Page 160 - DCAP103_Principle of operating system

Unit 5: Memory Management

memory-management solution. Modified versions of swapping, however, are found on many Notes
systems. A modification of swapping is used in many versions of UNIX. Swapping was normally
disabled, but would start if many processes were running and were using a threshold amount
of memory. Swapping would again be halted if the load on the system were reduced. Early PCs
lacked sophisticated hardware (or operating systems that take advantage of the hardware) to
implement more advanced memory management methods, but they were used to run multiple
large processes by a modified version of swapping. A prime example is the Microsoft Windows
3.1 operating system, which supports concurrent execution of processes in memory. If a new
process is loaded and there is insufficient main memory, an old process is swapped to disk. This
operating system, however, does not provide full swapping, because the user, rather than the
scheduler, decides when it is time to preempt one process for another. Any swapped-out process
remains swapped out (and not executing) until the user selects that process to run. Follow-on
Microsoft operating systems, such as Windows NT, take advantage of advanced MMU features
now found even on PCs.

Swapping is a simple memory/process management technique used by the
operating system(os) to increase the utilization of the processor by moving
some blocked process from the main memory to the secondary memory(hard
disk); thus forming a queue of temporarily suspended process and the
execution continues with the newly arrived process. After performing the
swapping process,the operating system has two options in selecting a process
for execution.

5.4 Contiguous Memory Allocation

The main memory must accommodate both the operating system and the various user processes.
We therefore need to allocate different parts of the main memory in the most efficient way
possible. This section will explain one common method, contiguous memory allocation. The
memory is usually divided into two partitions—one for the resident operating system, and
one for the user processes. We may place the operating system in either low memory or high
memory. The major factor affecting this decision is the location of the interrupt vector. Since the
interrupt vector is often in low memory, programmers usually place the operating system in
low memory as well. Thus, in this text, we shall discuss only the situation where the operating
system resides in low memory. The development of the other situation is similar. We usually
want several user processes to reside in memory at the same time. We therefore need to consider
how to allocate available memory to the processes that are in the input queue waiting to be
brought into memory. In this contiguous memory allocation, each process is contained in a
single contiguous section of memory.

5.5 Paging

Paging is a memory-management scheme that permits the physical-address space of a process to
be noncontiguous. Paging avoids the considerable problem of fitting the varying-sized memory
chunks onto the backing store, from which most of the previous memory-management schemes
suffered. When some code fragments or data residing in main memory need to be swapped out,
space must be found on the backing store. The fragmentation problems discussed in connection

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