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Part three: Memory ManagementPart three: Memory Management

programs, together with the data they access, must be in main memory (at least partially) during execution.

the computer keeps several processes in memory.

Many memory-management schemes exist, reflecting various approaches, and the effectiveness of each algorithm depends on the situation.

Selection of a memory-management scheme for a system depends on many factors, especially on the hardware design of the system.

Each algorithm requires its own hardware support.

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Chapter 8: Main MemoryChapter 8: Main Memory

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Chapter 8: Main Memory Chapter 8: Main Memory

Background

Contiguous Memory Allocation

Paging

Structure of the Page Table

Segmentation

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ObjectivesObjectives

To provide a detailed description of various ways of organizing memory hardware

To discuss various memory-management techniques, including paging and segmentation

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BackgroundBackground

Program must be brought (from disk) into memory and placed within a process for it to be run

Main memory and registers are only storage CPU can access directly

Register access in one CPU clock (or less)

Main memory can take many cycles

Cache sits between main memory and CPU registers

Protection of memory required to ensure correct operation

ensure correct operation ha to protect the operating system from access by user processes and, in addition, to protect user processes from one another.

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Base and Limit RegistersBase and Limit Registers

A pair of base and limit registers define the logical address space

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Binding of Instructions and Data to MemoryBinding of Instructions and Data to Memory

Address binding of instructions and data to memory addresses can happen at three different stages

Compile time: If memory location known a priori, absolute code can be generated; must recompile code if starting location changes

Load time: Must generate relocatable code if memory location is not known at compile time

Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another. Need hardware support for address maps (e.g., base and limit registers)

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Multistep Processing of a User Program Multistep Processing of a User Program

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Logical vs. Physical Address SpaceLogical vs. Physical Address Space 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

We now have two different types of addresses: logical addresses (in the range 0 to max ) and physical addresses (for example, in the range R + 0 to R + max). The user generates only logical addresses and thinks that the process runs in locations 0 to max. these logical addresses must be mapped to physical addresses before they are used.

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Logical vs. Physical Address SpaceLogical vs. Physical Address Space

Logical and physical addresses are the same in compile-time and load-time address-binding schemes; logical (virtual) and physical addresses differ in execution-time address-binding scheme

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Memory-Management Unit (MMU)Memory-Management Unit (MMU)

Hardware device that maps virtual to physical address

In MMU scheme, for example, the value in the relocation register is added to every address generated by a user process at the time it is sent to memory

The user program deals with logical addresses; it never sees the real physical addresses

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Dynamic relocation using a relocation registerDynamic relocation using a relocation register

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Dynamic LoadingDynamic Loading

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 operating system is required implemented through program design

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Dynamic LinkingDynamic 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

all processes that use a language library execute only one copy of the library code.

System also known as shared libraries

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SwappingSwapping

A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution

Backing store – fast disk large enough to accommodate copies of all memory images for all users; must provide direct access to these memory images

Roll out, roll in – swapping variant used for priority-based scheduling algorithms; lower-priority process is swapped out so higher-priority process can be loaded and executed

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Schematic View of SwappingSchematic View of Swapping

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Contiguous AllocationContiguous Allocation

Main memory usually into two partitions:

Resident operating system, usually held in low memory with interrupt vector

User processes then held in high memory

each process is contained in a single contiguous section of memory.

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Contiguous AllocationContiguous Allocation

Relocation registers used to protect user processes from each other, and from changing operating-system code and data

Relocation register contains value of smallest physical address

Limit register contains range of logical addresses – each logical address must be less than the limit register

MMU maps logical address dynamically

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Hardware Support for Relocation and Limit RegistersHardware Support for Relocation and Limit Registers

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Memory AllocationMemory Allocation

fixed-sized partitions.

将内存空间划分成若干个固定大小的区域，在每个区域可装入一个进程。

the degree of multiprogramming is bound by the number of partitions.

