DIRECTORY
	1 page directory (4k block holding 1024 entries)
	each entry points to a page table
TABLE
	total of 1024 page tables
	each table is a 4k block holding 1024 entries
	each entry points to a page frame (see PAGE FRAME ADDRESS)
FRAME
	total of 1048576 page frames
	each frame is a 4k block
	the frame is where the data/code is stored

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LINEAR ADDRESS
	dir:10, page:10, offset:12


FORMAT of PAGE DIRECTORY and PAGE TABLE entries
	TABLE:
	addr:20, avail:3,g:1, RES:1,d:1,a:1,pcd:1, pwt:1,u:1,w:1,p:1
	DIRECTORY:
	addr:20, avail:3,g:1, ps:1,RES:1,a:1,pcd:1, pwt:1,u:1,w:1,p:1

	avail:3= OS usage
	g=blobal page, ignored for pgdir

	ps= page size (pgdir only)
	RES= reserved, must be zero
	d= dirty (page has been written to) (pgtbl only)
	a= accessed (page has been used, r/w)
	pcd= if set then do not cache page on-chip

	pwt= write-through if set
	u= user (cpl3)
	w= writable
	p= page present

	if P bit is zero then the page is not in memory and the other 31 bits
	 may be used by the OS to locate the missing page.

	if P bit is zero and an attempt is made to access the page, a 
	 page-fault is generated.

	any unused entries in the page directory or page table should be cleared.
	 if a page-fault is generated check for a cleared entry. if clear 
	 then an iilegal access was made. if not clear then find the page in 
	 swap and load it.

	the A bit can be used to track how often the page is used. if not 
	 used it can be swapped out.

	the D bit could be used when saving a file. check the D bits for 
	 each page of the file. if the page hasn't been written to, then do not
	 write it to storage. after saving the file clear all D bits. this 
	 would speed up the time it takes to save a file.

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set bit 31 of CR0 to enable paging

CR3 (PDBR) is the physical address of the current page directory

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if a page is physically moved or reloaded to a new location in memory the TLB
entry for that page must be invalidated.

a task switch or reloading CR3 will cause the TLB's to be invalidated.

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page tables may be swapped out, but the page directory must always exist in 
memory while CR3 points to it. when a page is accesed the A bit of both the 
page table and page frame is set. the D bit is only used in the page frame entry.

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