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Memory Manaement

In: Computers and Technology

Submitted By sunny33
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OPERATING SYSTEMS MEMORY MANAGEMENT

Jerry Breecher
8: Memory Management 1

OPERATING SYSTEM Memory Management
What Is In This Chapter?
Just as processes share the CPU, they also share physical memory. This chapter is about mechanisms for doing that sharing.

8: Memory Management

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MEMORY MANAGEMENT
Just as processes share the CPU, they also share physical memory. This section is about mechanisms for doing that sharing. EXAMPLE OF MEMORY USAGE: Calculation of an effective address
Fetch from instruction Use index offset

Example: ( Here index is a pointer to an address ) loop:

load add store inc skip_equal branch loop ... continue ....

register, index 42, register register, index index index, final_address

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MEMORY MANAGEMENT

Definitions

• The concept of a logical address space that is bound to a separate physical address space is central to proper memory management. • Logical address – generated by the CPU; also referred to as virtual address • Physical address – address seen by the memory unit • Logical and physical addresses are the same in compile-time and loadtime address-binding schemes; logical (virtual) and physical addresses differ in execution-time address-binding scheme

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MEMORY MANAGEMENT
Relocatable Binding

Definitions

Means that the program image can reside anywhere in physical memory. Programs need real memory in which to reside. When is the location of that real memory determined? • This is called mapping logical to physical addresses. • This binding can be done at compile/link time. Converts symbolic to relocatable. Data used within compiled source is offset within object module. If it’s known where the program will reside, then absolute code is generated. Otherwise compiler produces relocatable code. Binds relocatable to physical. Can find best physical location. The code can be moved around during execution. Means flexible virtual mapping.

Compiler:

Load: Execution:

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MEMORY MANAGEMENT
This binding can be done at compile/link time. Converts symbolic to relocatable. Data used within compiled source is offset within object module. Can be done at load time. Binds relocatable to physical. Can be done at run time. Implies that the code can be moved around during execution. The next example shows how a compiler and linker actually determine the locations of these effective addresses.

Binding Logical To Physical
Source Compiler Object

Other Objects

Linker Executable Loader In-memory Image Libraries

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MEMORY MANAGEMENT

Binding Logical To Physical

4 void main() 5 { 6 printf( "Hello, from main\n" ); 7 b(); 8} 9 10 11 voidb() 12 { 13 printf( "Hello, from 'b'\n" ); 14 }

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MEMORY MANAGEMENT
ASSEMBLY LANGUAGE LISTING 000000B0: 000000B4 000000B8 000000BC 000000C0 000000C4 000000C8 000000CC 000000D0 000000D4 000000D8 000000DC 6BC23FD9 37DE0080 E8200000 D4201C1E 34213E81 E8400028 B43A0040 E8400040 6BC23FD9 4BC23F59 E840C000 37DE3F81 stw ldo bl depi ldo bl addi bl stw ldw bv ldo bl addil be,n nop stw ldo bl depi ldo bl addi ldw bv ldo

Binding Logical To Physical
; main() ; get current addr=BC ; ; ; ; ; get code start area call printf calc. String loc. call b store return addr

%r2,-20(%sp 64(%sp),%sp 0x000000C0,%r1 0,31,2,%r1 -192(%r1),%r1 0x000000E0,%r2 32,%r1,%r26 0x000000F4,%r2 %r2,-20(%sp) -84(%sp),%r2 %r0(%r2) -64(%sp),%sp

; return from main STUB(S) FROM LINE 6

000000E0: E8200000 000000E4 28200000 000000E8: E020E002 000000EC 000000F0: 000000F4: 000000F8 000000FC 00000100 00000104 00000108 0000010C 00000110 00000114 08000240 6BC23FD9 37DE0080 E8200000 D4201C1E 34213E01 E85F1FAD B43A0010 4BC23F59 E840C000 37DE3F81

0x000000E8,%r1 L%0,%r1 0x00000000(%sr7,%r1) %r2,-20(%sp) 64(%sp),%sp 0x00000100,%r1 0,31,2,%r1 -256(%r1),%r1 0x000000E0,%r2 8,%r1,%r26 -84(%sp),%r2 %r0(%r2) -64(%sp),%sp

void ; get current addr=F8 ; get code start area ; call printf ; return from b

b()

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MEMORY MANAGEMENT
00002000 00002004 00002008 0000200C 00002010 00002014 00002018 0000201C 00002020 00002024 000020B0 000020B4 000020B8 000020BC 000020C0 000020C4 000020C8 000020CC 000020D0 000020D4 000020D8 000020DC 000020E0 000020E4 000020E8 000020EC

