Lecture 7
Memory Management
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Lecture Contents
1. Memory Management Requirements
2. Memory Partitioning
3. Paging
4. Segmentation
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Memory
• Memory is an array of words or bytes, each with
its own address.
• Main memory and the registers built into the
processor itself are the only storage that the CPU
can access directly.
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1. Memory Management Requirements
Five requirements of memory management are
1. Relocation
2. Protection
3. Sharing
4. Logical organisation
5. Physical organisation
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1. Relocation
• In a multiprogramming system, the available main
memory is generally shared among a number of
processes.
• The programmer does not know where the
program will be placed in memory when it is
executed.
- It may be swapped to disk and return to main
memory at a different location (relocated).
• Memory references in program code must be
translated to the actual physical memory address
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Memory Management Terms
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Addressing
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2. Protection
• We need to make sure that each process has a
separate memory space.
• Processes should not be able to reference memory
locations in another process for reading or writing
purposes without permission
• It is impossible to check absolute addresses at
compile time to assure protection because
location of a program in main memory is
unpredictable
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2. Protection
• Hence all memory references generated by a
process must be checked at run time (i.e., at the
time of execution) to ensure that they refer only
to memory space allocated to that process
• Note that the memory protection requirement
must be satisfied by the processor (hardware)
rather than the operating system (software)
because the OS cannot anticipate all of the
memory references that a program will make.
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2. Protection
• We can provide protection by using two registers:
- base register: specifies starting address
- limit register: specifies the memory size.
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3. Sharing
• Any protection mechanism must have the
flexibility to allow several processes to access the
same portion of main memory
• Processes that are cooperating on some task may
need to share access to the same data structure
• The memory management system must therefore
allow controlled access to shared areas of
memory without compromising essential
protection
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4. Logical Organization
• Main memory is organized as a linear (or onedimensional) address space consisting of a
sequence of bytes or words
• Most programs are organized into modules
• Advantages of modularization
- Modules can be written and compiled
independently, with all references from one
module to another resolved by the system at
run time
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4. Logical Organization
- Different degrees of protection (e.g., read-only,
execute-only, read-write) can be given to
different modules.
- Modules can be shared among processes
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5. Physical Organization
•
Cannot leave the programmer with the
responsibility to manage memory
• The main memory available for a program plus its
data may be insufficient
- Overlaying allows various modules to be
assigned the same region of memory but
overlaying is time-consuming to program
• A programmer does not know at the time of
coding how much space will be available or
where that space will be
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Lecture Contents
1. Memory Management Requirements
2. Memory Partitioning
3. Paging
4. Segmentation
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Memory Management Techniques
Six memory management techniques are
1. Fixed Partitioning
2. Dynamic Partitioning
3. Simple (or pure) Paging
4. Simple (or pure) Segmentation
5. Virtual Memory Paging
6. Virtual Memory Segmentation
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2. Memory Partitioning
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1. Fixed-Partitioning Scheme
• Equal-size partitions
- Any process whose size is less than or equal to
the partition size can be loaded into an
available partition
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Equal-size Partitions
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Two Problems of Equal-size Partitions
• A program may too big to fit into a partition.
- The programmer must design the program with
the use of overlays so that only a portion of the
program needs be in main memory at any one
time
• Main memory utilization is extremely inefficient
- Any program, no matter how small, occupies an
entire partition. This results in internal
fragmentation (i.e., unused memory internal to
a partition)
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1. Fixed-Partitioning Scheme
• Unequal-size partitions
- Lessens both problems (i.e., program too big or
small)
- but doesn’t solve the problems completely
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Unequal-size Partitions
• In this figure,
- Programs up to 16 Mbytes can
be accommodated without overlay
- Smaller programs can be placed
in smaller partitions, reducing
internal fragmentation (i.e.,
wasted memory internal to a partition).
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Q&A
• Q: In a fixed-partitioning scheme, what are the
advantages of using unequal-size partitions?
• A: The advantages of using unequal-size fixed
partitions are
1. It is possible to provide one or two quite large
partitions and still have a large number of
partitions. The large partitions can allow the
entire loading of large programs.
