AMD x86 manual Write-Combining Definitions and Abbreviations, What is Write Combining?

AMD Athlon™ Processor x86 Code Optimization 22007E/0 — November 1999

Write-Combining Definitions and Abbreviations

This appendix uses the following definitions and abbreviations:

■UC—Uncacheable memory type

■WC—Write-combining memory type

■WT—Writethrough memory type

■WP—Write-protected memory type

■WB—Writeback memory type

■One Byte—8 bits

■One Word—16 bits

■Longword—32 bits (same as a x86 doubleword)

■Quadword—64 bits or 2 longwords

■Octaword—128 bits or 2 quadwords

■Cache Block—64 bytes or 4 octawords or 8 quadwords

What is Write Combining?

Write combining is the merging of multiple memory write cycles that target locations within the address range of a write buffer. The AMD Athlon processor combines multiple memory-write cycles to a 64-byte buffer whenever the memory address is within a WC or WT memory type region. The processor continues to combine writes to this buffer without writing the data to the system, as long as certain rules apply (see Table 9 on page 158 for more information).

Programming Details

The steps required for programming write combining on the

AMD Athlon processor are as follows:

1.Verify the presence of an AMD Athlon processor by using the CPUID instruction to check for the instruction family code and vendor identification of the processor. Standard function 0 on AMD processors returns a vendor identification string of “AuthenticAMD” in registers EBX, EDX, and ECX. Standard function 1 returns the processor

156 Write-Combining Definitions and Abbreviations