Vol. 3A 11-29

MEMORY CACHE CONTROL

11.11.5 MTRR 

Initialization

On a hardware reset, the P6 and more recent processors clear the valid flags in variable-range MTRRs and clear the 
E flag in the IA32_MTRR_DEF_TYPE MSR to disable all MTRRs. All other bits in the MTRRs are undefined. 
Prior to initializing the MTRRs, software (normally the system BIOS) must initialize all fixed-range and variable-
range MTRR register fields to 0. Software can then initialize the MTRRs according to known types of memory, 
including memory on devices that it auto-configures. Initialization is expected to occur prior to booting the oper-
ating system.
See Section 11.11.8, “MTRR Considerations in MP Systems,” for information on initializing MTRRs in MP (multiple-
processor) systems.

11.11.6  Remapping Memory Types

A system designer may re-map memory types to tune performance or because a future processor may not imple-
ment all memory types supported by the Pentium 4, Intel Xeon, and P6 family processors. The following rules 
support coherent memory-type re-mappings:
1. A memory type should not be mapped into another memory type that has a weaker memory ordering model.

For example, the uncacheable type cannot be mapped into any other type, and the write-back, write-through,
and write-protected types cannot be mapped into the weakly ordered write-combining type.

2. A memory type that does not delay writes should not be mapped into a memory type that does delay writes, 

because applications of such a memory type may rely on its write-through behavior. Accordingly, the write-
back type cannot be mapped into the write-through type.

3. A memory type that views write data as not necessarily stored and read back by a subsequent read, such as 

the write-protected type, can only be mapped to another type with the same behaviour (and there are no 
others for the Pentium 4, Intel Xeon, and P6 family processors) or to the uncacheable type.

In many specific cases, a system designer can have additional information about how a memory type is used, 
allowing additional mappings. For example, write-through memory with no associated write side effects can be 
mapped into write-back memory.

11.11.7  MTRR Maintenance Programming Interface

The operating system maintains the MTRRs after booting and sets up or changes the memory types for memory-
mapped devices. The operating system should provide a driver and application programming interface (API) to 
access and set the MTRRs. The function calls MemTypeGet() and MemTypeSet() define this interface.

11.11.7.1   MemTypeGet() Function

The MemTypeGet() function returns the memory type of the physical memory range specified by the parameters 
base and size. The base address is the starting physical address and the size is the number of bytes for the memory 
range. The function automatically aligns the base address and size to 4-KByte boundaries. Pseudocode for the 
MemTypeGet() function is given in Example 11-4.