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Chapter 8: Memory
Management Strategies
Operating System Concepts – 8th Edition
Silberschatz, Galvin and Gagne ©2009
Chapter 8: Memory Management Strategies
 Background
 Swapping
 Contiguous Memory Allocation
 Paging
 Structure of the Page Table
 Segmentation
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Objectives
 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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Background

Memory is central to the operation of a modern computer system.

Memory consists of a large array of words or bytes, each with its
own address

Selection of a memory-management method for a specific system
depends on many factors, especially on the hardware design of the
system.
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Basic Hardware

Main memory , registers and cash are ONLY storage CPU can access
directly

Program must be brought (from disk) into memory and placed within a
process for it to be run

Direct storage access time:
Huge problem… because
of the frequency of
memory accesses.
 Register access in one CPU clock (or less)
 Main memory can take many cycles (i.e. slowly)
 Solution: Cache (fast memory) sits between main memory and
CPU registers

Protection of memory:
 Protection of memory is necessary to ensure correct operation
 Protection from what (/possible risks)????
– protect the operating system from access by user processes
– protect user processes from one another.
 This protection must be provided by the hardware &can be
implemented in several ways.
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Basic Hardware (Cont.)
 One method to implement protection: use Base and Limit Registers
 We need to make sure that each process has a separate memory
space….How???
 Main idea: we need to determine the range of legal addresses
that can be accessed only by the process.
 We can provide this protection by using two registers (base
& limit)
– The base register>> holds the physical address of the
first byte in the legal range.
– The limit register >> holds the size of the range.
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Basic Hardware (Cont.)
– Example.: If the base register holds
300040 and the limit register is
120900….what is the range of legal
addresses ??
»
The program can legally access all addresses from
300040 through 420939 (inclusive)
»
NOTE: Last physical address = base +limit -1
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Basic Hardware (Cont.)
 How base & limit registers help to provide memory
protection??
 By applying (2) procedures:
–
Procedure (1): The CPU hardware compare every address
generated in user mode with the registers.
– Procedure (2): restrict the ability to load base & limit registers
only to OS..
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Basic Hardware (Cont.)
 Procedure (1): The CPU hardware compare every address
generated in user mode with the registers.
 If (CPU generated address ≥ base) & (CPU generated address <
base +limit) …
– Then …..the CPU generated address is legal and allowed to
access the memory
– Else…… the CPU generated address is illegal and NOT
allowed to access the memory…..(causing a trap (/error) to OS)

This scheme prevents a user program from (accidentally or
deliberately) modifying the code or data structures of either the
operating system or other users…..(solution to protection problem)
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Basic Hardware (Cont.)
Fig. (8.2): Hardware address protection with base and limit registers
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Basic Hardware (Cont.)
 Procedure (2): restrict the ability to load base & limit registers
ONLY to OS.
 This restriction applied by using a special privileged instruction.
 Since privileged instructions can be executed only in kernel mode,
and since only the operating system executes in kernel mode…..So,
ONLY the operating system can load the base and limit registers.
 This scheme allows the operating system to change the value of the
registers but prevents user programs from changing the registers’
contents.
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Address Binding
 Address binding( or relocation): The process of associating program
instructions and data to physical memory addresses
 A user program will go through several steps -some of which may be
optional-before being executed ….. ( steps are: compiling>>>
linking>>>execution)
 Addresses may be represented in different ways during these steps.

Addresses in the source program are generally symbolic (such as
count, sum).

A compiler will typically bind these symbolic addresses to relocatable
addresses (such as "14 bytes from the beginning of this module").

The linkage editor or loader will in turn bind the relocatable addresses
to absolute addresses (physical address) (such as 74014).
 Each binding is a mapping from one address space to another.
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Address Binding (cont.)
Figure 8.3: Multistep processing of a user program.
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Address Binding (cont.)
 The binding of instructions and data to memory addresses can be
done at any step along the way:

Compile time. The compiler translates symbolic addresses to absolute
addresses. If you know at compile time where the process will reside in
memory, then absolute code can be generated (Static).

