72 lines
2.5 KiB
Markdown
72 lines
2.5 KiB
Markdown
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This fun little homework tests if you understand how a multi-level page table
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works. And yes, there is some debate over the use of the term fun in the
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previous sentence. The program is called:
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paging-multilevel-translate.py
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Some basic assumptions:
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- The page size is an unrealistically-small 32 bytes
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- The virtual address space for the process in question (assume there is
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only one) is 1024 pages, or 32 KB
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- physical memory consists of 128 pages
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Thus, a virtual address needs 15 bits (5 for the offset, 10 for the VPN).
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A physical address requires 12 bits (5 offset, 7 for the PFN).
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The system assumes a multi-level page table. Thus, the upper five bits of a virtual
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address are used to index into a page directory; the page directory entry (PDE), if valid,
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points to a page of the page table. Each page table page holds 32 page-table entries
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(PTEs). Each PTE, if valid, holds the desired translation (physical frame number, or PFN)
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of the virtual page in question.
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The format of a PTE is thus:
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VALID | PFN6 ... PFN0
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and is thus 8 bits or 1 byte.
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The format of a PDE is essentially identical:
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VALID | PT6 ... PT0
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You are given two pieces of information to begin with.
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First, you are given the value of the page directory base register (PDBR),
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which tells you which page the page directory is located upon.
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Second, you are given a complete dump of each page of memory. A page dump
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looks like this:
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page 0: 08 00 01 15 11 1d 1d 1c 01 17 15 14 16 1b 13 0b ...
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page 1: 19 05 1e 13 02 16 1e 0c 15 09 06 16 00 19 10 03 ...
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page 2: 1d 07 11 1b 12 05 07 1e 09 1a 18 17 16 18 1a 01 ...
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...
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which shows the 32 bytes found on pages 0, 1, 2, and so forth. The first byte
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(0th byte) on page 0 has the value 0x08, the second is 0x00, the third 0x01,
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and so forth.
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You are then given a list of virtual addresses to translate.
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Use the PDBR to find the relevant page table entries for this virtual page.
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Then find if it is valid. If so, use the translation to form a final physical
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address. Using this address, you can find the VALUE that the memory reference
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is looking for.
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Of course, the virtual address may not be valid and thus generate a fault.
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Some useful options:
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-s SEED, --seed=SEED the random seed
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-n NUM, --addresses=NUM number of virtual addresses to generate
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-c, --solve compute answers for me
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Change the seed to get different problems, as always.
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Change the number of virtual addresses generated to do more translations
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for a given memory dump.
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Use -c (or --solve) to show the solutions.
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