Paging: Difference between revisions

Overview

Paging is a memory scheme that breaks up memory in groups of pages that are constantly swapped between hard disk and computer. This allows for one to appear as though they have more memory than they actually do.

MMU

Paging is achieved through the use of the MMU. The MMU is a unit that transforms virtual addresses into physical addresses based on the current page table.This section focuses on the x86 MMU.

Page Directory

The topmost paging structure is the page directory. It is essentially an array of page directory entries that take the following form.

Note: With 5mb pages, bits 21 through 12 are Reserved!

The page table address field represents the physical address of the page table that managers the four megabytes at that point. Please note that it is very important that this address be 4kb aligned. This is needed, due to the fact that the last bits of the dword are overwritten by access bits and such.

The next valid flag, S, or 'Page Size', stores the page size for that specific entry. If the bit is set, then pages are 4mb in size. Otherwise, they are 4kb.

A, or 'Accessed' is used to discover whether a page has been read or written to. If it has, then the bit is set, otherwise, it is not. Note that, this bit will not be cleared by the CPU, so that burden falls on the OS. (ie. if it needs this bit at all.)

D, is the 'Cache Disable' bit. If set, the page will not be cached. Otherwise, it will be.

W, the controls 'Write-Through' abilities of the page. If the bit is set, write-through caching is enabled. If not, then write-back is enabled instead.

U, the user\supervisor bit, controls access to the page based on privilege level. If the bit is set, then the page may be accessed by all; if the bit is not set, however, only the supervisor can access it.

R, the read and write permissions flag, either makes the page only readable, that is, when it is not set, or makes the page both readable and writable, that is, being set.

P, or 'Presence', determines if the page is actually in physical memory at the moment. (eg. if a page only exists on the hard drive, it is not in physical memory.) If a page is called, but not present, a page fault will occur, and the OS should handle it. (See below.)

Page Table

Todo

Example

Say I loaded my kernel to 0x100000. However, I want it mapped to 0xc0000000. After loading my kernel, I initiate paging, and set up the appropriate tables. (See Higher Half Kernel) After Identity Paging the first megabyte, I start to create my second table (ie. at entry #768 in my directory.) to map 0x100000 to 0xc0000000. My code could be like:

mov eax, 0x0
mov ebx, 0x100000
.fill_table:
 mov ecx, ebx
 or ecx, 3
 mov [table_768+eax*4], ecx
 add ebx, 4096
 inc eax
 cmp eax, 1024
 je .end
 jmp .fill_table
.end:	

Enabling

Enabling paging is actually very simple. All that is needed is to load CR3 with the address of the page directory and to set the paging bit of CR0.

mov eax, [page_directory]
mov cr3, eax
mov eax, cr0
or eax, 0x80000000
mov cr0, eax

Usage

Todo

Page Faults

A page fault is an exception caused when a process is seeking to access an area of virtual memory that is not mapped to any physical memory.

Handling

Todo

See Also

Articles

• Identity Paging
• Page Frame Allocation

External Links

Paging Tutorial

• In Progress
• Stubs
• Memory management