tables in the memory into registers: MOV CL, BYTE_TABLE[2] ; Gets the 3rd element of the BYTE_TABLE
MOV CL, BYTE_TABLE + 2 ; Gets the 3rd element of the BYTE_TABLE
MOV CX, WORD_TABLE[3] ; Gets the 4th element of the WORD_TABLE
MOV CX, WORD_TABLE + 3 ; Gets the 4th element of the WORD_TABLE

Indirect Memory Addressing
This addressing mode utilizes the computer's ability of Segment:Offset addressing. Generally, the base registers EBX, EBP (or BX, BP) and the index registers (DI, SI), coded within square brackets for memory references, are used for this purpose.
25

Assembly Programming

Indirect addressing is generally used for variables containing several elements like, arrays. Starting address of the array is stored in, say, the EBX register.
The following code snippet shows how to access different elements of the variable. MY_TABLE TIMES 10 DW 0

; Allocates 10 words (2 bytes) each initialized to 0

MOV EBX, [MY_TABLE]

; Effective Address of MY_TABLE in EBX

MOV [EBX], 110

; MY_TABLE[0] = 110

ADD EBX, 2

; EBX = EBX +2

MOV [EBX], 123

; MY_TABLE[1] = 123

The MOV Instruction
We have already used the MOV instruction that is used for moving data from one storage space to another. The MOV instruction takes two operands.

Syntax
The syntax of the MOV instruction is:
MOV

destination, source

The MOV instruction may have one of the following five forms:
MOV

register, register

MOV

register, immediate

MOV

memory, immediate

MOV

register, memory

MOV

memory, register

Please note that:


Both the operands in MOV operation should be of same size



The value of source operand remains unchanged

The MOV instruction causes ambiguity at times. For example, look at the statements: MOV

EBX, [MY_TABLE]

; Effective Address of MY_TABLE in EBX

MOV

[EBX], 110

; MY_TABLE[0] = 110

It is not clear whether you want to move a byte equivalent or word equivalent of the number 110. In such cases, it is wise to use a type specifier.
Following table shows some of the common type specifiers:
26

Assembly Programming

Type Specifier

Bytes addressed

BYTE

1

WORD

2

DWORD

4

QWORD

8

TBYTE

10

Example
The following program illustrates some of the concepts discussed above. It stores a name 'Zara Ali' in the data section of the memory, then changes its value to another name 'Nuha Ali' programmatically and displays both the names. section global
_start:

.text
_start

;must be declared for linker (ld)

;tell linker entry point

;writing the name 'Zara Ali' mov edx,9

;message length

mov

ecx, name

;message to write

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

mov

[name],

dword 'Nuha'

; Changed the name to Nuha Ali

;writing the name 'Nuha Ali' mov edx,8

;message length

mov

ecx,name

;message to write

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

mov

eax,1

;system call number (sys_exit)
27

Assembly Programming

int

section

0x80

;call kernel

.data

name db 'Zara Ali '
When the above code is compiled and executed, it produces the following result:
Zara Ali Nuha Ali

28

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8. VARIABLES

NASM provides various define directives for reserving storage space for variables. The define assembler directive is used for allocation of storage space.
It can be used to reserve as well as initialize one or more bytes.

Allocating Storage Space for Initialized Data
The syntax for storage allocation statement for initialized data is:
[variable-name]

define-directive

initial-value

[,initial-value]...

Where, variable-name is the identifier for each storage space. The assembler associates an offset value for each variable name defined in the data segment.
There are five basic forms of the define directive:
Directive

Purpose

Storage Space

DB

Define Byte

allocates 1 byte

DW

Define Word

allocates 2 bytes

DD

Define Doubleword

allocates 4 bytes

DQ

Define Quadword

allocates 8 bytes

DT

Define Ten Bytes

allocates 10 bytes

Following are some examples of using define directives: choice DB

'y'

number

DW

12345

neg_number

DW

-12345

big_number

DQ

123456789

real_number1

DD

1.234

real_number2

DQ

123.456

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Please note that:


Each byte of character is stored as its ASCII value in hexadecimal.



Each decimal value is automatically converted to its 16-bit binary equivalent and stored as a hexadecimal number.



Processor uses the little-endian byte ordering.



Negative numbers are converted to its 2's complement representation.



Short and long floating-point numbers are represented using 32 or 64 bits, respectively.

The following program shows the use of define directive: section .text global _start
_start:

;must be declared for linker (gcc)

;tell linker entry point

mov

edx,1

;message length

mov

ecx,choice

;message to write

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

mov

eax,1

;system call number (sys_exit)

int

0x80

;call kernel

section .data choice DB 'y'
When the above code is compiled and executed, it produces the following result: y Allocating Storage Space for Uninitialized Data
The reserve directives are used for reserving space for uninitialized data. The reserve directives take a single operand that specifies the number of units of space to be reserved. Each define directive has a related reserve directive.
There are five basic forms of the reserve directive:

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Assembly Programming

Directive

Purpose

RESB

Reserve a Byte

RESW

Reserve a Word

RESD

Reserve a Doubleword

RESQ

Reserve a Quadword

REST

Reserve a Ten Bytes

Multiple Definitions
You can have multiple data definition statements in a program. For example: choice DB

'Y'

;ASCII of y = 79H

number1

DW

12345

;12345D = 3039H

12345679

;123456789D = 75BCD15H

number2

DD

The assembler allocates contiguous memory for multiple variable definitions.

Multiple Initializations
The TIMES directive allows multiple initializations to the same value. For example, an array named marks of size 9 can be defined and initialized to zero using the following statement: marks TIMES

9

DW

0

The TIMES directive is useful in defining arrays and tables. The following program displays 9 asterisks on the screen: section .text

global _start
_start:

;must be declared for linker (ld)

;tell linker entry point

mov

edx,9

;message length

mov

ecx, stars

;message to write

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel
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mov

eax,1

;system call number (sys_exit)

int

0x80

;call kernel

section stars .data times 9 db '*'

When the above code is compiled and executed, it produces the following result:
*********

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9. CONSTANTS

There are several directives provided by NASM that define constants. We have already used the EQU directive in previous chapters. We will particularly discuss three directives:


EQU



%assign



%define

The EQU Directive
The EQU directive is used for defining constants. The syntax of the EQU directive is as follows:
CONSTANT_NAME EQU expression
For example,
TOTAL_STUDENTS equ 50
You can then use this constant value in your code, like: mov ecx,

TOTAL_STUDENTS

cmp

eax,

TOTAL_STUDENTS

The operand of an EQU statement can be an expression:
LENGTH equ 20
WIDTH

equ 10

AREA

equ length * width

Above code segment would define AREA as 200.

