Intel IA-32 - Manual (Page 468)

10-28 Vol. 3A
MEMORY CACHE CONTROL
•
FE (fixed MTRRs enabled) flag, bit 10 — Fixed-range MTRRs are enabled when set;
fixed-range MTRRs are disabled when clear. When the fixed-range MTRRs are enabled,
they take priority over the variable-range MTRRs when overlaps in ranges occur. If the
fixed-range MTRRs are disabled, the variable-range MTRRs can still be used and can map
the range ordinarily covered by the fixed-range MTRRs.
•
E (MTRRs enabled) flag, bit 11 — MTRRs are enabled when set; all MTRRs are
disabled when clear, and the UC memory type is applied to all of physical memory. When
this flag is set, the FE flag can disable the fixed-range MTRRs; when the flag is clear, the
FE flag has no affect. When the E flag is set, the type specified in the default memory type
field is used for areas of memory not already mapped by either a fixed or variable MTRR.
Bits 8 and 9, and bits 12 through 63, in the IA32_MTRR_DEF_TYPE MSR are reserved; the
processor generates a general-protection exception (#GP) if software attempts to write nonzero
values to them.
10.11.2.2 Fixed Range MTRRs
The fixed memory ranges are mapped with 11 fixed-range registers of 64 bits each. Each of these
registers is divided into 8-bit fields that are used to specify the memory type for each of the sub-
ranges the register controls:
•
Register IA32_MTRR_FIX64K_00000 — Maps the 512-KByte address range from 0H
to 7FFFFH. This range is divided into eight 64-KByte sub-ranges.
•
Registers IA32_MTRR_FIX16K_80000 and IA32_MTRR_FIX16K_A0000 — Maps
the two 128-KByte address ranges from 80000H to BFFFFH. This range is divided into
sixteen 16-KByte sub-ranges, 8 ranges per register.
•
Registers IA32_MTRR_FIX4K_C0000 through IA32_MTRR_FIX4K_F8000 —
Maps eight 32-KByte address ranges from C0000H to FFFFFH. This range is divided into
sixty-four 4-KByte sub-ranges, 8 ranges per register.
Table 10-9 shows the relationship between the fixed physical-address ranges and the corre-
sponding fields of the fixed-range MTRRs; Table 10-8 shows memory type encoding for
MTRRs.
Figure 10-5. IA32_MTRR_DEF_TYPE MSR
Type — Default memory type
FE — Fixed-range MTRRs enable/disable
E — MTRR enable/disable
63 0
Reserved
F
E
7 10 11
Type
8 9 12
E
Reserved