Download Memory-Management Strategies 8.1 Background Base and Limit

Chapter 8:
Memory-Management Strategies
Memory-Management Strategies
8.1 Background
8.2 Swapping
Objectives
„ To provide a detailed description of various
ways of organizing memory hardware
„ To discuss various memory-management
8.3 Contiguous Memory Allocation
8.4 Paging
8.5 Structure of the Page Table
8.6 Segmentation
techniques, including paging and
segmentation
8.7 Example: The Intel Pentium
„ To provide a detailed description of the Intel
Pentium, which supports both pure
segmentation and segmentation with paging
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8.1
Operating System Principles
8.2
Operating System Principles
8.1 Background
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Base and Limit Registers
„ Program must be brought (from disk) into memory
and placed within a process for it to be run
„ A pair of base and limit registers define the
logical address space
„ Main memory and registers are the 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
size
„ Protection of memory required to ensure correct
operation
Please refresh the
Instruction
Execution Cycle
Operating System Principles
8.3
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Operating System Principles
8.4
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Binding of Instructions and Data to Memory
HW address protection with base and limit registers
„ Address binding of instructions and data to memory
addresses can happen at three different stages
z
Compile time: If memory location known
beforehand, absolute code can be generated;
must recompile code if starting location changes
z
Load time: Must generate relocatable code if
memory location is not known at compile time
z
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)
physical
mostly
used
Conceptually
(概念上)
Operating System Principles
8.5
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Multistep Processing of a User Program
Operating System Principles
8.6
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Logical 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
z
Logical address – generated by the CPU; also referred
to as virtual address
z
Physical address – address seen by the memory unit
„ 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
Operating System Principles
8.7
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Operating System Principles
8.8
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Memory-Management Unit (MMU)
Example: Assembly Program (supplemented)
„ Hardware device that maps virtual to physical address
„ In MMU scheme, 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
Operating System Principles
8.9
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Example: Machine Code
Operating System Principles
8.10
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Dynamic relocation using a relocation register
MS-DOS used four relocation registers in loading and
running processes. Why four, instead of one ???
Operating System Principles
8.11
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Operating System Principles
8.12
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Dynamic Loading
Dynamic Linking
„ Linking postponed until execution time
„ 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
„ Small piece of code, called 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
„ No special support from the operating system is
required. It is implemented through program design
processes’ memory address
„ Dynamic linking is particularly useful for libraries with
several versions
Operating System Principles
8.13
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z
Only programs compiled with the new library version are
affected by the incompatible changes incorporated in it.
Other programs will continue using the older library.
z
This system is also known as shared libraries
Operating System Principles
8.2 Swapping
8.14
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Swapping
„ 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
„ Major part of swap time is transfer time; total transfer
time is directly proportional to the amount of memory
swapped
„ Modified versions of swapping are found on many
systems (i.e., UNIX, Linux, and Windows)
„ System maintains a ready queue of ready-to-run
processes which have memory images on disk
„ Context-switch time in such a swapping system is fairly
high, see example in p. 323
Operating System Principles
8.15
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Operating System Principles
8.16
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Schematic View of Swapping
8.3 Contiguous Allocation
„ Main memory usually into two partitions:
• Modification of
swapping: only enabled
when many processes
are using a threshold
amount of memory
z
Resident operating system, usually held in low memory
with interrupt vector
z
User processes then held in high memory
„ Relocation registers used to protect user processes from
each other, and from changing operating-system code
and data
• I/O problem: if I/O is asynchronously accessing the user memory
for I/O buffers, then the process cannot be swapped.
> Latch job in memory while it is involved in I/O
> Do I/O only into OS buffers
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8.17
Operating System Principles
HW support for relocation and limit registers
limit
register
z
Base register contains value of smallest physical address
z
Limit register contains range of logical addresses – each
logical address must be less than the limit register
z
MMU maps logical address dynamically
Contiguous Allocation
„ Fixed-sized partition
z
relocation
register
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8.18
Operating System Principles
Multiple-partition allocation
„ Hole – block of available memory; holes of various
size are scattered throughout memory
logical
+
<
physical
address
z
When a process arrives, it is allocated memory from a
hole large enough to accommodate it
z
Operating system maintains information about:
a) allocated partitions b) free partitions (hole)
OS
OS
OS
OS
process 5
process 5
process 5
process 5
process 9
process 9
process 8
process 10
In reality
process 2
Operating System Principles
8.19
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Operating System Principles
process 2
process 2
8.20
process 2
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Fragmentation
Dynamic Storage-Allocation Problem
„ External Fragmentation – total memory space exists to
„ How to satisfy a request of size n from a list of
free holes
z
z
First-fit: Allocate the first hole that is big enough
z
Best-fit: Allocate the smallest hole that is big enough;
must search entire list, unless ordered by size
Produces
z
the smallest leftover hole
Statistical analysis of first fit reveals a 50-percent rule
property
„ 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
Worst-fit: Allocate the largest hole; must also search
entire list
Produces
satisfy a request, but it is not contiguous
the largest leftover hole
• First-fit and best-fit better than worst-fit in terms of
speed and storage utilization. First-fit is normally faster.
z
Shuffle memory contents to place all free memory together in
one large block
z Compaction is possible only if relocation is dynamic, and is
done at execution time
z I/O problem
Latch
Do
8.21
Operating System Principles
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job in memory while it is involved in I/O
I/O only into OS buffers
8.4 Paging
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8.22
Operating System Principles
Address Translation Scheme
„ Logical address space of a process can be noncontiguous;
process is allocated physical memory whenever the latter is
available
„ Address generated by CPU is divided into:
z
Page number (p) – used as an index into a page table which
contains base address of each page in physical memory
z
Page offset (d) – combined with base address to define the
physical memory address that is sent to the memory unit
„ 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
page number
„ 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
z
page offset
p
d
m-n
n
For given logical address space 2m and page size 2n
„ Internal fragmentation
Operating System Principles
8.23
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Operating System Principles
8.24
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Paging Hardware
Operating System Principles
8.25
Paging Model of Logical and Physical Memory
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Paging Example
Free Frames
32-byte memory and 4-byte pages
Operating System Principles
8.27
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8.26
Operating System Principles
after allocation
before allocation
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Operating System Principles
8.28
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