Example
The following example illustrates the use of the EQU directive:
SYS_EXIT

equ 1

SYS_WRITE equ 4
STDIN

equ 0

STDOUT

equ 1

section

.text
33

Assembly Programming

global _start
_start:

;must be declared for using gcc

;tell linker entry point

mov eax, SYS_WRITE mov ebx, STDOUT mov ecx, msg1 mov edx, len1 int 0x80

mov eax, SYS_WRITE mov ebx, STDOUT mov ecx, msg2 mov edx, len2 int 0x80

mov eax, SYS_WRITE mov ebx, STDOUT mov ecx, msg3 mov edx, len3 int 0x80 mov eax,SYS_EXIT int 0x80

section msg1 db

;system call number (sys_exit)
;call kernel

.data
'Hello, programmers!',0xA,0xD

len1 equ $ - msg1 msg2 db 'Welcome to the world of,', 0xA,0xD len2 equ $ - msg2 msg3 db 'Linux assembly programming! ' len3 equ $- msg3
When the above code is compiled and executed, it produces the following result:
Hello, programmers!
Welcome to the world of,
Linux assembly programming!
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The %assign Directive
The %assign directive can be used to define numeric constants like the EQU directive. This directive allows redefinition. For example, you may define the constant TOTAL as:
%assign TOTAL 10
Later in the code, you can redefine it as:
%assign

TOTAL

20

This directive is case-sensitive.

The %define Directive
The %define directive allows defining both numeric and string constants. This directive is similar to the #define in C. For example, you may define the constant PTR as:
%define PTR [EBP+4]
The above code replaces PTR by [EBP+4].
This directive also allows redefinition and it is case-sensitive.

35

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10. ARITHMETIC INSTRUCTIONS
The INC Instruction

The INC instruction is used for incrementing an operand by one. It works on a single operand that can be either in a register or in memory.

Syntax
The INC instruction has the following syntax:
INC destination
The operand destination could be an 8-bit, 16-bit or 32-bit operand.

Example
INC EBX

; Increments 32-bit register

INC DL

; Increments 8-bit register

INC [count]

; Increments the count variable

The DEC Instruction
The DEC instruction is used for decrementing an operand by one. It works on a single operand that can be either in a register or in memory.

Syntax
The DEC instruction has the following syntax:
DEC destination
The operand destination could be an 8-bit, 16-bit or 32-bit operand.

Example segment .data count dw

0

value db

15

segment .text inc [count] dec [value] mov ebx, count
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Assembly Programming

inc word [ebx] mov esi, value dec byte [esi]

The ADD and SUB Instructions
The ADD and SUB instructions are used for performing simple addition/subtraction of binary data in byte, word and doubleword size, i.e., for adding or subtracting 8-bit, 16-bit or 32-bit operands, respectively.

Syntax
The ADD and SUB instructions have the following syntax:
ADD/SUB

destination, source

The ADD/SUB instruction can take place between:


Register to register



Memory to register



Register to memory



Register to constant data



Memory to constant data

However, like other instructions, memory-to-memory operations are not possible using ADD/SUB instructions. An ADD or SUB operation sets or clears the overflow and carry flags.

Example
The following example will ask two digits from the user, store the digits in the
EAX and EBX register, respectively, add the values, store the result in a memory location 'res' and finally display the result.
SYS_EXIT

equ 1

SYS_READ

equ 3

SYS_WRITE equ 4
STDIN

equ 0

STDOUT

equ 1

segment .data

msg1 db "Enter a digit ", 0xA,0xD
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Assembly Programming

len1 equ $- msg1

msg2 db "Please enter a second digit", 0xA,0xD len2 equ $- msg2

msg3 db "The sum is: " len3 equ $- msg3

segment .bss

num1 resb 2 num2 resb 2 res resb 1

section

.text

global _start
_start:

;must be declared for using gcc

;tell linker entry point

mov eax, SYS_WRITE mov ebx, STDOUT mov ecx, msg1 mov edx, len1 int 0x80

mov eax, SYS_READ mov ebx, STDIN mov ecx, num1 mov edx, 2 int 0x80

mov eax, SYS_WRITE mov ebx, STDOUT mov ecx, msg2 mov edx, len2 int 0x80
38

Assembly Programming

mov eax, SYS_READ mov ebx, STDIN mov ecx, num2 mov edx, 2 int 0x80

mov eax, SYS_WRITE mov ebx, STDOUT mov ecx, msg3 mov edx, len3 int 0x80

; moving the first number to eax register and second number to ebx
; and subtracting ascii '0' to convert it into a decimal number mov eax, [number1] sub eax, '0' mov ebx, [number2] sub ebx, '0'

; add eax and ebx add eax, ebx
; add '0' to to convert the sum from decimal to ASCII add eax, '0'

; storing the sum in memory location res mov [res], eax

; print the sum mov eax, SYS_WRITE mov ebx, STDOUT mov ecx, res mov edx, 1 int 0x80
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Assembly Programming

exit: mov eax, SYS_EXIT xor ebx, ebx int 0x80
When the above code is compiled and executed, it produces the following result:
Enter a digit:
3
Please enter a second digit:
4
The sum is:
7
The program with hardcoded variables: section .text

global _start
_start:
mov sub mov sub ;must be declared for using gcc

;tell linker entry point eax,'3' eax, '0' ebx, '4' ebx, '0'

add

eax, ebx

add

eax, '0'

mov

[sum], eax

mov

ecx,msg

mov

edx, len

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

ecx,sum

mov

edx, 1

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

eax,1 ;system call number (sys_exit)

;call kernel

;call kernel
40

Assembly Programming

int

0x80

;call kernel

section .data msg db "The sum is:", 0xA,0xD len equ $ - msg segment .bss sum resb 1
When the above code is compiled and executed, it produces the following result:
The sum is:
7

The MUL/IMUL Instruction
There are two instructions for multiplying binary data. The MUL (Multiply) instruction handles unsigned data and the IMUL (Integer Multiply) handles signed data. Both instructions affect the Carry and Overflow flag.

Syntax
The syntax for the MUL/IMUL instructions is as follows:
MUL/IMUL multiplier
Multiplicand in both cases will be in an accumulator, depending upon the size of the multiplicand and the multiplier and the generated product is also stored in two registers depending upon the size of the operands. Following section explains MUL instructions with three different cases:
SN

Scenarios

1

When two bytes are multiplied The multiplicand is in the AL register, and the multiplier is a byte in the memory or in another register. The product is in AX. High-order 8 bits of the product is stored in AH and the low-order 8 bits are stored in AL.

2

When two one-word values are multiplied The multiplicand should be in the AX register, and the multiplier is a word
41

Assembly Programming

in memory or another register. For example, for an instruction like MUL
DX, you must store the multiplier in DX and the multiplicand in AX.
The resultant product is a doubleword, which will need two registers. The high-order (leftmost) portion gets stored in DX and the lower-order
(rightmost) portion gets stored in AX.

3

When two doubleword values are multiplied When two doubleword values are multiplied, the multiplicand should be in
EAX and the multiplier is a doubleword value stored in memory or in another register. The product generated is stored in the EDX:EAX registers, i.e., the high order 32 bits gets stored in the EDX register and the low order 32-bits are stored in the EAX register.

Example
MOV AL, 10
MOV DL, 25
MUL DL
...
MOV DL, 0FFH

; DL= -1

MOV AL, 0BEH

; AL = -66

IMUL DL

Example
The following example multiplies 3 with 2, and displays the result: section .text

global _start
_start:

mov sub mov

;must be declared for using gcc

;tell linker entry point

al,'3' al, '0' bl, '2'
42

Assembly Programming

sub

bl, '0'

mul

bl

add

al, '0'

mov

[res], al

mov

ecx,msg

mov

edx, len

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

ecx,res

mov

edx, 1

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

eax,1 ;system call number (sys_exit)

int

0x80

;call kernel

;call kernel

;call kernel

section .data msg db "The result is:", 0xA,0xD len equ $- msg segment .bss res resb 1
When the above code is compiled and executed, it produces the following result:
The result is:
6

The DIV/IDIV Instructions
The division operation generates two elements - a quotient and a remainder.
In case of multiplication, overflow does not occur because double-length registers are used to keep the product. However, in case of division, overflow may occur. The processor generates an interrupt if overflow occurs.
The DIV (Divide) instruction is used for unsigned data and the IDIV (Integer
Divide) is used for signed data.