Binding Logical To Physical
. @ l f . l f n

EXECUTABLE IS DISASSEMBLED HERE 0009000F ; . . . 08000240 ; . . . 48656C6C ; H e l 6F2C2066 ; o , 726F6D20 ; r o m 620A0001 ; b . . 48656C6C ; H e l 6F2C2066 ; o , 726F6D20 ; r o m 6D61696E ; m a i 6BC23FD9 stw %r2,-20(%sp) ; main 37DE0080 ldo 64(%sp),%sp E8200000 bl 0x000020C0,%r1 D4201C1E depi 0,31,2,%r1 34213E81 ldo -192(%r1),%r1 E84017AC bl 0x00003CA0,%r2 B43A0040 addi 32,%r1,%r26 E8400040 bl 0x000020F4,%r2 6BC23FD9 stw %r2,-20(%sp) 4BC23F59 ldw -84(%sp),%r2 E840C000 bv %r0(%r2) 37DE3F81 ldo -64(%sp),%sp E8200000 bl 0x000020E8,%r1 ; stub 28203000 addil L%6144,%r1 E020E772 be,n 0x000003B8(%sr7,%r1) 08000240 nop

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MEMORY MANAGEMENT
000020F0 000020F4 000020F8 000020FC 00002100 00002104 00002108 0000210C 00002110 00002114 00003CA0 00003CA4 00003CA8 00003CAC 00003CB0 00003CB4 00003CB8 00003CBC 00003CC0 00003CC4 00003CC8 00003CCC 00003CD0 00003CD4 00003CD8 00003CDC 00003CE0 00003CE8

Binding Logical To Physical
; b

EXECUTABLE IS DISASSEMBLED HERE 6BC23FD9 stw %r2,-20(%sp) 37DE0080 ldo 64(%sp),%sp E8200000 bl 0x00002100,%r1 D4201C1E depi 0,31,2,%r1 34213E01 ldo -256(%r1),%r1 E840172C bl 0x00003CA0,%r2 B43A0010 addi 8,%r1,%r26 4BC23F59 ldw -84(%sp),%r2 E840C000 bv %r0(%r2) 37DE3F81 ldo -64(%sp),%sp 6BC23FD9 37DE0080 6BDA3F39 2B7CFFFF 6BD93F31 343301A8 6BD83F29 37D93F39 6BD73F21 4A730009 B67700D0 E8400878 08000258 4BC23F59 E840C000 37DE3F81 E8200000 E020E852 stw ldo stw addil stw ldo stw ldo stw ldw addi bl copy ldw bv ldo bl be,n %r2,-20(%sp) 64(%sp),%sp %r26,-100(%sp) L%-26624,%dp %r25,-104(%sp) 212(%r1),%r19 %r24,-108(%sp) -100(%sp),%r25 %r23,-112(%sp) -8188(%r19),%r19 104,%r19,%r23 0x00004110,%r2 %r0,%r24 -84(%sp),%r2 %r0(%r2) -64(%sp),%sp 0x00003CE8,%r1 0x00000428(%sr7,%r1)

; printf

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MEMORY MANAGEMENT
Dynamic loading
+ + + +

More Definitions

Routine is not loaded until it is called Better memory-space utilization; unused routine is never loaded. Useful when large amounts of code are needed to handle infrequently occurring cases. No special support from the OS is required - implemented through program design.

Dynamic Linking
+ + + + + Linking postponed until execution time. Small piece of code, stub, used to locate the appropriate memory-resident library routine. Stub replaces itself with the address of the routine, and executes the routine. Operating system needed to check if routine is in processes’ memory address. Dynamic linking is particularly useful for libraries. Performs the above operations. Usually requires hardware support. 8: Memory Management 11

Memory Management

MEMORY MANAGEMENT
BARE MACHINE:

SINGLE PARTITION ALLOCATION

No protection, no utilities, no overhead. This is the simplest form of memory management. Used by hardware diagnostics, by system boot code, real time/dedicated systems. logical == physical User can have complete control. Commensurably, the operating system has none. DEFINITION OF PARTITIONS: Division of physical memory into fixed sized regions. (Allows addresses spaces to be distinct = one user can'muck with another user, or the system.) t The number of partitions determines the level of multiprogramming. Partition is given to a process when it' scheduled. s Protection around each partition determined by bounds ( upper, lower ) base / limit. These limits are done in hardware. 8: Memory Management 12

MEMORY MANAGEMENT
RESIDENT MONITOR: Primitive Operating System.

