GXLV Datasheet, PDF (77/247 Pages) National Semiconductor (TI) – Geode™ GXLV Processor Series Low Power Integrated x86 Solutions

Processor Programming (Continued)
3.5.4 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.
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
locate an entry in the page directory table. The page
directory table acts as a 32-bit master index to up to 1 KB
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 physical
address of the current page directory table is stored in the
CR3 control register, also referred to as the Page Direc-
tory Base Register (PDBR).
Bits [21:12] of the 32-bit linear address, referred to as the
Page Table Index (PTI), locate a 32-bit entry in the sec-
ond-level page table. This page table entry (PTE) contains
the base address of the desired page frame. The second-
level page table addresses up to 1K individual page
frames. A second-level page table is 4 KB in size and is
itself a page. Bits [11:0] of the 32-bit linear address, the
Page Frame Offset (PFO), locate the desired physical
data within the page frame.
Since the page directory table can point to 1 KB page
tables, and each page table can point to 1 KB page
frames, a total of 1 MB page frames can be implemented.
Each page frame contains 4 KB, therefore, up to 4 GB of
virtual memory can be addressed by the CPU with a sin-
gle page directory table.
Linear
Address
31
22 21
12 11
0
Directory Table Index
(DTI)
Page Table Index
(PTI)
Page Frame Offset
(PFO)
CR3
Control
Register
1
DTE Cache
2-Entry
Fully Associative
0
4 KB
DTE
0
Directory Table
Main TLB ³¹
32-Entry
4-Way Set
Associative ₀
4 GB
4 KB
-4 KB
Physical Page
PTE
-0
0
Page Table
0
Memory
External Memory
Figure 3-8. Paging Mechanism
Revision 1.1
77
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