User's Manual

172 Memory Type Range Register (MTRR) Mechanism
AMD Athlon™ Processor x86 Code Optimization
22007E/0—November 1999
There are two types of address ranges: fixed and variable. (See
Figure 12.) For each address range, there is a memory type. For
each 4K, 16K or 64K segment within the first 1 Mbyte of
memory, there is one fixed address MTRR. The fixed address
ranges all exist in the first 1 Mbyte. There are eight variable
address ranges above 1 Mbytes. Each is programmed to a
specific memory starting address, size and alignment. If a
variable range overlaps the lower 1 MByte and the fixed
MTRRs are enabled, then the fixed-memory type dominates.
The address regions have the following priority with respect to
each other:
1. Fixed address ranges
2. Variable address ranges
3. Default memory type (UC at reset)