Page Table
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The page table maps virtual page numbers to physical page numbers per process. Any data structure could be used for this. The simplest choice is a linear page table: an array with virtual page numbers as indices. While simple, it is also space-inefficient (Multi-level-Page-Tables).
Besides the physical page number that maps to the virtual page number, additional bits are stored in each entry:
| Name | Description |
|---|---|
| Valid bit | Is this page currently allocated or unused? |
| Protection bits | Can this be read from, written to, or both? |
| Present bit | Is this page swapped out to disk? |
| Dirty bit | Has this page been modified since its allocation? |
| Reference bit | Has this page been accessed since its allocation? |
| Mode bit | Can this page be accessed by user-mode processes? |
The valid bit in particular helps reduce memory usage: physical page numbers are huge, so we only want to map them to virtual page numbers when memory is actually allocated.
Tags: programming ostep