2. Internal fragmentation is reduced because a small
program can be put into a small partition.
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Placement Algorithm
• For equal-size partitions
- Placement of processes in memory is trivial (no
options)
• For unequal-size partitions
- can assign each process to the smallest partition
within which it will fit
- advantage of this approach is that processes are
always assigned in such a way as to minimize
internal fragmentation (i.e., wasted memory
within a partition).
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Remaining Problems with Fixed Partitions
• The number of active processes is limited by the
system (i.e., limited by the pre-determined
number of partitions)
• Small processes will not use the partition space
efficiently (i.e., internal fragmentation still
exists)
• The use of fixed partitioning is almost unknown
today
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2. Dynamic Partitioning (DP)
• With dynamic partitioning, the partitions are of
variable length and number
• When a process is brought into main memory, it is
allocated exactly as much memory as it requires
and no more
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Dynamic Partitioning Example
OS (8M)
P2
P1
(14M)
(20M)
Empty (6M)
Empty
P4(8M)
P2
(56M)
(14M)
Empty (6M)
P3
(18M)
Empty (4M)
Refer to Figure 7.4
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2. Dynamic Partitioning (DP)
• DP starts out well, but eventually it leads to a
situation in which there are a lot of small
discontiguous holes in memory.
• As time goes on, memory becomes more and more
fragmented, and memory utilization declines.
This phenomenon is referred to as external
fragmentation (i.e., wasted memory external to
all partitions). That is, there is enough total
memory space to satisfy a request but the
available spaces are not contiguous.
Refer to Figure 7.4
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2. Dynamic Partitioning (DP)
• External fragmentation can be resolved using
compaction (e.g., defragmenter).
- From time to time, OS moves processes so that
they are contiguous and so that all of free
memory is together in one block
- Compaction is a time consuming procedure and
wasteful of processor time
Refer to Figure 7.4
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Q&A
• Q: What is the difference between internal and
external fragmentation?
• A:
- Internal fragmentation is a phenomenon
associated with fixed partitioning. Internal
fragmentation refers to the wasted/unused
space/memory internal to a partition (i.e., wasted
memory within a partition) due to the fact that the
block of data loaded is smaller than the partition.
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Q&A
-
External fragmentation is a phenomenon
associated with dynamic partitioning. External
fragmentation refers to the fact that many small
discontiguous areas of main memory external to
any partition are wasted (i.e., wasted spaces
external to all partitions, or wasted memory
outside all partitions). That is, there is enough
total memory space to satisfy a request but the
available spaces are not contiguous.
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Placement Algorithm
• When it is time to load or swap a process into
main memory, and if there is more than one free
block of memory of sufficient size, then OS must
decide which free block to allocate to the process
•Placement algorithms are best-fit, worst-fit, nextfit, and first-fit.
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Placement Algorithm
• Best-fit placement algorithm
- searches entire list and chooses the block that is
closest in size to the request
- is usually the worst performer because leftover
hole is small
- since smallest block is found for process, main
memory is quickly segmented by blocks too
small to satisfy memory allocation requests
- memory compaction must be done more
frequently than with the other algorithms
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Placement Algorithm
• Worst-fit placement algorithm
- Searches entire list and chooses the block that
is largest in size to the request
- produces largest leftover hole, which may be
more useful than smaller leftover hole created
by best-fit approach.
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Placement Algorithm
• Next-fit placement algorithm
- scans memory from the location of the last
placement, and chooses the next available
block that is large enough
- tends to produce slightly worse results than the
first-fit replacement algorithm
- will more frequently lead to an allocation from
a free block at the end of memory
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Placement Algorithm
• Next-fit placement algorithm
- largest block of free memory, which usually
appears at the end of the memory space, is
quickly broken up into small fragments
- compaction may be required more frequently
with next-fit
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Placement Algorithm
• First-fit placement algorithm
- scans memory from the beginning and chooses
the first available block that is large enough
- is not only the simplest but usually the best and
fastest as well
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Placement Algorithm
? 19 M ←
→ 19 M
Next fit
Worst fit
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Buddy System
• Both fixed and dynamic partitioning schemes
have drawbacks.