Syntax
43

Assembly Programming

The format for the DIV/IDIV instruction:
DIV/IDIV

divisor

The dividend is in an accumulator. Both the instructions can work with 8-bit, 16bit or 32-bit operands. The operation affects all six status flags. Following section explains three cases of division with different operand size:
SN

Scenarios

1

When the divisor is 1 byte The dividend is assumed to be in the AX register (16 bits). After division, the quotient goes to the AL register and the remainder goes to the AH register. 2

When the divisor is 1 word The dividend is assumed to be 32 bits long and in the DX:AX registers.
The high-order 16 bits are in DX and the low-order 16 bits are in AX.
After division, the 16-bit quotient goes to the AX register and the 16-bit remainder goes to the DX register.

3

When the divisor is doubleword The dividend is assumed to be 64 bits long and in the EDX:EAX registers.
The high-order 32 bits are in EDX and the low-order 32 bits are in EAX.
After division, the 32-bit quotient goes to the EAX register and the 32-bit remainder goes to the EDX register.
44

Assembly Programming

Example
The following example divides 8 with 2. The dividend 8 is stored in the 16-bit
AX register and the divisor 2 is stored in the 8-bit BL register. section .text

global _start
_start:
mov

;tell linker entry point ax,'8' sub mov ;must be declared for using gcc

ax, '0' bl, '2'

sub

bl, '0'

div

bl

add

ax, '0'

mov

[res], ax

mov

ecx,msg

mov

edx, len

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

ecx,res

mov

edx, 1

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

eax,1 ;system call number (sys_exit)

int

0x80

;call kernel

;call kernel

;call kernel

section .data
45

Assembly Programming

msg db "The result is:", 0xA,0xD len equ $- msg segment .bss res resb 1
When the above code is compiled and executed, it produces the following result:
The result is:
4

46

Assembly Programming

11. LOGICAL INSTRUCTIONS

The processor instruction set provides the instructions AND, OR, XOR, TEST, and
NOT Boolean logic, which tests, sets, and clears the bits according to the need of the program.
The format for these instructions:
SN

Instruction

Format

1

AND

AND operand1, operand2

2

OR

OR operand1, operand2

3

XOR

XOR operand1, operand2

4

TEST

TEST operand1, operand2

5

NOT

NOT operand1

The first operand in all the cases could be either in register or in memory. The second operand could be either in register/memory or an immediate (constant) value. However, memory-to-memory operations are not possible. These instructions compare or match bits of the operands and set the CF, OF, PF, SF and ZF flags.

The AND Instruction
The AND instruction is used for supporting logical expressions by performing bitwise AND operation. The bitwise AND operation returns 1, if the matching bits from both the operands are 1, otherwise it returns 0. For example:
Operand1:

0101

Operand2:

0011

---------------------------After AND -> Operand1: 0001
The AND operation can be used for clearing one or more bits. For example, say the BL register contains 0011 1010. If you need to clear the high-order bits to zero, you AND it with 0FH.
47

Assembly Programming

AND BL,

0FH

; This sets BL to 0000 1010

Let's take up another example. If you want to check whether a given number is odd or even, a simple test would be to check the least significant bit of the number. If this is 1, the number is odd, else the number is even.
Assuming the number is in AL register, we can write:
AND AL, 01H
JZ

; ANDing with 0000 0001

EVEN_NUMBER

The following program illustrates this:

Example section .text

global _start

;must be declared for using gcc

_start:

;tell linker entry point

mov

ax,

8h

;getting 8 in the ax

and

ax, 1

jz

evnn

mov

eax, 4

;system call number (sys_write)

mov

ebx, 1

;file descriptor (stdout)

mov

ecx, odd_msg

;message to write

mov

edx, len2

;length of message

int

0x80

;call kernel

jmp

outprog

;and ax with 1

evnn: mov ah,

09h

mov

eax, 4

;system call number (sys_write)

mov

ebx, 1

;file descriptor (stdout)

mov

ecx, even_msg

;message to write

mov

edx, len1

;length of message

int

0x80

;call kernel

mov

eax,1

;system call number (sys_exit)

int

0x80

;call kernel

outprog:

section

.data

even_msg

db

'Even Number!' ;message showing even number
48

Assembly Programming

len1

equ

$ - even_msg

odd_msg db len2 equ

'Odd Number!'

;message showing odd number

$ - odd_msg

When the above code is compiled and executed, it produces the following result:
Even Number!
Change the value in the ax register with an odd digit, like: mov ax, 9h

; getting 9 in the ax

The program would display:
Odd Number!
Similarly, to clear the entire register, you can AND it with 00H.

The OR Instruction
The OR instruction is used for supporting logical expression by performing bitwise OR operation. The bitwise OR operator returns 1, if the matching bits from either or both operands are one. It returns 0, if both the bits are zero.
For example,
Operand1:

0101

Operand2:

0011

---------------------------After OR -> Operand1:

0111

The OR operation can be used for setting one or more bits. For example, let us assume the AL register contains 0011 1010, you need to set the four low-order bits, you can OR it with a value 0000 1111, i.e., FH.
OR BL, 0FH

; This sets BL to

0011 1111

Example
The following example demonstrates the OR instruction. Let us store the value 5 and 3 in the AL and the BL registers, respectively, then the instruction,
OR AL, BL should store 7 in the AL register: section .text
49

Assembly Programming

global _start

;must be declared for using gcc

_start:

;tell linker entry point

mov

al, 5

;getting 5 in the al

mov

bl, 3

;getting 3 in the bl

or

al, bl

;or al and bl registers, result should be 7

add

al, byte '0'

;converting decimal to ascii

mov

[result],

mov

eax, 4

mov

ebx, 1

mov

ecx, result

mov

edx, 1

int

0x80

al

outprog: mov eax,1

;system call number (sys_exit)

int

0x80

;call kernel

section

.bss

result resb 1
When the above code is compiled and executed, it produces the following result:
7

The XOR Instruction
The XOR instruction implements the bitwise XOR operation. The XOR operation sets the resultant bit to 1, if and only if the bits from the operands are different.
If the bits from the operands are same (both 0 or both 1), the resultant bit is cleared to 0.
For example,
Operand1:

0101

Operand2:

0011

---------------------------After XOR -> Operand1:

0110

XORing an operand with itself changes the operand to 0. This is used to clear a register. 50

Assembly Programming

XOR

EAX, EAX

The TEST Instruction
The TEST instruction works same as the AND operation, but unlike AND instruction, it does not change the first operand. So, if we need to check whether a number in a register is even or odd, we can also do this using the
TEST instruction without changing the original number.
TEST

AL, 01H

JZ

EVEN_NUMBER

The NOT Instruction
The NOT instruction implements the bitwise NOT operation. NOT operation reverses the bits in an operand. The operand could be either in a register or in the memory.
For example,
Operand1:

0101 0011

After NOT -> Operand1:

1010 1100

51

Assembly Programming

12. CONDITIONS

Conditional execution in assembly language is accomplished by several looping and branching instructions. These instructions can change the flow of control in a program. Conditional execution is observed in two scenarios:
SN

Conditional Instructions

1

Unconditional jump
This is performed by the JMP instruction. Conditional execution often involves a transfer of control to the address of an instruction that does not follow the currently executing instruction. Transfer of control may be forward, to execute a new set of instructions or backward, to re-execute the same steps.

