72 lines
2.5 KiB
Markdown
72 lines
2.5 KiB
Markdown
|
|
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.
|
|
|