Page 136 - DCAP403_Operating System

Unit 7: Memory Management

descriptor points to page table for that segment. This give some of the advantages of paging (easy Notes
placement) with some of the advantages of segments (logical division of the program).
Figure 7.9: Intel 80386 Address Translation

logical address selector offset

descriptor table

segment descriptor +

linear address directory page offest page frame

physical address

page directory page table

directory entry page table entry

page directory
base register

Some operating systems allow for the combination of segmentation with paging. If the size of a
segment exceeds the size of main memory, the segment may be divided into equal size pages.
The virtual address consists of three parts: (1) segment number (2) the page within the segment

and (3) the offset within the page. The segment number is used to find the segment descriptor

and the address within the segment is used to find the page frame and the offset within that
page.
7.8 Virtual Memory

Many of us use computers on a daily basis. Although you use it for many different purposes
in many different ways, you share one common reason of using them; to make our job more
efficient and easier.

However, there are times when computers cannot run as fast as you want it to or just cannot
handle certain processes effectively, due to the shortage of system resources. When the limitations
of system resources become a major barrier to achieving your maximum productivity, you often
consider the apparent ways of upgrading the system, such as switching to a faster CPU, adding
more physical memory (RAM), installing utility programs, and so on. As a computer user, you
want to make the most of the resources available; the process of preparing plans to coordinate the

total system to operate in the most efficient manner. This is called a system optimization.
When it comes to system optimization, there is one great invention of modern computing called
virtual memory. It is an imaginary memory area supported by some operating system (for

LOVELY PROFESSIONAL UNIVERSITY 129