Load time : When it is not known at compile time where the process
will reside in memory, then The compiler translates symbolic addresses
to relative (relocatable) addresses. The loader translates these to
absolute addresses (Static).

Execution time. If the process can be moved during its execution from
one memory segment to another, then binding must be delayed until run
time. The absolute addresses are generated by specialhardware
(e.g.MMU)

Most general-purpose OSs use this method (Dynamic).
 Static-new locations are determined before execution.
 Dynamic-new locations are determined during execution.
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Logical vs. Physical Address Space
 Logical/virtual address: address generated by CPU
 Physical address: address seen by memory hardware
 Compile-time / load-time binding >>> logical address = physical address
 Run-time binding >>> logical address≠ physical address

Logical address space: is the set of all logical addresses generated by a
program

Physical address space: is the set of all physical addresses corresponding to
these logical addresses.

MMU (Memory-Management Unit ): h/w device that maps virtual addresses to
physical addresses at run time

Different methods of address mapping (/memory management strategies):

Continuous memory allocation

Paging

segmentation
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Simple Address Mapping Method
 For NOW, we illustrate address mapping with a simple MMU scheme:

The base register is now called a relocation register.

Basic methodology>>>> Physical address= logical address + relocation register

For example:

Base register contains 14000

If logical address =0 >>>> Physical address= 0+14000=14000

If logical address =346 >>>> Physical address= 346+14000=14346
 The user program never sees the real physical addresses. The user
program deals with logical addresses. MMU converts logical addresses into
physical addresses.
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Example of Simple Address Mapping Method
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Swapping
 Swapping is a mechanism in which a process can be swapped temporarily
out of main memory to a backing store , and then brought back into memory
for continued execution.
 Backing store is a usually a hard disk drive or any other secondary storage
which fast in access and large .
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Swapping (Cont.)
 How swapping performed???

The system maintains a ready queue consisting of all processes whose memory
images are on the backing store or in memory and are ready to run.

Whenever the CPU scheduler decides to execute a process, it calls the
dispatcher.

The dispatcher checks to see whether the next process in the queue is in
memory.

If it is not, and if there is no free memory region, the dispatcher swaps out a
process currently in memory and swaps in the desired process.

Then, Dispatcher reloads registers and transfers control to the selected process.


Major time consuming part of swapping is transfer time
Total transfer time ∝ amount of memory swapped.

Context-switch time in such a swapping system is high.

Swapping is normally disabled but will start if many processes are running and are
using a threshold amount of memory.

Swapping is again halted when the load on the system is reduced.
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Basic Memory Management schemes
Mem. Manegment schems
(/strategies)
Paging
Continuous
Allocation
Multiple-partition
method
(Fixed-partition
allocation)
variable size
method
Conventional
paging
(basic method)
Hierarchical
Page
Segmentation
Hashed Page
Inverted Page
>>>>These strategies can also be combined.
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Contiguous Memory Allocation (CMA)
 The memory is usually divided into two partitions:

One for the resident OS

One for the user processes.
 We can place the OS in either low memory or high memory .
 Basic methodology:

Each process is contained in a single contiguous section of memory.
 CMA strategy can be applied in (2) methods:

Multiple-partition method/(Fixed-partition allocation) method

variable-partition allocation method
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Memory Allocation
 Method #(1): Multiple-partition method (/Fixed-partition method)

Main idea: divide memory into several fixed-sized partitions. Each
partition may contain exactly one process.

. In this multiple-partition method:

When a partition is free, a process is selected from the input queue and is
loaded into the free partition.

When the process terminates, the partition becomes available for another
process.

