Mapping a virtual address to a physical address is relatively easy once the correct formulas are known. There are also some pieces of information that you will need, such as the page size and a page table.
There are four steps to converting a virtual address to a physical address:
It is not always possible to get a physical address from a virtual address. If the page table has a page listed as invalid, then a segmentation fault would occur, instead of accessing the physical address.
Q: Given the following page table, map the (base 10) virtual addresses to physical addresses.
Virtual Addresses: 500, 8200, 102400
(2011 Mid semester exam, Q15)
Relevant information: Page size is 4KB (4096B)
| Page | Frame |
|---|---|
| 0 | 200 |
| 1 | 201 |
| 2 | 22 |
| 3 | invalid |
| ... | invalid |
| 25 | 24 |
| ... | invalid |
For each of these virtual addresses, we need to calculate the physical address using the process outlined above.
Virtual address 500:
$$page = 500 / 4096 = 0$$
$$offset = 500\ \%\ 4096 = 500$$
From our page table, we can see that page 0 maps to frame 200.
$$physical\ address = 200 * 4096 + 500 = 819700$$
Virtual address 8200:
$$page = 8200 / 4096 = 2$$
$$offset = 8200\ \%\ 4096 = 8$$
From our page table, we can see that page 2 maps to frame 22.
$$physical\ address = 22 * 4096 + 8 = 90120$$
Virtual address 102400:
$$page = 102400 / 4096 = 25$$
$$offset = 102400\ \%\ 4096 = 0$$
From our page table, we can see that page 25 maps to frame 24.
$$physical\ address = 24 * 4096 + 0 = 98304$$
From this, we can see that it is possible to get a physical address for every virtual address we were asked to convert. This means that there were no segmentation faults.
It is possible for a page fault to occur, but we have not been given enough information to determine whether this is the case.