2

Conditional jump
This is performed by a set of jump instructions j<condition> depending upon the condition. The conditional instructions transfer the control by breaking the sequential flow and they do it by changing the offset value in IP.

Let us discuss the CMP instruction before discussing the conditional instructions.

CMP Instruction
The CMP instruction compares two operands. It is generally used in conditional execution. This instruction basically subtracts one operand from the other for comparing whether the operands are equal or not. It does not disturb the destination or source operands. It is used along with the conditional jump instruction for decision making.

Syntax
CMP destination, source
CMP compares two numeric data fields. The destination operand could be either in register or in memory. The source operand could be a constant (immediate) data, register or memory.
Example
CMP DX,

00

; Compare the DX value with zero
52

Assembly Programming

JE

L7

; If yes, then jump to label L7

.
.
L7: ...
CMP is often used for comparing whether a counter value has reached the number of times a loop needs to be run. Consider the following typical condition:
INC EDX
CMP EDX, 10

; Compares whether the counter has reached 10

JLE LP1

; If it is less than or equal to 10, then jump to LP1

Unconditional Jump
As mentioned earlier, this is performed by the JMP instruction. Conditional execution often involves a transfer of control to the address of an instruction that does not follow the currently executing instruction. Transfer of control may be forward, to execute a new set of instructions or backward, to re-execute the same steps.

Syntax
The JMP instruction provides a label name where the flow of control is transferred immediately. The syntax of the JMP instruction is:
JMP label

Example
The following code snippet illustrates the JMP instruction:
MOV

AX, 00

; Initializing AX to 0

MOV

BX, 00

; Initializing BX to 0

MOV

CX, 01

; Initializing CX to 1

ADD

AX, 01

; Increment AX

ADD

BX, AX

; Add AX to BX

SHL

CX, 1

; shift left CX, this in turn doubles the CX value

JMP

L20

; repeats the statements

L20:

53

Assembly Programming

Conditional Jump
If some specified condition is satisfied in conditional jump, the control flow is transferred to a target instruction. There are numerous conditional jump instructions depending upon the condition and data.
Following are the conditional jump instructions used on signed data used for arithmetic operations:
Instruction

Description

Flags tested

JE/JZ

Jump Equal or Jump Zero

ZF

JNE/JNZ

Jump not Equal or Jump Not Zero

ZF

JG/JNLE

Jump Greater or Jump Not Less/Equal

OF, SF, ZF

JGE/JNL

Jump Greater or Jump Not Less

OF, SF

JL/JNGE

Jump Less or Jump Not Greater/Equal

OF, SF

JLE/JNG

Jump Less/Equal or Jump Not Greater

OF, SF, ZF

Following are the conditional jump instructions used on unsigned data used for logical operations:
Instruction

Description

Flags tested

JE/JZ

Jump Equal or Jump Zero

ZF

JNE/JNZ

Jump not Equal or Jump Not Zero

ZF

JA/JNBE

Jump Above or Jump Not Below/Equal

CF, ZF

JAE/JNB

Jump Above/Equal or Jump Not Below

CF

JB/JNAE

Jump Below or Jump Not Above/Equal

CF

JBE/JNA

Jump Below/Equal or Jump Not Above

AF, CF

54

Assembly Programming

The following conditional jump instructions have special uses and check the value of flags:
Instruction

Description

Flags tested

JXCZ

Jump if CX is Zero

none

JC

Jump If Carry

CF

JNC

Jump If No Carry

CF

JO

Jump If Overflow

OF

JNO

Jump If No Overflow

OF

JP/JPE

Jump Parity or Jump Parity Even

PF

JNP/JPO

Jump No Parity or Jump Parity Odd

PF

JS

Jump Sign (negative value)

SF

JNS

Jump No Sign (positive value)

SF

The syntax for the J<condition> set of instructions:

Example
CMP AL, BL
JE

EQUAL

CMP AL, BH
JE

EQUAL

CMP AL, CL
JE

EQUAL

NON_EQUAL: ...
EQUAL: ...

55

Assembly Programming

Example
The following program displays the largest of three variables. The variables are double-digit variables. The three variables num1, num2 and num3 have values
47, 72 and 31, respectively: section .text

global _start

_start:

;must be declared for using gcc

;tell linker entry point

mov

ecx, [num1] cmp ecx, [num2]

jg

check_third_num

mov

ecx, [num3]

check_third_num: cmp ecx, [num3]

jg

_exit

mov

ecx, [num3]

_exit: mov mov

ecx,msg

mov

edx, len

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

mov

ecx,largest

mov

edx, 2

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

mov

eax, 1

int

section

[largest], ecx

80h

.data

msg db "The largest digit is: ", 0xA,0xD
56

Assembly Programming

len equ $- msg num1 dd '47' num2 dd '22' num3 dd '31'

segment .bss largest resb 2
When the above code is compiled and executed, it produces the following result:
The largest digit is:
47

57

13. LOOPS

Assembly Programming

The JMP instruction can be used for implementing loops. For example, the following code snippet can be used for executing the loop-body 10 times.
MOV CL, 10
L1:
<LOOP-BODY>
DEC CL
JNZ L1
The processor instruction set, however, includes a group of loop instructions for implementing iteration. The basic LOOP instruction has the following syntax:
LOOP

label

Where, label is the target label that identifies the target instruction as in the jump instructions. The LOOP instruction assumes that the ECX register contains the loop count. When the loop instruction is executed, the ECX register is decremented and the control jumps to the target label, until the ECX register value, i.e., the counter reaches the value zero.
The above code snippet could be written as: mov ECX,10 l1: <loop body> loop l1

Example
The following program prints the number 1 to 9 on the screen: section .text

global _start
_start:

;must be declared for using gcc
;tell linker entry point

mov ecx,10 mov eax, '1'

l1:
58

Assembly Programming

mov [num], eax mov eax, 4 mov ebx, 1 push ecx mov ecx, num mov edx, 1 int 0x80 mov eax, [num] sub eax, '0' inc eax add eax, '0' pop ecx loop l1 mov eax,1

;system call number (sys_exit)

int 0x80

;call kernel

section

.bss

num resb 1
When the above code is compiled and executed, it produces the following result:
123456789:

59

Assembly Programming

14. NUMBERS

Numerical data is generally represented in binary system. Arithmetic instructions operate on binary data. When numbers are displayed on screen or entered from keyboard, they are in ASCII form.
So far, we have converted this input data in ASCII form to binary for arithmetic calculations and converted the result back to binary. The following code shows this: section

.text

global _start
_start:

