Intel ARCHITECTURE IA-32 User Manual download pdf (Page 467)

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Vol. 3A 10-27

MEMORY CACHE CONTROL

• WC (write combining) flag, bit 10 — The write-combining (WC) memory type is

supported when set; the WC type is not supported when clear.

Bit 9 and bits 11 through 63 in the IA32_MTRRCAP MSR are reserved. If software attempts to

write to the IA32_MTRRCAP MSR, a general-protection exception (#GP) is generated.

For the Pentium 4, Intel Xeon, and P6 family processors, the IA32_MTRRCAP MSR always

contains the value 508H.

10.11.2 Setting Memory Ranges with MTRRs

The memory ranges and the types of memory specified in each range are set by three groups of

registers: the IA32_MTRR_DEF_TYPE MSR, the fixed-range MTRRs, and the variable range

MTRRs. These registers can be read and written to using the RDMSR and WRMSR instruc-

tions, respectively. The IA32_MTRRCAP MSR indicates the availability of these registers on

the processor (see Section 10.11.1, “MTRR Feature Identification”).

10.11.2.1 IA32_MTRR_DEF_TYPE MSR

The IA32_MTRR_DEF_TYPE MSR (named MTRRdefType MSR for the P6 family proces-

sors) sets the default properties of the regions of physical memory that are not encompassed by

MTRRs. The functions of the flags and field in this register are as follows:

• Type field, bits 0 through 7 — Indicates the default memory type used for those physical

memory address ranges that do not have a memory type specified for them by an MTRR

(see Table 10-8 for the encoding of this field). The legal values for this field are 0, 1, 4, 5,

and 6. All other values result in a general-protection exception (#GP) being generated.

Intel recommends the use of the UC (uncached) memory type for all physical memory

addresses where memory does not exist. To assign the UC type to nonexistent memory

locations, it can either be specified as the default type in the Type field or be explicitly

assigned with the fixed and variable MTRRs.

Figure 10-4. IA32_MTRRCAP Register

VCNT — Number of variable range registers

FIX — Fixed range registers supported

WC — Write-combining memory type supported

Reserved

71011

VCNT

Reserved

1 2 ... 462 463 464 465 466 467 468 469 470 471 472 ... 635 636

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