SINGLE PARTITION ALLOCATION

Usually in low memory where interrupt vectors are placed. Must check each memory reference against fence ( fixed or variable ) in hardware or register. If user generated address < fence, then illegal. User program starts at fence -> fixed for duration of execution. Then user code has fence address built in. But only works for static-sized monitor. If monitor can change in size, start user at high end and move back, OR use fence as base register that requires address binding at execution time. Add base register to every generated user address. Isolate user from physical address space using logical address space. Concept of "mapping addresses” shown on next slide. 8: Memory Management 13

MEMORY MANAGEMENT
Limit Register

SINGLE PARTITION ALLOCATION
Relocation Register

Yes CPU Logical Address

<
No

+

Physical Address

MEMORY

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MEMORY MANAGEMENT

CONTIGUOUS ALLOCATION
All pages for a process are allocated together in one chunk.

JOB SCHEDULING

Must take into account who wants to run, the memory needs, and partition availability. (This is a combination of short/medium term scheduling.) Sequence of events: In an empty memory slot, load a program THEN it can compete for CPU time. Upon job completion, the partition becomes available. Can determine memory size required ( either user specified or "automatically" ).
8: Memory Management 15

DYNAMIC STORAGE

MEMORY MANAGEMENT

CONTIGUOUS ALLOCATION

(Variable sized holes in memory allocated on need.) Operating System keeps table of this memory - space allocated based on table. Adjacent freed space merged to get largest holes - buddy system. ALLOCATION PRODUCES HOLES
OS OS OS

process 1

process 1

process 1

process 2 process 3

Process 2 Terminates process 3

Process 4 Starts

process 4

process 3

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MEMORY MANAGEMENT

CONTIGUOUS ALLOCATION

HOW DO YOU ALLOCATE MEMORY TO NEW PROCESSES? First fit - allocate the first hole that' big enough. s Best fit - allocate smallest hole that' big enough. s Worst fit - allocate largest hole. (First fit is fastest, worst fit has lowest memory utilization.) Avoid small holes (external fragmentation). This occurs when there are many small pieces of free memory. What should be the minimum size allocated, allocated in what chunk size? Want to also avoid internal fragmentation. This is when memory is handed out in some fixed way (power of 2 for instance) and requesting program doesn'use it all. t

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MEMORY MANAGEMENT
If a job doesn'fit in memory, the scheduler can t wait for memory skip to next job and see if it fits. What are the pros and cons of each of these?

LONG TERM SCHEDULING

There' little or no internal fragmentation (the process uses the memory given to it s the size given to it will be a page.) But there can be a great deal of external fragmentation. This is because the memory is constantly being handed cycled between the process and free.

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MEMORY MANAGEMENT
Trying to move free memory to one large block.

COMPACTION

Only possible if programs linked with dynamic relocation (base and limit.) There are many ways to move programs in memory. Swapping: if using static relocation, code/data must return to same place. But if dynamic, can reenter at more advantageous memory.
OS P1 OS P1 P2 P3 P2 P3 8: Memory Management 19 OS P1 P3 P2

MEMORY MANAGEMENT


PAGING
New Concept!!

• • • • • •

Logical address space of a process can be noncontiguous; process is allocated physical memory whenever that memory is available and the program needs it. Divide physical memory into fixed-sized blocks called frames (size is power of 2, between 512 bytes and 8192 bytes). Divide logical memory into blocks of same size called pages. Keep track of all free frames. To run a program of size n pages, need to find n free frames and load program. Set up a page table to translate logical to physical addresses. Internal fragmentation.

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MEMORY MANAGEMENT
Address Translation Scheme

PAGING

Address generated by the CPU is divided into: • Page number (p) – used as an index into a page table which contains base address of each page in physical memory. • Page offset (d) – combined with base address to define the physical memory address that is sent to the memory unit.
4096 bytes = 2^12 – it requires 12 bits to contain the Page offset

p

d

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MEMORY MANAGEMENT
Frames = physical blocks Pages = logical blocks

PAGING

Permits a program' memory to be physically noncontiguous so it can be allocated s from wherever available. This avoids fragmentation and compaction.