Internal Fragmentation

Multiple partition method

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Contiguous Allocation (Cont.)Contiguous Allocation (Cont.)

Multiple-partition allocation

Hole – block of available memory; holes of various size are scattered throughout memory

When a process arrives, it is allocated memory from a hole large enough to accommodate it

Operating system maintains information about:a) allocated partitions b) free partitions (hole)

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OS

process 5

process 2

OS

process 5

process 8

process 2

OS

process 5

process 9

process 2

process 10

OS

process 5

process 2

process 9

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Dynamic Storage-Allocation ProblemDynamic Storage-Allocation Problem

First-fit: Allocate the first hole that is big enough Best-fit: Allocate the smallest hole that is big enough;

must search entire list, unless ordered by size Produces the smallest leftover hole

Worst-fit: Allocate the largest hole; must also search entire list Produces the largest leftover hole

How to satisfy a request of size n from a list of free holes?

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FragmentationFragmentation

External Fragmentation – total memory space exists to satisfy a request, but it is not contiguous

Internal Fragmentation – allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used

Reduce external fragmentation by compaction

Shuffle memory contents to place all free memory together in one large block

Compaction is possible only if relocation is dynamic, and is done at execution time

This scheme can be expensive.

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PagingPaging

连续分配造成碎片，而 Compaction 对系统性能十分不利。 Physical address space of a process can be noncontiguous;

process is allocated physical memory whenever the latter is available

Divide physical memory into fixed-sized blocks called frames (size is power of 2, between 512 bytes and 8,192 bytes)

Divide logical memory into blocks of same size called pages

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PagingPaging

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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Paging Model of Logical and Physical MemoryPaging Model of Logical and Physical Memory

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Address Translation SchemeAddress Translation Scheme

Address generated by 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

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Address Translation SchemeAddress Translation Scheme

For given logical address space 2m and page size 2n

page number page offset

p d

m - n n

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Paging HardwarePaging Hardware

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Paging ExamplePaging Example

32-byte memory and 4-byte pages

0

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0

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Page SizePage Size

If process size is independent of page size, we expect internal fragmentation to average one-half page per process. This consideration suggests that small page sizes are desirable.

However, overhead is involved in each page-table entry, and this overhead is reduced as the size of the pages increases.

Also, disk I/O is more efficient when the number of data being transferred is larger. Generally, page sizes have grown over time as processes, data sets, and main memory have become larger.

Today, pages typically are between 4 KB and 8 KB in size

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Page SizePage Size

Today, pages typically are between 4 KB and 8 KB in size

Usually, each page-table entry is 4 bytes long, but that size can vary as well.

A 32-bit entry can point to one of 232 physical page frames. If frame size is 4 KB, then a system with 4-byte entries can address 244 bytes (or 16 TB) of physical memory.

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Free FramesFree Frames

Before allocation After allocation

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User’s view of memoryUser’s view of memory

the clear separation between the user's view of memory and the actual physical memory.

The user program views memory as one single space, containing only this one program.

In fact, the user program is scattered throughout physical memory, which also holds other programs.

The difference between the user's view of memory and the actual physical memory is reconciled by the address-translation hardware.

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User’s view of memoryUser’s view of memory

The logical addresses are translated into physical addresses. This mapping is hidden from the user and is controlled by the operating system.

Notice that the user process by definition is unable to access memory it does not own. It has no way of addressing memory outside of its page table, and the table includes only those pages that the process owns.

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Frame tableFrame table

Since the operating system is managing physical memory, it must be aware of the allocation details of physical memory-which frames arc allocated, which frames are available, how many total frames there are, and so on.

This information is generally kept in a data structure called a frame table.

The frame table has one entry for each physical page frame, indicating whether the latter is free or allocated and, if it is allocated, to which page of which process or processes.

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context-switch timecontext-switch time

If a user makes a system call (to do I/O, for example) and provides an address as a parameter (a buffer, for instance), that address must be mapped to produce the correct physical address.