;must be declared for using gcc

;tell linker entry point

mov

eax,'3'

sub

eax, '0'

mov

ebx, '4'

sub

ebx, '0'

add

eax, ebx

add

eax, '0'

mov

[sum], eax

mov

ecx,msg

mov

edx, len

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

ecx,sum

mov

edx, 1

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

eax,1 ;system call number (sys_exit)

int

0x80

;call kernel

;call kernel

;call kernel

section .data msg db "The sum is:", 0xA,0xD len equ $ - msg
60

Assembly Programming

segment .bss sum resb 1
When the above code is compiled and executed, it produces the following result:
The sum is:
7
Such conversions, however, have an overhead, and assembly language programming allows processing numbers in a more efficient way, in the binary form. Decimal numbers can be represented in two forms:


ASCII form



BCD or Binary Coded Decimal form

ASCII Representation
In ASCII representation, decimal numbers are stored as string of ASCII characters. For example, the decimal value 1234 is stored as:
31

32

33

34H

Where, 31H is ASCII value for 1, 32H is ASCII value for 2, and so on. There are four instructions for processing numbers in ASCII representation:


AAA - ASCII Adjust After Addition



AAS - ASCII Adjust After Subtraction



AAM - ASCII Adjust After Multiplication



AAD - ASCII Adjust Before Division

These instructions do not take any operands and assume the required operand to be in the AL register.
The following example uses the AAS instruction to demonstrate the concept: section .text

global _start
_start:

;must be declared for using gcc

;tell linker entry point

sub

ah, ah

mov

al, '9'

sub

al, '3'

aas or al, 30h
61

Assembly Programming

mov

[res], ax

mov

edx,len

;message length

mov

ecx,msg

;message to write

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

edx,1 ;message length

mov

ecx,res

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

eax,1 ;system call number (sys_exit)

int

0x80

section

;call kernel

;message to write

;call kernel

;call kernel

.data

msg db 'The Result is:',0xa len equ $ - msg section .bss res resb 1
When the above code is compiled and executed, it produces the following result:
The Result is:
6

BCD Representation
There are two types of BCD representation:


Unpacked BCD representation



Packed BCD representation

In unpacked BCD representation, each byte stores the binary equivalent of a decimal digit. For example, the number 1234 is stored as:
01

02

03

04H
62

Assembly Programming

There are two instructions for processing these numbers:


AAM - ASCII Adjust After Multiplication



AAD - ASCII Adjust Before Division

The four ASCII adjust instructions, AAA, AAS, AAM, and AAD, can also be used with unpacked BCD representation. In packed BCD representation, each digit is stored using four bits. Two decimal digits are packed into a byte. For example, the number 1234 is stored as:
12

34H

There are two instructions for processing these numbers:


DAA - Decimal Adjust After Addition



DAS - decimal Adjust After Subtraction

There is no support for multiplication and division in packed BCD representation.

Example
The following program adds up two 5-digit decimal numbers and displays the sum. It uses the above concepts: section .text

global _start

_start:

;must be declared for using gcc

;tell linker entry point

mov

esi, 4

;pointing to the rightmost digit

mov

ecx, 5

;num of digits

clc add_loop: mov

al, [num1 + esi]

adc

al, [num2 + esi]

aaa pushf or

al, 30h

popf mov [sum + esi], al

dec

esi

loop

add_loop
63

Assembly Programming

mov

edx,len

;message length

mov

ecx,msg

;message to write

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

edx,5 ;message length

mov

ecx,sum

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

eax,1 ;system call number (sys_exit)

int

0x80

section

;call kernel

;message to write

;call kernel

;call kernel

.data

msg db 'The Sum is:',0xa len equ $ - msg num1 db '12345' num2 db '23456' sum db '

'

When the above code is compiled and executed, it produces the following result:
The Sum is:
35801

64

Assembly Programming

15. STRINGS

We have already used variable length strings in our previous examples. The variable length strings can have as many characters as required. Generally, we specify the length of the string by either of the two ways:


Explicitly storing string length



Using a sentinel character

We can store the string length explicitly by using the $ location counter symbol that represents the current value of the location counter. In the following example: msg

db

len

equ

'Hello, world!',0xa ;our dear string
$ - msg

;length of our dear string

$ points to the byte after the last character of the string variable msg.
Therefore, $-msg gives the length of the string. We can also write msg db 'Hello, world!',0xa ;our dear string len equ 13

;length of our dear string

Alternatively, you can store strings with a trailing sentinel character to delimit a string instead of storing the string length explicitly. The sentinel character should be a special character that does not appear within a string.
For example: message DB 'I am loving it!', 0

String Instructions
Each string instruction may require a source operand, a destination operand or both. For 32-bit segments, string instructions use ESI and EDI registers to point to the source and destination operands, respectively.
For 16-bit segments, however, the SI and the DI registers are used to point to the source and destination, respectively.
There are five basic instructions for processing strings. They are:


MOVS - This instruction moves 1 Byte, Word or Doubleword of data from memory location to another.

65

Assembly Programming



LODS - This instruction loads from memory. If the operand is of one byte, it is loaded into the AL register, if the operand is one word, it is loaded into the AX register and a doubleword is loaded into the EAX register.



STOS - This instruction stores data from register (AL, AX, or EAX) to memory. 

CMPS - This instruction compares two data items in memory. Data could be of a byte size, word or doubleword.



SCAS - This instruction compares the contents of a register (AL, AX or
EAX) with the contents of an item in memory.

Each of the above instruction has a byte, word, and doubleword version; and string instructions can be repeated by using a repetition prefix.
These instructions use the ES:DI and DS:SI pair of registers, where DI and SI registers contain valid offset addresses that refers to bytes stored in memory. SI is normally associated with DS (data segment) and DI is always associated with
ES (extra segment).
The DS:SI (or ESI) and ES:DI (or EDI) registers point to the source and destination operands, respectively. The source operand is assumed to be at
DS:SI (or ESI) and the destination operand at ES:DI (or EDI) in memory.
For 16-bit addresses, the SI and DI registers are used, and for 32-bit addresses, the ESI and EDI registers are used.
The following table provides various versions of string instructions and the assumed space of the operands.
Basic
Instruction

Operands at Byte
Operation

Word
Operation

Double word Operation

MOVS

ES:DI,
DS:EI

MOVSB

MOVSW

MOVSD

LODS

AX, DS:SI

LODSB

LODSW

LODSD

STOS

ES:DI, AX

STOSB

STOSW

STOSD

CMPS

DS:SI, ES:
DI

CMPSB

CMPSW

CMPSD

SCAS

ES:DI, AX

SCASB

SCASW

SCASD

66

Assembly Programming

MOVS
The MOVS instruction is used to copy a data item (byte, word or doubleword) from the source string to the destination string. The source string is pointed by
DS:SI and the destination string is pointed by ES:DI.
The following example explains the concept: section .text

global _start
_start:

;must be declared for using gcc

;tell linker entry point

mov

ecx, len

mov

esi, s1

mov

edi, s2

cld rep movsb

mov

edx,20

mov

ecx,s2 ;message to write

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80 ;call kernel

mov

eax,1 ;system call number (sys_exit)

int

0x80 ;call kernel

;message length

section .data s1 db 'Hello, world!',0 ;string 1 len equ $-s1 section s2 resb

.bss
20

;destination

When the above code is compiled and executed, it produces the following result:
Hello, world!

