Page 181 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes
this value indicates that the associated page is both legal and in memory. If the bit is set to
“invalid,” this value indicates that the page either is not valid (that is, not in the logical address
space of the process), or is valid but is currently on the disk. The page-table entry for a page that
is brought into memory is set as usual, but the page-table entry for a page that is not currently
in memory is simply marked invalid, or contains the address of the page on disk. This situation
is depicted in figure given below. Notice that marking a page invalid will have no effect if the
process never attempts to access that page. Hence, if we guess right and page in all and only
those pages that are actually needed, the process will run exactly as though we had brought in
all pages. While the process executes and accesses pages that are memory resident, execution
proceeds normally. But what happens if the process tries to access a page that was not brought
into memory? Access to a page marked invalid causes a page-fault trap. The paging hardware,
in translating the address through the page table, will notice that the invalid bit is set, causing
a trap to the operating system. This trap is the result of the operating system’s failure to bring
the desired page into memory (in an attempt to minimize disk-transfer overhead and memory
requirements), rather than an invalid address error as a result of an attempt to use an illegal
memory address (such as an incorrect array subscript). We must therefore correct this oversight.
The procedure for handling this page fault is straightforward (Figure 5.24).

Figure 5.24: Page Table when Some Pages are not in Main Memory

0
1
0 A 2
Valid- invalid
1 B frame bit 3
2 C 4 A
0 4 v
3 D 1 i 5
2 6 v 6 C A B
4 E
3 i
7
5 F 4 i C D E
5 9 v 8
6 G
6 i F
9 F
7 H 7 i
10
Page table
Logical
memory 11
12
13
14
15
Physical memory

1. We check an internal table (usually kept with the process control block) for this process,
to determine whether the reference was a valid or invalid memory access.
2. If the reference was invalid, we terminate the process. If it was valid, but we have not yet
brought in that page, we now page it in.
3. We find a free frame (by taking one from the free-frame list, for example).

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