Motorola M68000 User's Guide download pdf (Page 221)

The 68030 207

easy to take a binary logical address and partition it into a page number

and an offset within the page.

Logical Address (example)

31 10 S 0

offset

Since addresses on the 68030 are all 32 bits, we can divide the logical

address into two pieces as long as the sizes add up to 32. For example, if

we desire pages to be IK (1024 bytes), the offset requires 10 bits, leaving

22 bits for the page number. Therefore, there would be 222 pages, each

1024 bytes long.

How do we then map the pages into the corresponding physical page

frames? Also, what if all the pages are not currently in memory? When we

implement a virtual memory, we store the pages that are not currently in

memory on a secondary storage device such as a disk. The hardware must

be aware of what pages are actually in physical memory at a given time.

Both of these problems are solved by the use of a page table. A page table

is simply a translation table. The physical frame number corresponding

to a particular page can be found by looking it up in the page table. A

simple organization of a page table is that of an array of frame numbers.

The page number is used as an index into the array.

The problem of determining if a page is currently mapped to physical

memory is solved by providing an extra bit with each page table entry.

This bit can be set if the frame number is valid and reset if the page

is not in memory. In the latter case, the frame number can be used to

provide information to the operating system about where the page is

located on secondary storage. The hardware doesn’t handle this case. The

hardware only handles the mapping of pages to physical memory. What

the hardware does when it encounters a non-resident page is to generate

a page fault. This page fault is detected by the operating system, the

missing page is brought into memory, and the instruction that caused the

page fault is allowed to continue. This time it will find the missing page.

On the following page is an example of the first few entries of a page

table. It is easy to see that page 0 is mapped to frame 23, page 1 is

mapped to frame 12, and page 3 is not in memory.

While this scheme seems simple, the implementation is rather com

plex. For each memory reference, the CPU must determine what page

is being referenced, find the proper entry in the page table, compute the

actual physical address by adding the offset to the frame’s location, and

finally perform the memory access. Two subtle problems emerge when

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Motorola M68000 User's Guide Page 221