- fixed partitioning scheme limits the number of
active processes and may use space
inefficiently if there is a poor match between
available partition sizes and process sizes.
- dynamic partitioning scheme is more complex
to maintain and includes overhead of
compaction.
• An interesting compromise is buddy system
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Buddy System
• Entire space available for allocation is treated as a
single block of size 2n (e.g., n = 10, 2n = 1024)
• If a request of size m, where 2n-1 < m ≤ 2n, is
made, then entire block is allocated
• Otherwise, block is split into two equal buddies
- Process continues until smallest block greater
than or equal to m is generated
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Example of Buddy System
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Tree Representation of Buddy System
B=
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Buddy System
• Buddy system is a reasonable compromise to
overcome the disadvantages of both the fixed and
variable partitioning schemes
• In contemporary OSs, virtual memory based on
paging and segmentation is superior.
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Buddy System
• Buddy system has found application in parallel
systems as an efficient means of allocation and
release for parallel programs. A modified form of
the buddy system is used for UNIX kernel
memory allocation
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Relocation
• When program is loaded into memory, the actual
(or absolute) memory locations are determined
• A process may occupy different partitions which
means different absolute memory locations
during execution. This is due to
- swapping out and in (by swapper - swaps entire
process, not pager - swap a page)
- compaction
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Types of Addresses
• Logical address (virtual address): is a reference
to a memory location independent of the current
assignment of data to memory
- A translation from a logical address to a
physical address must be made before the
memory access can be achieved.
- Logical address is generated by CPU.
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Types of Addresses
• Physical address (or absolute address): is an
actual location in main memory.
- Physical address is managed by memorymanagement unit (MMU)
• Relative address: is expressed as a location
relative to some known point, usually a value in a
processor register.
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Types of Addresses
• Base register is also called relocation register.
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Address Translation
>
≤
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Address Translation
• Base register indicates starting address in main
memory of the program
• Bounds register indicates ending address in main
memory of the program
• Value in base register is added to relative address
to produce absolute address.
• Resulting address is compared to value in bounds
register. If the address is within bounds, then
instruction execution may proceed. Otherwise, an
interrupt is generated to OS for error processing
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Lecture Contents
1. Memory Management Requirements
2. Memory Partitioning
3. Paging
4. Segmentation
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3. Paging
• Main memory is divided into equal, fixed-size
blocks. Each block is called a frame.
• Each program stored on disk is divided into equal,
fixed-size blocks. Each block is called a page.
• Page size equals frame size.
• In paging scheme, wasted space in memory for
each process is due to internal fragmentation
consisting of only a fraction of the last page of a
process. There is no external fragmentation.
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3. Paging
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3. Paging
• Traditionally, support for paging has been handled
by hardware. However, recent designs have
implemented paging by closely integrating the
hardware and operating system, especially on 64bit microprocessors.
• OS maintains a page table for each process. Page
table contains the frame location for each page in
the process. That is, the page table contains the
base address of each page in physical memory
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3. Paging
• A logical address (generated by CPU) is a
reference to a memory location independent of
the current assignment of data to memory.
- A translation to a physical address must be made
before the memory access can be achieved.
- Logical address consists of a page number p and
page offset within the page d
(page#, offset) = (p, d).
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3. Paging
• A physical address (or absolute address managed by MMU) is an actual location in main
memory. Physical address consists of a frame
number f and frame offset d within the frame
(frame#, offset) = (f, d).
• A relative address is a particular example of
logical address. That is, it is the (n + m)-bit
decimal equivalent of p2||d2 or s2||d2.
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Paging Hardware
.
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Page Table
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Assignment of Processes and Free Frames
A.0
A.1
A.2
A.3
D.0
B.0
D.1
B.1
D.2
B.2
C.0
C.1
C.2
C.3
D.3
D.4
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3. Paging
• Paging is similar to fixed partitioning.