67

Assembly Programming

LODS
In cryptography, a Caesar cipher is one of the simplest known encryption techniques. In this method, each letter in the data to be encrypted is replaced by a letter some fixed number of positions down the alphabet.
In this example, let us encrypt a data by simply replacing each alphabet in it with a shift of two alphabets, so a will be substituted by c, b with d and so on.
We use LODS to load the original string 'password' into the memory. section .text global _start
_start:

;must be declared for using gcc

;tell linker entry point

mov

ecx, len

mov

esi, s1

mov

edi, s2

loop_here: lodsb add al, 02 stosb loop

loop_here

cld rep movsb

mov

edx,20

;message length

mov

ecx,s2

;message to write

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int mov int

0x80 eax,1 0x80

;call kernel
;system call number (sys_exit)
;call kernel

section .data s1 db 'password', 0 ;source len equ $-s1 section .bss
68

Assembly Programming

s2 resb 10

;destination

When the above code is compiled and executed, it produces the following result: rcuuyqtf STOS
The STOS instruction copies the data item from AL (for bytes - STOSB), AX (for words - STOSW) or EAX (for doublewords - STOSD) to the destination string, pointed to by ES:DI in memory.
The following example demonstrates use of the LODS and STOS instruction to convert an upper case string to its lower case value: section .text

global _start
_start:

;must be declared for using gcc

;tell linker entry point

mov

ecx, len

mov

esi, s1

mov

edi, s2

loop_here: lodsb or

al, 20h

stosb loop loop_here

cld rep movsb

mov

edx,20

mov

ecx,s2 ;message to write

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80 ;call kernel

mov

eax,1 ;system call number (sys_exit)

int

0x80 ;call kernel

section

;message length

.data
69

Assembly Programming

s1 db 'HELLO, WORLD', 0 ;source len equ $-s1 section .bss

s2 resb 20

;destination

When the above code is compiled and executed, it produces the following result: hello, world

CMPS
The CMPS instruction compares two strings. This instruction compares two data items of one byte, word or doubleword, pointed to by the DS:SI and ES:DI registers and sets the flags accordingly. You can also use the conditional jump instructions along with this instruction.
The following example demonstrates comparing two strings using the CMPS instruction: section

.text

global _start
_start:

;must be declared for using gcc

;tell linker entry point

mov esi, s1 mov edi, s2 mov ecx, lens2 cld repe cmpsb jecxz equal

;jump when ecx is zero

;If not equal then the following code mov eax, 4 mov ebx, 1 mov ecx, msg_neq mov edx, len_neq int 80h jmp exit equal: 70

Assembly Programming

mov eax, 4 mov ebx, 1 mov ecx, msg_eq mov edx, len_eq int 80h exit: mov eax, 1 mov ebx, 0 int 80h section .data

s1 db 'Hello, world!',0

;our first string

lens1 equ $-s1 s2 db 'Hello, there!', 0

;our second string

lens2 equ $-s2 msg_eq db 'Strings are equal!', 0xa len_eq equ $-msg_eq msg_neq db 'Strings are not equal!' len_neq equ $-msg_neq
When the above code is compiled and executed, it produces the following result:
Strings are not equal!

SCAS
The SCAS instruction is used for searching a particular character or set of characters in a string. The data item to be searched should be in AL (for
SCASB), AX (for SCASW) or EAX (for SCASD) registers. The string to be searched should be in memory and pointed by the ES:DI (or EDI) register.
Look at the following program to understand the concept: section .text global _start
_start:

;must be declared for using gcc

;tell linker entry point

71

Assembly Programming

mov ecx,len mov edi,my_string mov al , 'e' cld repne scasb je found ; when found
; If not not then the following code mov eax,4 mov ebx,1 mov ecx,msg_notfound mov edx,len_notfound int 80h jmp exit found: mov eax,4 mov ebx,1 mov ecx,msg_found mov edx,len_found int 80h exit: mov eax,1 mov ebx,0 int 80h section .data my_string db 'hello world', 0 len equ $-my_string msg_found db 'found!', 0xa len_found equ $-msg_found msg_notfound db 'not found!' len_notfound equ $-msg_notfound
72

Assembly Programming

When the above code is compiled and executed, it produces the following result: found! Repetition Prefixes
The REP prefix, when set before a string instruction, for example - REP MOVSB, causes repetition of the instruction based on a counter placed at the CX register.
REP executes the instruction, decreases CX by 1, and checks whether CX is zero.
It repeats the instruction processing until CX is zero.
The Direction Flag (DF) determines the direction of the operation.


Use CLD (Clear Direction Flag, DF = 0) to make the operation left to right.



Use STD (Set Direction Flag, DF = 1) to make the operation right to left.

The REP prefix also has the following variations:


REP: It is the unconditional repeat. It repeats the operation until CX is zero. 

REPE or REPZ: It is conditional repeat. It repeats the operation while the zero flag indicates equal/zero. It stops when the ZF indicates not equal/zero or when CX is zero.



REPNE or REPNZ: It is also conditional repeat. It repeats the operation while the zero flag indicates not equal/zero. It stops when the ZF indicates equal/zero or when CX is decremented to zero.

73

Assembly Programming

16. ARRAYS

We have already discussed that the data definition directives to the assembler are used for allocating storage for variables. The variable could also be initialized with some specific value. The initialized value could be specified in hexadecimal, decimal or binary form.
For example, we can define a word variable ‘months’ in either of the following way: MONTHS

DW

12

MONTHS

DW

0CH

MONTHS

DW

0110B

The data definition directives can also be used for defining a one-dimensional array. Let us define a one-dimensional array of numbers.
NUMBERS

DW

34,

45,

56,

67,

75, 89

The above definition declares an array of six words each initialized with the numbers 34, 45, 56, 67, 75, 89. This allocates 2x6 = 12 bytes of consecutive memory space. The symbolic address of the first number will be NUMBERS and that of the second number will be NUMBERS + 2 and so on.
Let us take up another example. You can define an array named inventory of size 8, and initialize all the values with zero, as:
INVENTORY

DW

0

DW

0

DW

0

DW

0

DW

0

DW

0

DW

0

DW

0

Which can be abbreviated as:
INVENTORY

DW

0, 0 , 0 , 0 , 0 , 0 , 0 , 0

The TIMES directive can also be used for multiple initializations to the same value. Using TIMES, the INVENTORY array can be defined as:
74

Assembly Programming

INVENTORY TIMES 8 DW 0

Example
The following example demonstrates the above concepts by defining a 3-element array x, which stores three values: 2, 3 and 4. It adds the values in the array and displays the sum 9: section .text

global _start

;must be declared for linker (ld)

_start:

mov

;number bytes to be summed

mov

ebx,0

;EBX will store the sum

mov

ecx, x

;ECX will point to the current element to be summed

add

ebx, [ecx]

add

ecx,1

;move pointer to next element

dec

eax

;decrement counter

jnz

top:

eax,3

top

;if counter not 0, then loop again

done: add ebx, '0'

mov

[sum], ebx ;done, store result in "sum"

display: mov edx,1

;message length

mov

ecx, sum

;message to write

mov

ebx, 1

;file descriptor (stdout)

mov

eax, 4

;system call number (sys_write)

int

0x80

;call kernel

mov

eax, 1

;system call number (sys_exit)

int

0x80

;call kernel

section

.data

global x x: db

2

db

4

db

3
75

Assembly Programming

sum: db 0

When the above code is compiled and executed, it produces the following result:
9

76

Assembly Programming

17. PROCEDURES

Procedures or subroutines are very important in assembly language, as the assembly language programs tend to be large in size. Procedures are identified by a name. Following this name, the body of the procedure is described which performs a well-defined job. End of the procedure is indicated by a return statement. Syntax
Following is the syntax to define a procedure: proc_name: procedure body
...
ret
The procedure is called from another function by using the CALL instruction. The
CALL instruction should have the name of the called procedure as an argument as shown below:
CALL proc_name
The called procedure returns the control to the calling procedure by using the
RET instruction.

