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11-50 Vol. 3
MEMORY CACHE CONTROL
11.12.3 Selecting a Memory Type from the PAT
To select a memory type for a page from the PAT, a 3-bit index made up of the PAT,
PCD, and PWT bits must be encoded in the page-table or page-directory entry for the
page.
Table 11-11 shows the possible encodings of the PAT, PCD, and PWT bits and
the PAT entry selected with each encoding. The PAT bit is bit 7 in page-table entries
that point to 4-KByte pages and bit 12 in paging-structure entries that point to larger
pages. The PCD and PWT bits are bits 4 and 3, respectively, in paging-structure
entries that point to pages of any size.
The PAT entry selected for a page is used in conjunction with the MTRR setting for the
region of physical memory in which the page is mapped to determine the effective
memory type for the page, as shown in
Table 11-7.
11.12.4 Programming the PAT
Table 11-12 shows the default setting for each PAT entry following a power up or
reset of the processor. The setting remain unchanged following a soft reset (INIT
reset).
Table 11-11. Selection of PAT Entries with PAT, PCD, and PWT Flags
PAT PCD PWT PAT Entry
0 0 0 PAT0
0 0 1 PAT1
0 1 0 PAT2
0 1 1 PAT3
1 0 0 PAT4
1 0 1 PAT5
1 1 0 PAT6
1 1 1 PAT7
Table 11-12. Memory Type Setting of PAT Entries Following a Power-up or Reset
PAT Entry Memory Type Following Power-up or Reset
PAT0 WB
PAT1 WT
PAT2 UC-
PAT3 UC
PAT4 WB
PAT5 WT
PAT6 UC-
PAT7 UC