The degree of multiprogramming is bound by the number of partitions

One of the simplest methods for allocating memory

This method is called (Multiprogramming with a Fixed number of
Tasks/ MFT)

This method is no longer in use
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Memory Allocation (cont.)
 Method #(2): variable-partition method:

Memory is divided into variable-sized partitions

OS maintains a list of allocated / free partitions (holes)

When a process arrives, it is allocated memory from a hole large
enough to accommodate it

Memory is allocated to processes until requirements of next process in
queue cannot be met

OS may skip down the queue to allocate memory to a smaller process
that fits in available memory

When process exits, memory is returned to the set of holes and merged
with adjacent holes, if any

The method is a generalization of the fixed-partition scheme (called
Multiprogramming with a Variable number of Tasks (MVT))
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Memory Allocation (cont.)
 Example:
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Memory Allocation (cont.)
 New problem appear in variable-partition method…(how to satisfy a
request of size (n) from a list of free holes??? )
 There are many solutions to this problem:


First fit.

Allocate the first hole that is big enough.

Searching can start either at the beginning of the set of holes or where the previous
first-fit search ended. We can stop searching as soon as we find a free hole that is large
enough.
Best fit




Allocate the smallest hole that is big enough.
We must search the entire list, unless the list is ordered by size.
This strategy produces the smallest leftover hole.
Worst fit.

Allocate the largest hole.

we must search the entire list, unless it is sorted by size.

This strategy produces the largest leftover hole, which may be more useful than the
smaller leftover hole from a best-fit approach.
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Memory Allocation (cont.)

Exercise (8.16): Given five memory partitions of 100 KB, 500 KB, 200 KB, 300
KB, and 600 KB (in order), how would each of the first-fit, best-fit, and worst-fit
algorithms place processes of 212 KB, 417 KB, 112 KB, and 426 KB (in
order)?Which algorithm makes the most efficient use of memory?
>>> Let p1, p2, p3 & p4 are the names of the processes
Best-fit:
First-fit:
P1>>> 100, 500, 200, 300, 600
P2>>> 100, 288, 200, 300, 600
P3>>> 100, 288, 200, 300, 183
100, 176, 200, 300, 183
P4 (426K) must wait
final set
of holes
P1>>> 100, 500, 200, 300, 600
P2>>> 100, 500, 200, 88, 600
P3>>> 100, 83, 200, 88, 600
P4>>> 100, 83, 88, 88, 600
100, 83, 88, 88, 174
Worst-fit:
P1>>> 100, 500, 200, 300, 600
P2>>> 100, 500, 200, 300, 388
P3>>> 100, 83, 200, 300, 388
100, 83, 200, 300, 276 << final set of hole
P4 (426K) must wait
>>> In this example, Best-fit turns out to be the best because there is no wait processes.
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Fragmentation (Cont.)
 Fragmentation is a problem appears in memory allocation censers
about unusable memory space.
 Fragmentation types:


External fragmentation: memory space to satisfy a request is available, but
is not contiguous…(i.e. storage is fragmented into a large number of small
holes)

Both the first-fit and best-fit strategies for memory allocation suffer from external
fragmentation.

(1/3) of memory may be unusable!!!
Internal Fragmentation: allocated memory may be larger than requested
memory….(i.e. some memory within partition may be left unused)

may be used to avoid overhead required to keep track of small holes
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Fragmentation (Cont.)
 External fragmentation solution:


Compaction: Memory contents shuffled to place all free memory
together in one large block

Dynamic relocation (run-time binding) needed

Compaction is expensive (high overhead)
Permit the logical address space of the processes to be noncontiguous, thus allowing a process to be allocated physical memory
wherever it is available.

Two techniques achieve this solution:
–
Paging
–
Segmentation
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End of Chapter 7
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Helpful Information
 The computer system has two modes of operation :


User mode

Kernel mode (also called supervisor mode/ system mode/ privileged mode).
There is a bit (mode bit) indicate the current mode: kernel (0) or user (1).
 When the computer system is executing on behalf of a user application, the
system is in user mode.
 Whenever the operating system gains control of the computer, it is in kernel
mode.
 Only the operating system executes in kernel mode
 Privileged instructions: are a machine code instructions that may only be
executed when the processor is running in kernel mode.
• Example of privileged instructions: I/O operations and interrupt management.
•If
an attempt is made to execute a privileged instruction in user mode>>> the
hardware does not execute the instruction & traps it to the operating system.
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