Example
Let us write a very simple procedure named sum that adds the variables stored in the ECX and EDX register and returns the sum in the EAX register: section .text

global _start
_start:

;must be declared for using gcc

;tell linker entry point

mov

ecx,'4'

sub

ecx, '0'

mov

edx, '5'

sub

edx, '0'

call

sum

mov

[res], eax

mov

ecx, msg

;call sum procedure

77

Assembly Programming

mov

edx, len

mov

ebx,1 ;file descriptor (stdout)

mov

eax,4 ;system call number (sys_write)

int

0x80

mov

ecx, res

mov

edx, 1

mov

ebx, 1

;file descriptor (stdout)

mov

eax, 4

;system call number (sys_write)

int

0x80

mov

eax,1 ;system call number (sys_exit)

int

0x80

;call kernel

;call kernel

;call kernel

sum: mov eax, ecx

add

eax, edx

add

eax, '0'

ret section .data msg db "The sum is:", 0xA,0xD len equ $- msg segment .bss res resb 1
When the above code is compiled and executed, it produces the following result:
The sum is:
9

Stacks Data Structure
A stack is an array-like data structure in the memory in which data can be stored and removed from a location called the 'top' of the stack. The data that needs to be stored is 'pushed' into the stack and data to be retrieved is 'popped' out from the stack. Stack is a LIFO data structure, i.e., the data stored first is retrieved last.
Assembly language provides two instructions for stack operations: PUSH and
POP. These instructions have syntaxes like:
PUSH

operand
78

Assembly Programming

POP

address/register

The memory space reserved in the stack segment is used for implementing stack. The registers SS and ESP (or SP) are used for implementing the stack.
The top of the stack, which points to the last data item inserted into the stack is pointed to by the SS:ESP register, where the SS register points to the beginning of the stack segment and the SP (or ESP) gives the offset into the stack segment. The stack implementation has the following characteristics:


Only words or doublewords could be saved into the stack, not a byte.



The stack grows in the reverse direction, i.e., toward the lower memory address. 

The top of the stack points to the last item inserted in the stack; it points to the lower byte of the last word inserted.

As we discussed about storing the values of the registers in the stack before using them for some use; it can be done in following way:
; Save the AX and BX registers in the stack
PUSH

AX

PUSH

BX

; Use the registers for other purpose
MOV AX, VALUE1
MOV BX, VALUE2
...
MOV VALUE1, AX
MOV VALUE2, BX
; Restore the original values
POP AX
POP BX

Example
The following program displays the entire ASCII character set. The main program calls a procedure named display, which displays the ASCII character set. section

.text

global _start
_start:
call

;must be declared for using gcc

;tell linker entry point display 79

Assembly Programming

mov

eax,1 ;system call number (sys_exit)

int

0x80

;call kernel

display: mov ecx, 256

next: push ecx

mov

eax, 4

mov

ebx, 1

mov

ecx, achar

mov

edx, 1

int

80h

pop

ecx

mov

dx, [achar]

cmp

byte [achar], 0dh

inc

byte [achar]

loop

next

ret section .data achar db '0'
When the above code is compiled and executed, it produces the following result:
0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwx
yz{|}
...
...

80

Assembly Programming

18. RECURSION

A recursive procedure is one that calls itself. There are two kind of recursion: direct and indirect. In direct recursion, the procedure calls itself and in indirect recursion, the first procedure calls a second procedure, which in turn calls the first procedure.
Recursion could be observed in numerous mathematical algorithms. For example, consider the case of calculating the factorial of a number. Factorial of a number is given by the equation:
Fact (n) = n * fact (n-1) for n > 0
For example: factorial of 5 is 1 x 2 x 3 x 4 x 5 = 5 x factorial of 4 and this can be a good example of showing a recursive procedure. Every recursive algorithm must have an ending condition, i.e., the recursive calling of the program should be stopped when a condition is fulfilled. In the case of factorial algorithm, the end condition is reached when n is 0.
The following program shows how factorial n is implemented in assembly language. To keep the program simple, we will calculate factorial 3. section .text

global _start
_start:

;must be declared for using gcc

;tell linker entry point

mov bx, 3 call proc_fact

add

ax, 30h

mov

;for calculating factorial 3

[fact], ax

mov

edx,len

;message length

mov

ecx,msg

;message to write

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

mov mov edx,1

;message length

ecx,fact

;message to write
81

Assembly Programming

mov

ebx,1

;file descriptor (stdout)

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

mov

eax,1

;system call number (sys_exit)

int

0x80

;call kernel

proc_fact: cmp bl, 1

jg

do_calculation

mov

ax, 1

ret do_calculation: dec

bl

call

proc_fact

inc

bl

mul

bl

;ax = al * bl

ret

section

.data

msg db 'Factorial 3 is:',0xa len equ $ - msg

section .bss fact resb 1
When the above code is compiled and executed, it produces the following result:
Factorial 3 is:
6

82

Assembly Programming

19. MACROS

Writing a macro is another way of ensuring modular programming in assembly language. 

A macro is a sequence of instructions, assigned by a name and could be used anywhere in the program.



In NASM, macros are defined with %macro and %endmacro directives.



The macro begins with the %macro directive and ends with the
%endmacro directive.

The Syntax for macro definition:
%macro macro_name

number_of_params

<macro body>
%endmacro
Where, number_of_params specifies the number parameters, macro_name specifies the name of the macro.
The macro is invoked by using the macro name along with the necessary parameters. When you need to use some sequence of instructions many times in a program, you can put those instructions in a macro and use it instead of writing the instructions all the time.
For example, a very common need for programs is to write a string of characters in the screen. For displaying a string of characters, you need the following sequence of instructions: mov edx,len

;message length

mov ecx,msg

;message to write

mov ebx,1

;file descriptor (stdout)

mov eax,4

;system call number (sys_write)

int 0x80

;call kernel

In the above example of displaying a character string, the registers EAX, EBX,
ECX and EDX have been used by the INT 80H function call. So, each time you need to display on screen, you need to save these registers on the stack, invoke
INT 80H and then restore the original value of the registers from the stack. So, it could be useful to write two macros for saving and restoring data.

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We have observed that, some instructions like IMUL, IDIV, INT, etc., need some of the information to be stored in some particular registers and even return values in some specific register(s). If the program was already using those registers for keeping important data, then the existing data from these registers should be saved in the stack and restored after the instruction is executed.

