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.
Technique
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 expected 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).
In order to avoid the problem of read/write order described above, the write buffer can be treated as a fully associative cache and added into the memory hierarchy of the device in which it is implemented.2 Adding complexity slows down the memory hierarchy so this technique is often only used for memory which does not need strong ordering (always correct) like the frame buffers of video cards.
See also
- Framebuffer (FB), and when linear: LFB
- Memory type range registers (MTRR) – the older x86 cache control mechanism
- Page attribute table (PAT) – x86 page table extension that allows fine-grained cache control, including write combining
- Page table
- Uncacheable speculative write combining (USWC)
- Video Graphics Array (VGA), and Banked (BVGA) Frame Buffer
External links
- 6x86opt, ctppro, CTU, DirectNT, FastVid, fstorion, K6Speed, MTRRLFBE, S3 Speed Up & Write Allocate Monitor enable LFB and BVGA Write Combining on Intel Pentium Pro/2/3/4 and AMD K6 CPUs in Windows 9x, Windows NTx, DOS, OS/2 and Linux
- MTRRLFBE enable LFB and BVGA Write Combining on Intel Pentium Pro/2/3/4 CPUs in Windows 9x and DOS
- CTU (Internet Archive cached copy) enable LFB and Banked VGA Write Combining on AMD K6 CPUs in Windows 9x and DOS
References
Intel (November 1998). "Write Combining Memory Implementation Guidelines" (PDF). Intel. Retrieved 2010-11-02. http://download.intel.com/design/PentiumII/applnots/24442201.pdf ↩
BearWindows (2008-09-01). "Video Hardware Acceleration in DOS Environment". BearWindows Vault. Archived from the original on July 24, 2008. Retrieved 2010-11-02. https://web.archive.org/web/20080724002604/http://www.bearwindows.boot-land.net/af.htm ↩