The operating system maintains a copy of the page table for each process, just as it maintains a copy of the instruction counter and register contents.

It is also used by the CPU dispatcher to define the hardware page table when a process is to be allocated the CPU.

Paging therefore increases the context-switch time.

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Implementation of Page TableImplementation of Page Table

the page table is implemented as a set of dedicated registers.

These registers should be built with very high-speed logic to make the paging-address translation efficient.

Every access to memory must go through the paging map, so efficiency is a major consideration.

The CPU dispatcher reloads these registers, just as it reloads the other registers.

The use of registers for the page table is satisfactory if the page table is reasonably small (for example, 256 entries).

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Implementation of Page TableImplementation of Page Table

Page table is kept in main memory

Page-table base register (PTBR) points to the page table

Page-table length register (PTLR) indicates size of the page table

In this scheme every data/instruction access requires two memory accesses. One for the page table and one for the data/instruction.

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TLBTLB

The two memory access problem can be solved by the use of a special fast-lookup hardware cache called associative memory or translation look-aside buffer (TLB)

The TLB is associative high-speed memory. Each entry in the TLB consists of two parts: a key (or tag) and a value. When the associative memory is presented with an item, the item is compared with all keys simultaneously. If the item is found, the corresponding value field is returned.

The search is fast; the hardware, however, is expensive. Typically, the number of entries in a TLB is small, often numbering between 64 and 1,024.

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Associative MemoryAssociative Memory

Associative memory – parallel search

Address translation (p, d)

If p is in associative register, get frame # out

Otherwise get frame # from page table in memory

Page # Frame #

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How to use TLBHow to use TLB

The TLB contains only a few of the page-table entries.

When a logical address is generated by the CPU, its page number is presented to the TLB.

If the page number is found, its frame number is immediately available and is used to access memory. The whole task may take less than 10 percent longer than it would if an unmapped memory reference were used.

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How to use TLBHow to use TLB

If the page number is not in the TLB (known as a TLB miss), a memory reference to the page table must be made. When the frame number is obtained, we can use it to access memory . In addition, we add the page number and frame number to the TLB, so that they will be found quickly on the next reference.

If the TLB is already full of entries, the operating system must select one for replacement.

every time a new page table is selected (for instance, with each context switch), the TLB must be flushed (or erased) to ensure that the next executing process does not use the wrong translation information.

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Paging Hardware With TLBPaging Hardware With TLB

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Effective Access TimeEffective Access Time

Associative Lookup = time unit

Assume memory cycle time is 1 microsecond

Hit ratio – percentage of times that a page number is found in the associative registers; ratio related to number of associative registers

Hit ratio = Effective Access Time (EAT)

EAT = (1 + ) + (2 + )(1 – )

= 2 + –

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Effective Access TimeEffective Access Time

An 80-percent hit ratio means that we find the desired page number in the TLB 80 percent of the time.

If it takes 20 nanoseconds to search the TLB and 100 nanoseconds to access memory,

then a mapped-memory access takes 120 nanoseconds when the page number is in the TLB.

If we fail to find the page number in the TLB (20 nanoseconds), then we must first access memory for the page table and frame number (100 nanoseconds) and then access the desired byte in memory (100 nanoseconds ),for a total of 220 nanoseconds.

EAT = 0.80 x 120 + 0.20 x 220= 140 nanoseconds.

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Memory ProtectionMemory Protection

Memory protection implemented by associating protection bit with each frame

One bit can define a page to be read-write or read-only

easily expand this approach to provide a finer level of protection.

We can create hardware to provide read-only, read-write, or execute-only protection

or, by providing separate protection bits for each kind of access, we can allow any combination of these accesses.

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Memory ProtectionMemory Protection

Valid-invalid bit attached to each entry in the page table:

“valid” indicates that the associated page is in the process’ logical address space, and is thus a legal page

Rarely does a process use all its address range. In fact, many processes use only a small fraction of the address space available to them.

