Page 175 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes the entry is pointing is in memory or on disk. If the table is on disk, the operating system can
use the other 31 bits to specify the disk location of the table; the table then can be brought into
memory on demand.

All CPU address is differed one another.

Segmentation is one approach to memory management and protection in the
operating system. It has been superseded by paging for most purposes, but
much of the terminology of segmentation is still used, “segmentation fault”
being an example. Some operating systems still have segmentation at some
logical level although paging is used as the main memory management policy.

Self Assessment

Multiple choice questions:
1. Primary memory stores ..................... .
( a) Data alone

( b) Programs alone

( c) Results alone
( d) All of these
2. Memory is made up of ..................... .

( a) Set of wires
( b) Set of circuits
( c) Large number of cells

( d) All of these
3. The principal of locality of reference justifies the use of
( a) re-enterable (b) non-reusable
( c) virtual memory (d) cache memory

5.7 Virtual Memory

5.7.1 Paging and Swapping
Virtual memory is a way of making the physical memory of a computer system effectively larger
than it really is. Rather than using mirrors, the system does this by determining which parts of
its memory are often sitting idle, and then makes a command decision to empty their contents
onto a disk, thereby freeing up useful RAM.
As we noted earlier, it is quite seldom that every byte of every program is in use all of the time.
More often programs are large and contain sections of code which are visited rarely if ever at
all by the majority of users — so if they are not used, why keep them in RAM?

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