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