• The differences are
- partitions are rather small
- a program may occupy more than one partition
- and these partitions need not be contiguous
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16-bit Logical and Relative Addresses
Page size is
1K (byte) = 210
= 1024 bytes
So m = 10 bits are needed
for offset and n = 6 bits
are needed for page
number
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Logical-to-Physical Translation in Paging
= 1502
0111011110
478
frame 6
6622 =
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Logical-to-Physical Translation in Paging
• Consider an address of n + m bits, where leftmost
n bits are page number, rightmost m bits are offset.
In our example, n = 6 and m = 10.
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Logical-to-Physical Translation in Paging
• The following steps are needed for address
translation:
- Extract the page number as the leftmost n bits
of the logical address.
- Use the page number as an index into the
process page table to find the frame number k.
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Logical-to-Physical Translation in Paging
- The starting physical address of the frame is k
× 2m, and the physical address of the referenced
byte is that number plus the offset. This
physical address needs not be calculated; it is
easily constructed by appending the offset to
the frame number.
• Example: logical address = 0000010111011110,
frame number k = 6 = 000110. → physical address
= 6 × 210 + 478 = 6144 + 478 = 6622
= 0001100111011110
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Lecture Contents
1. Memory Management Requirements
2. Memory Partitioning
3. Paging
4. Segmentation
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4. Segmentation
• A program can be divided into segments
- Segments may vary in length
- There is a maximum segment length (2m)
• Logical address consist of two parts: a segment
number s and segment offset d (i.e., offset within
the segment) (segment#, offset) = (s, d).
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4. Segmentation
• Segmentation is similar to dynamic partitioning
• The difference is that with segmentation a
program may occupy more than one partition, and
these partitions need not be contiguous
• Segmentation eliminates internal fragmentation
but, like dynamic partitioning, it suffers from
external fragmentation. However, because a
process is broken up into a number of smaller
pieces, the external fragmentation should be less
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4. Segmentation
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Segmentation
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Segmentation
= Relative address: 4848
752
8224
1950 > 752
maximum segment
size is 212 = 4096
bytes. Thus, m =
12 bits, n = 4 bits
8976
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Logical-to-Physical Translation in Segmentation
• Consider an address of n + m bits, where leftmost
n bits are segment number and rightmost m bits are
the offset. In our example, n = 4 and m = 12. Thus
the maximum segment size is 212 = 4096.
• The following steps are needed for address
translation:
- Extract segment number as leftmost n bits of
logical address (e.g., 0001|001011110000).
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Logical-to-Physical Translation in Segmentation
- Use segment number as an index into process
segment table to find starting physical address
of the segment (i.e., the value of base).
- Compare the offset, expressed in rightmost m
bits, to the length of the segment. If offset is
greater than or equal to the segment length, the
address is invalid.
- Desired physical address is sum of starting
physical address of the segment and the offset.
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Summary
• One of the most important and complex tasks of
OS is memory management.
• Main memory is a resource to be allocated to and
shared among a number of active processes.
• To use processor and I/O facilities efficiently, it is
desirable to maintain as many processes in main
memory as possible.
• It is desirable to free programmers from memory
restrictions in program development.
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Summary
• The basic tools of memory management are
paging and segmentation.
- With paging, each process is divided into
relatively small, fixed-size pages.
- Segmentation provides for the use of pieces of
varying size.
• It is also possible to combine paging and
segmentation in a single memory management
scheme.
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Review Questions
Q. What are the differences among a frame, a page,
and a segment?
A.
• In a paging system, main memory is divided into
equal, fixed-size blocks. Each block is called a
frame.
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Review Questions
• In a paging system, a program stored on disk is
divided into equal, fixed-size blocks. Each block is
called a page.
The sizes of a page and a frame are equal. Thus,
one page can be loaded into one frame exactly.
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Review Questions
• In a segmentation system, a program stored on
disk is divided into variable-size blocks. Each block
is called a segment.
Although there is a maximum segment length, it is
not required that all segments of all programs be of
the same length.
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chmod Command
• Assume that the access permissions of the file
hello.exe are -rw-------.
• How to set permissions r (read), w (write), and x
(execute) on the file hello.exe for a user
- chmod u=rwx hello.exe
- chmod u+x hello.exe
- chmod 700 hello.exe
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chmod Command
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