Memory Management
1. Memory manager
Memory consists of a large array of bytes (words) each with its own address. Main memory
is the only storage that CPU can access directly. The CPU fetches instructions from the main
memory. Memory Management is very critical for the performance of a computer system.
The Memory manager is responsible for allocating and de-allocating memory to the
processes that are running on a computer system.
2. Static Memory Allocation Methods
2.1 Single-user Contiguous
Each program to be processes is loaded in its entirety into memory. It is given all the memory
it requires and the space is contiguous. If the program is bigger than the available memory it
is not executed. The program stays in the memory until it is fully executed. The programs are
processed sequentially one after the other. No more than one program can be executed at the
same time therefore multiprogramming is not supported. This method was used in early
computers systems at 1940’s and 1950’s but since it was not efficient it was not used in
newer systems.
2.2 Fixed Partitions
In this method the main memory is divided into fixed partitions. One partition is used for
each program. The size of the partition could only be configured when the machine was
shutdown. Once the machine is running then the partitions are fixed. Each job is allocated a
partition and it stayed there until it was executed.
If a program was smaller than the partition assigned to it then the remainder memory space is
wasted. The phenomenon of blocks of unused memory is referred to as fragmentation
(internal).
A protection scheme is used so a fixed partition used by a program is not allocated to another
program. This method introduced multiprogramming but it was more complex than Singleuser Contiguous method.
2.3 Dynamic partitions
The programs are given only as much as memory as they request when they are loaded for
execution. The memory that they are allocated is still in contiguous blocks. In this way they
make more efficient use of the memory since no space is wasted on partitions. However
when a job is executed and leaves the memory, the free memory partition may not be the
same size as new job and fragments of free memory are created between the memory blocks.
The memory manager must keep track of which programs are running on which memory
locations and has the responsibility to allocate the free partitions to the incoming programs.
There are two popular strategies for allocating the partitions. First fit and Best Fit
Best fit: The allocator places a process in the smallest block of unallocated memory in which
it will fit. For example, suppose a process requests 12KB of memory and the memory
manager currently has a list of unallocated blocks of 6KB, 14KB, 19KB, 11KB, and 13KB
blocks. The best-fit strategy will allocate 12KB of the 13KB block to the process. 3
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First fit: There may be many holes in the memory, so the operating system, to reduce the
amount of time it spends analyzing the available spaces, begins at the start of primary
memory and allocates memory from the first hole it encounters large enough to satisfy the
request. Using the same example as above, first fit will allocate 12KB of the 14KB block to
the process.
External fragmentation
External fragmentation is the development of unusable portions of memory in a dynamic
partition memory management scheme. As programs terminate they release their allocated
section of main memory which is promptly allocated to other programs ready to execute. If
the size of the new program is smaller than that of the dynamic partition, then a hole develops
between two allocated memory partitions. This hole is the external fragment.
2.4 Relocatable Dynamic Partitions
This method minimize the problem of defragmentation by relocating programs to gather
together all the empty blocks and compact them make one block of memory large enough to
accommodate some or all the jobs waiting to get in.
3. Compaction
Compaction is used to consolidate all the external fragments (free areas in memory) into one
contiguous block. In some cases this enhances throughput by allowing more programs to be
active at the same time.
4. Relocation
Relocation is the process by which programs are repositioned in main memory to allow
compaction of free memory areas. When relocation takes place, the addresses specified by a
program for either branching or data reference is modified, during execution, to allow the
program to execute correctly. Relocation can be time consuming; therefore it should be done
sparingly. Some suggestions are: (1) when a certain percentage of main memory is used up,
(2) when the number of programs waiting for execution reaches a prescribed upper limit, (3)
when a prescribed amount of time has elapsed, (4) combinations of 1, 2 and 3.
Quick Quiz
1. Mention 4 advantages of Main Memory Management?
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Allocation/de-allocation for processes, files, I/O.
Maintenance of several processes at a time
Keep track of who's using what memory
Movement of process memory to/from secondary storage.
2. Explain the term Process?
A process is a program in execution: (A program is passive, a process active.) A process
has resources (CPU time, files) and attributes that must be managed.
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