Processor Programming (Continued)
3.9 PAGING MECHANISM
The paging mechanism translates a linear address to its
corresponding physical address. If the required page is
not currently present in RAM, an exception is generated.
When the operating system services the exception, the
required page can be loaded into memory and the instruc-
tion restarted. Pages are either 4 KB or 1 MB in size. The
CPU defaults to 4 KB pages that are aligned to 4 KB
boundaries.
locate an entry in the page directory table. The page
directory table acts as a 32-bit master index to up to 1 K
individual second-level page tables. The selected entry in
the page directory table, referred to as the directory table
entry (DTE), identifies the starting address of the second-
level page table. The page directory table itself is a page
and is, therefore, aligned to a 4 KB boundary. The physi-
cal address of the current page directory table is stored in
the CR3 control register, also referred to as the Page
Directory Base Register (PDBR).
A page is addressed by using two levels of tables as illus-
trated in Figure 3-8. Bits[31:22] of the 32-bit linear
address, the Directory Table Index (DTI) are used to
Linear
Address
31
22 21
12 11
0
Directory Table Index
(DTI)
Page Table Index
(PTI)
Page Frame Offset
(PFO)
4 GB
31
0
1
0
Main TLB
32-Entry
4-Way Set
Associative
DTE Cache
2-Entry
Fully Associative
-4 KB
4 KB
4 KB
Physical Page
Memory
DTE
PTE
-0
0
CR3
0
0
Control
Register
Directory Table
Page Table
External Memory
Figure 3-8. Paging Mechanism
www.national.com
72
Revision 3.1
NSC [ National Semiconductor ]
