Paging

COMPUTER ORGANIZATION AND ARCHITECTURE DESIGNING FOR PERFORMANCE NINTH EDITION

Both unequal fixed-size and variable-size partitions are inefficient in the use of

memory. Suppose, however, that memory is partitioned into equal fixed-size chunks

that are relatively small, and that each process is also divided into small fixed-size

chunks of some size. Then the chunks of a program, known as

pages

, could be

assigned to available chunks of memory, known as

frames

, or page frames. At most,

then, the wasted space in memory for that process is a fraction of the last page.

 

Figure 8.15 shows an example of the use of pages and frames. At a given point

in time, some of the frames in memory are in use and some are free. The list of free

frames is maintained by the OS. Process A, stored on disk, consists of four pages.

When it comes time to load this process, the OS finds four free frames and loads the

four pages of the process A into the four frames.

 

 

Now suppose, as in this example, that there are not sufficient unused con-

tiguous frames to hold the process. Does this prevent the OS from loading A?

The answer is no, because we can once again use the concept of logical address. A

simple base address will no longer suffice. Rather, the OS maintains a

page table

for each process. The page table shows the frame location for each page of the

process. Within the program, each logical address consists of a page number and

a relative address within the page. Recall that in the case of simple partitioning, a

logical address is the location of a word relative to the beginning of the program;

the processor translates that into a physical address. With paging, the logical-

to-physical address translation is still done by processor hardware. The processor

must know how to access the page table of the current process. Presented with a

logical address (page number, relative address), the processor uses the page table

to produce a physical address (frame number, relative address). An example is

shown in Figure 8.16.

 

This approach solves the problems raised earlier. Main memory is divided

into many small equal-size frames. Each process is divided into frame-size pages:

smaller processes require fewer pages, larger processes require more. When a

process is brought in, its pages are loaded into available frames, and a page table

is set up.