11-30 Vol. 3A

MEMORY CACHE CONTROL

Example 11-4.  MemTypeGet() Pseudocode

#define MIXED_TYPES -1     /* 0 < MIXED_TYPES || MIXED_TYPES > 256 */

IF CPU_FEATURES.MTRR /* processor supports MTRRs */

THEN

Align BASE and SIZE to 4-KByte boundary;
IF (BASE + SIZE) wrap 4-GByte address space 

THEN return INVALID;

FI;
IF MTRRdefType.E = 0

THEN return UC;

FI;
FirstType 

¨

 Get4KMemType (BASE);

/* Obtains memory type for first 4-KByte range. */
/* See Get4KMemType (4KByteRange) in Example 11-5. */
FOR each additional 4-KByte range specified in SIZE

NextType 

¨

 Get4KMemType (4KByteRange);

IF NextType 

¼

 FirstType

THEN return MixedTypes;

FI;

ROF;

return FirstType;

ELSE return UNSUPPORTED;

FI;

If the processor does not support MTRRs, the function returns UNSUPPORTED. If the MTRRs are not enabled, then 
the UC memory type is returned. If more than one memory type corresponds to the specified range, a status of 
MIXED_TYPES is returned. Otherwise, the memory type defined for the range (UC, WC, WT, WB, or WP) is 
returned.
The pseudocode for the Get4KMemType() function in Example 11-5 obtains the memory type for a single 4-KByte 
range at a given physical address. The sample code determines whether an PHY_ADDRESS falls within a fixed 
range by comparing the address with the known fixed ranges: 0 to 7FFFFH (64-KByte regions), 80000H to BFFFFH 
(16-KByte regions), and C0000H to FFFFFH (4-KByte regions). If an address falls within one of these ranges, the 
appropriate bits within one of its MTRRs determine the memory type.

Example 11-5.  Get4KMemType() Pseudocode

IF IA32_MTRRCAP.FIX AND MTRRdefType.FE /* fixed registers enabled */

THEN IF PHY_ADDRESS is within a fixed range

return IA32_MTRR_FIX.Type;

FI;
FOR each variable-range MTRR in IA32_MTRRCAP.VCNT

IF IA32_MTRR_PHYSMASK.V = 0

THEN continue;

FI;
IF (PHY_ADDRESS AND IA32_MTRR_PHYSMASK.Mask) =

(IA32_MTRR_PHYSBASE.Base 
AND IA32_MTRR_PHYSMASK.Mask)

THEN

return IA32_MTRR_PHYSBASE.Type;

FI;

ROF;
return MTRRdefType.Type;