Operating Systems

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4.3.2: Page tables

A discussion of page tables is also appropriate for (non-demand) paging, but the issues are more acute with demand paging since the tables can be much larger. Why?

  1. The total size of the active processes is no longer limited to the size of physical memory. Since the total size of the processes is greater, the total size of the page tables is greater and hence concerns over the size of the page table are more acute.

  2. With demand paging an important question is the choice of a victim page to page out. Data in the page table can be useful in this choice.

We must be able access to the page table very quickly since it is needed for every memory access.

Unfortunate laws of hardware.

So we can't just say, put the page table in fast processor registers, and let it be huge, and sell the system for $1000.

The simplest solution is to put the page table in main memory. However it seems to be both too slow and two big.

  1. Seems too slow since all memory references require two reference.
  2. The page table might be too big.

Contents of a PTE

Each page has a corresponding page table entry (PTE). The information in a PTE is for use by the hardware. Why must it be tailored for the hardware and not the OS?
Because it is accessed frequently.
But actually some systems (software TLB reload) do not have hardware access. Information set by and used by the OS is normally kept in other OS tables. The page table format is determined by the hardware, so access routines are not portable. The following fields are often present.

  1. The valid bit. This tells if the page is currently loaded (i.e., is in a frame). If set, the frame number is valid. It is also called the presence or presence/absence bit. If a page is accessed with the valid bit unset, a page fault is generated by the hardware.

  2. The frame number. This field is the main reason for the table. It gives the virtual to physical address translation.

  3. The Modified bit. Indicates that some part of the page has been written since it was loaded. This is needed if the page is evicted so that the OS can tell if the page must be written back to disk.

  4. The referenced bit. Indicates that some word in the page has been referenced. Used to select a victim: unreferenced pages make good victims by the locality property (discussed below).

  5. Protection bits. For example one can mark text pages as execute only. This requires that boundaries between regions with different protection are on page boundaries. Normally many consecutive (in logical address) pages have the same protection so many page protection bits are redundant. Protection is more naturally done with segmentation.

Multilevel page tables

Recall the previous diagram. Most of the virtual memory is the unused space between the data and stack regions. However, with demand paging this space does not waste real memory. But the single large page table does waste real memory.

The idea of multi-level page tables (a similar idea is used in Unix i-node-based file systems, which we study later when we do I/O) is to add a level of indirection and have a page table containing pointers to page tables.

Address translation with a 2-level page table

For a two level page table the virtual address is divided into three pieces

+-----+-----+-------+
| P#1 | P#2 | Offset|
+-----+-----+-------+

The VAX used a 2-level page table structure, but with some wrinkles (see Tanenbaum for details).

Naturally, there is no need to stop at 2 levels. In fact the SPARC has 3 levels and the Motorola 68030 has 4 (and the number of bits of Virtual Address used for P#1, P#2, P#3, and P#4 can be varied).