Write-combining

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Write combining (WC) is a computer bus technique for allowing data to be combined and temporarily stored in a buffer -- the write combine buffer (WCB) -- to be released together later in burst mode instead of writing (immediately) as single bits or small chunks.

Write combining cannot be used for general memory access (data or code regions) due to the 'weak ordering'. Write-combining does not guarantee that the combination of writes and reads is done in the correct order. For example, a Write/Read/Write Combination to a specific address would lead to the write combining order of Read/Write/Write which can lead to obtaining wrong values with the first read (which potentially relies on the write before). So this technique can only be used for Memory which does not need 'Strong Ordering' (always correct) like Frame Buffer of Video Cards.

[edit] See also

• MTRR - Registers to control the semantics of memory accesses of processors, incl. write-combining

[edit] External links

• FAST v1.10 Enable Write Combining for Pentium Pro/2 on Win9x
• CTU by Rob Mueller Enable Write Combining for K6-X on Win9x

[edit] References