It would be wasteful in these cases to create a page table with entries for every page in the address range. Most of this table would be unused but would take up valuable memory space.

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Valid (v) or Invalid (i) Bit In A Page TableValid (v) or Invalid (i) Bit In A Page Table

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Memory ProtectionMemory Protection

provide hardware in the form of a page-table length register (PTLR), to indicate the size of the page table.

This value is checked against every logical address to verify that the address is in the valid range for the process.

Failure of this test causes an error trap to the operating system.

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Shared PagesShared Pages

Shared code

One copy of read-only (reentrant) code shared among processes (i.e., text editors, compilers, window systems).

Shared code must appear in same location in the logical address space of all processes

Private code and data

Each process keeps a separate copy of the code and data

The pages for the private code and data can appear anywhere in the logical address space

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Shared Pages ExampleShared Pages Example

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Structure of the Page TableStructure of the Page Table

Hierarchical Paging

Hashed Page Tables

Inverted Page Tables

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Hierarchical Page TablesHierarchical Page Tables

a large logical address space needs a large page table

the page table needs access contiguously in main memory.

It is very difficult to find a contiguous space larger than page size to store page table.

Break up the logical address space into multiple page tables

A simple technique is a two-level page table

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Two-Level Page-Table SchemeTwo-Level Page-Table Scheme

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Two-Level Paging ExampleTwo-Level Paging Example

A logical address (on 32-bit machine with 4K page size) is divided into: a page number consisting of 20 bits a page offset consisting of 12 bits

Since the page table is paged, the page number is further divided into: a 10-bit page number a 10-bit page offset

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Two-Level Paging ExampleTwo-Level Paging Example

Thus, a logical address is as follows:

where p1 is an index into the outer page table, and p2 is the displacement within the page of the outer page table

page number page offsetp1 p2 d

10 10 12 设计非常精妙！

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Address-Translation SchemeAddress-Translation Scheme

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Three-level Paging SchemeThree-level Paging Scheme

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Hashed Page TablesHashed Page Tables

Common in address spaces > 32 bits

The virtual page number is hashed into a page table. This page table contains a chain of elements hashing to the same location.

Virtual page numbers are compared in this chain searching for a match. If a match is found, the corresponding physical frame is extracted.

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Hashed Page TableHashed Page Table

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Inverted Page TableInverted Page Table

Only one page table is in the system

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

Decreases memory needed to store each page table, but increases time needed to search the table when a page reference occurs

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Inverted Page Table ArchitectureInverted Page Table Architecture

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SegmentationSegmentation

paging --- the separation of the user's view of memory and the actual physical memory.

the user's view of memory is not the same as the actual physical memory.

A program is a collection of segments. A segment is a logical unit such as:

main program, procedure, function, method, object,

local variables, global variables, common block, stack,

symbol table, arrays

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User’s View of a ProgramUser’s View of a Program

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Logical View of SegmentationLogical View of Segmentation

1

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1

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user space physical memory space

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Segmentation Architecture Segmentation Architecture

Logical address consists of a two tuple:

<segment-number, offset>,

Segment table – maps two-dimensional physical addresses; each table entry has:

base – contains the starting physical address where the segments reside in memory

limit – specifies the length of the segment

Segment-table base register (STBR) points to the segment table’s location in memory

Segment-table length register (STLR) indicates number of segments used by a program;

segment number s is legal if s < STLRP303

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Segmentation HardwareSegmentation Hardware

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Segmentation Architecture (Cont.)Segmentation Architecture (Cont.)

Protection

With each entry in segment table associate:

validation bit = 0 illegal segment

read/write/execute privileges

Protection bits associated with segments; code sharing occurs at segment level

Since segments vary in length, memory allocation is a dynamic storage-allocation problem

A segmentation example is shown in the following diagram

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Example of SegmentationExample of Segmentation

HomeworkHomework ：： 44、、 55、、 66、、 77、、 99、、 1111 、、 1212、、 1313

End of Chapter 8End of Chapter 8