Size of frames/pages is defined by hardware (power of 2 to ease calculations)

HARDWARE An address is determined by: page number ( index into table ) + offset ---> mapping into ---> base address ( from table ) + offset. 8: Memory Management 22

MEMORY MANAGEMENT
Paging Example - 32-byte memory with 4-byte pages
0a 1b 2c 3d 4e 5f 6g 7h 8I 9j 10 k 11 l 12 m 13 n 14 o 15 p Logical Memory

PAGING
0 4 I j k l m n o p

8

0 1 2 3

5 6 1 2

12 16 20 a b c d e f g h

Page Table

24

Physical Memory
28

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MEMORY MANAGEMENT
• A 32 bit machine can address 4 gigabytes which is 4 million pages (at 1024 bytes/page). WHO says how big a page is, anyway? • Could use dedicated registers (OK only with small tables.) • Could use a register pointing to table in memory (slow access.) • Cache or associative memory • (TLB = Translation Lookaside Buffer): • simultaneous search is fast and uses only a few registers.

PAGING

IMPLEMENTATION OF THE PAGE TABLE TLB = Translation Lookaside Buffer

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MEMORY MANAGEMENT
IMPLEMENTATION OF THE PAGE TABLE Issues include: key and value hit rate 90 - 98% with 100 registers add entry if not found Effective access time = %fast * time_fast + %slow *

PAGING

time_slow

Relevant times: 2 nanoseconds to search associative memory – the TLB. 20 nanoseconds to access processor cache and bring it into TLB for next time. Calculate time of access: hit = 1 search + 1 memory reference miss = 1 search + 1 mem reference(of page table) + 1 mem reference.

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MEMORY MANAGEMENT
SHARED PAGES Data occupying one physical page, but pointed to by multiple logical pages. Useful for common code must be write protected. (NO write-able data mixed with code.) Extremely useful for read/write communication between processes.

PAGING

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MEMORY MANAGEMENT
INVERTED PAGE TABLE: One entry for each real page of memory. Entry consists of the virtual address of the page stored in that real memory location, with information about the process that owns that page. Essential when you need to do work on the page and must find out what process owns it. Use hash table to limit the search to one - or at most a few - page table entries. 8: Memory Management

PAGING

27

MEMORY MANAGEMENT
PROTECTION:

PAGING

•Bits associated with page tables. •Can have read, write, execute, valid bits. •Valid bit says page isn’t in address space. •Write to a write-protected page causes a fault. Touching an invalid page causes a fault. ADDRESS MAPPING: •Allows physical memory larger than logical memory. •Useful on 32 bit machines with more than 32-bit addressable words of memory. •The operating system keeps a frame containing descriptions of physical pages; if allocated, then to which logical page in which process.

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MEMORY MANAGEMENT
MULTILEVEL PAGE TABLE A means of using page tables for large address spaces.

PAGING

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MEMORY MANAGEMENT
USER'S VIEW OF MEMORY

Segmentation

A programmer views a process consisting of unordered segments with various purposes. This view is more useful than thinking of a linear array of words. We really don'care at what address a segment is located. t Typical segments include global variables procedure call stack code for each function local variables for each large data structures Logical address = segment name ( number ) + offset Memory is addressed by both segment and offset.
8: Memory Management 30

MEMORY MANAGEMENT
Segment Table Limit S Logical Address D Base

Segmentation

HARDWARE -- Must map a dyad (segment / offset) into one-dimensional address.

CPU

Yes

<
No

+
Physical Address MEMORY

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MEMORY MANAGEMENT
HARDWARE base / limit pairs in a segment table. Limit 1000 400 400 1100 1000

Segmentation

1 2 0 3 4

0 1 2 3 4

Base 1400 6300 4300 3200 4700

1 4

2 3

Logical Address Space
8: Memory Management

Physical Memory
32

MEMORY MANAGEMENT
PROTECTION AND SHARING Addresses are associated with a logical unit (like data, code, etc.) so protection is easy. Can do bounds checking on arrays Sharing specified at a logical level, a segment has an attribute called "shareable". Can share some code but not all - for instance a common library of subroutines. FRAGMENTATION Use variable allocation segment lengths vary. since

Segmentation

Again have issue of fragmentation; Smaller segments means less fragmentation. Can use compaction since segments are relocatable. 8: Memory Management

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MEMORY MANAGEMENT

Segmentation

PAGED SEGMENTATION Combination segmentation. of paging and

address = frame at ( page table base for segment + offset into page table ) offset into memory + Look at example of Intel architecture.

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MEMORY MANAGEMENT
WRAPUP
We’ve looked at how to do paging - associating logical with physical memory. This subject is at the very heart of what every operating system must do today.

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