Example
Following example shows defining and using macros:
; A macro with two parameters
; Implements the write system call
%macro write_string 2 mov eax, 4

mov

ebx, 1

mov

ecx, %1

mov

edx, %2

int

80h

%endmacro

section

.text

global _start

;must be declared for using gcc

_start:

;tell linker entry point

write_string msg1, len1 write_string msg2, len2 write_string msg3, len3 mov eax,1

;system call number (sys_exit)

int 0x80

;call kernel

section

.data

msg1 db

'Hello, programmers!',0xA,0xD

len1 equ $ - msg1 msg2 db 'Welcome to the world of,', 0xA,0xD len2 equ $- msg2 msg3 db 'Linux assembly programming! ' len3 equ $- msg3

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When the above code is compiled and executed, it produces the following result:
Hello, programmers!
Welcome to the world of,
Linux assembly programming!

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20. FILE MANAGEMENT

The system considers any input or output data as stream of bytes. There are three standard file streams:


Standard input (stdin),



Standard output (stdout), and



Standard error (stderr).

File Descriptor
A file descriptor is a 16-bit integer assigned to a file as a file id. When a new file is created or an existing file is opened, the file descriptor is used for accessing the file.
File descriptor of the standard file streams - stdin, stdout and stderr are 0, 1 and 2, respectively.

File Pointer
A file pointer specifies the location for a subsequent read/write operation in the file in terms of bytes. Each file is considered as a sequence of bytes. Each open file is associated with a file pointer that specifies an offset in bytes, relative to the beginning of the file. When a file is opened, the file pointer is set to zero.

File Handling System Calls
The following table briefly describes the system calls related to file handling:
%eax

Name

%ebx

%ecx

%edx

2

sys_fork

struct pt_regs

-

-

3

sys_read

unsigned int

char *

size_t

4

sys_write

unsigned int

const char *

size_t

5

sys_open

const char *

int

int

6

sys_close

unsigned int

-

-

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8

sys_creat

const char *

int

-

19

sys_lseek

unsigned int

off_t

unsigned int

The steps required for using the system calls are same, as we discussed earlier:
1. Put the system call number in the EAX register.
2. Store the arguments to the system call in the registers EBX, ECX, etc.
3. Call the relevant interrupt (80h).
4. The result is usually returned in the EAX register.

Creating and Opening a File
For creating and opening a file, perform the following tasks:
1. Put the system call sys_creat() number 8, in the EAX register.
2. Put the filename in the EBX register.
3. Put the file permissions in the ECX register.

The system call returns the file descriptor of the created file in the EAX register, in case of error, the error code is in the EAX register.

Opening an Existing File
For opening an existing file, perform the following tasks:
1. Put the system call sys_open() number 5, in the EAX register.
2. Put the filename in the EBX register.
3. Put the file access mode in the ECX register.
4. Put the file permissions in the EDX register.

The system call returns the file descriptor of the created file in the EAX register, in case of error, the error code is in the EAX register.
Among the file access modes, most commonly used are: read-only (0), writeonly (1), and read-write (2).

Reading from a File
For reading from a file, perform the following tasks:
1. Put the system call sys_read() number 3, in the EAX register.
2. Put the file descriptor in the EBX register.

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3. Put the pointer to the input buffer in the ECX register.
4. Put the buffer size, i.e., the number of bytes to read, in the EDX register.

The system call returns the number of bytes read in the EAX register, in case of error, the error code is in the EAX register.

Writing to a File
For writing to a file, perform the following tasks:
1. Put the system call sys_write() number 4, in the EAX register.
2. Put the file descriptor in the EBX register.
3. Put the pointer to the output buffer in the ECX register.
4. Put the buffer size, i.e., the number of bytes to write, in the EDX register.

The system call returns the actual number of bytes written in the EAX register, in case of error, the error code is in the EAX register.

Closing a File
For closing a file, perform the following tasks:
1. Put the system call sys_close() number 6, in the EAX register.
2. Put the file descriptor in the EBX register.

The system call returns, in case of error, the error code in the EAX register.

Updating a File
For updating a file, perform the following tasks:
1. Put the system call sys_lseek () number 19, in the EAX register.
2. Put the file descriptor in the EBX register.
3. Put the offset value in the ECX register.
4. Put the reference position for the offset in the EDX register.

The reference position could be:


Beginning of file - value 0



Current position - value 1



End of file - value 2

The system call returns, in case of error, the error code in the EAX register.

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Example
The following program creates and opens a file named myfile.txt, and writes a text 'Welcome to Tutorials Point' in this file. Next, the program reads from the file and stores the data into a buffer named info. Lastly, it displays the text as stored in info. section .text

global _start
_start:

;must be declared for using gcc

;tell linker entry point

;create the file mov eax, 8

mov

ebx, file_name

mov

ecx, 0777

;read, write and execute by all

int

0x80

;call kernel

mov [fd_out], eax

; write into the file mov edx,len

;number of bytes

mov

ecx, msg

;message to write

mov

ebx, [fd_out]

;file descriptor

mov

eax,4

;system call number (sys_write)

int

0x80

;call kernel

; close the file mov eax, 6 mov ebx, [fd_out]

; write the message indicating end of file write mov eax, 4 mov ebx, 1 mov ecx, msg_done mov edx, len_done int 0x80

;open the file for reading mov eax, 5
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mov ebx, file_name mov ecx, 0

;for read only access

mov edx, 0777

;read, write and execute by all

int

0x80

mov

[fd_in], eax

;read from file mov eax, 3 mov ebx, [fd_in] mov ecx, info mov edx, 26 int 0x80

; close the file mov eax, 6 mov ebx, [fd_in]

; print the info mov eax, 4 mov ebx, 1 mov ecx, info mov edx, 26 int 0x80

mov

eax,1

;system call number (sys_exit)

int

0x80

;call kernel

section

.data

file_name db 'myfile.txt' msg db 'Welcome to Tutorials Point' len equ

$-msg

msg_done db 'Written to file', 0xa len_done equ $-msg_done

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section .bss fd_out resb 1 fd_in resb 1

info resb

26

When the above code is compiled and executed, it produces the following result:
Written to file
Welcome to Tutorials Point

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21. MEMORY MANAGEMENT

The sys_brk() system call is provided by the kernel, to allocate memory without the need of moving it later. This call allocates memory right behind the application image in the memory. This system function allows you to set the highest available address in the data section.
This system call takes one parameter, which is the highest memory address needed to be set. This value is stored in the EBX register.
In case of any error, sys_brk() returns -1 or returns the negative error code itself. The following example demonstrates dynamic memory allocation.

Example
The following program allocates 16kb of memory using the sys_brk() system call: section

.text

global _start
_start:

;must be declared for using gcc
;tell linker entry point

mov

eax, 45

xor

ebx, ebx

int

80h

add

eax, 16384

mov

ebx, eax

mov

eax, 45

int

80h

cmp

eax, 0

jl

exit

;exit, if error

mov

edi, eax

;EDI = highest available address

sub

edi, 4

;pointing to the last DWORD

mov

ecx, 4096

;number of DWORDs allocated

xor

eax, eax

;clear eax

std rep cld

;sys_brk

;number of bytes to be reserved

;sys_brk

;backward stosd ;repete for entire allocated area
;put DF flag to normal state
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mov

eax, 4

mov

ebx, 1

mov

ecx, msg

mov

edx, len

int

80h

;print a message

exit: mov eax, 1

xor

ebx, ebx

int

80h

section

.data

msg

db

len

equ

"Allocated 16 kb of memory!", 10
$ - msg

When the above code is compiled and executed, it produces the following result:
Allocated 16 kb of memory!

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