From Wikipedia, the free encyclopedia Write-combining
Write-combining
Write combining (WC)[1] is a computer bus technique for • Page Attribute Table - x86 page table extension that
allowing data to be combined and temporarily stored in a allows fine-grained cache control, incl. write
buffer -- the write combine buffer (WCB) -- to be released combining
together later in burst mode instead of writing (immedi- • MTRR (Memory Type Range Registers) - The older
ately) as single bits or small chunks. x86 cache control mechanism.
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 combina-
External links
tion of writes and reads is done in the correct order. • 6x86opt, ctppro, CTU, DirectNT, FastVid, fstorion,
For example, a Write/Read/Write combination to a spe- K6Speed, MTRRLFBE, S3 Speed Up & Write Allocate
cific address would lead to the write combining order Monitor enable LFB and Banked VGA Write
of Read/Write/Write which can lead to obtaining wrong Combining on Intel Pentium Pro/2/3/4 and AMD K6
values with the first read (which potentially relies on the CPUs in Windows 9x, Windows NTx, DOS, OS/2 and
write before). Linux
In order to avoid the problem of read/write order de- • MTRRLFBE enable LFB and Banked VGA Write
scribed above, the write buffer can be treated as a fully Combining on Intel Pentium Pro/2/3/4 CPUs in
associative cache and added into the memory hierarchy Windows 9x and DOS
of the device in which it is implemented[2]. Adding com- • CTU enable LFB and Banked VGA Write Combining
plexity slows down the memory hierarchy so this tech- on AMD K6 CPUs in Windows 9x and DOS
nique is often only used for memory which does not need
’strong ordering’ (always correct) like the frame buffers
of video cards.
References
[1] Intel (November 1998). "Write Combining Memory
Implementation Guidelines". Intel.
See also http://download.intel.com/design/PentiumII/
• Write buffer - A buffer used to facilitate apparently applnots/24442201.pdf. Retrieved 2010-11-02.
simultaneous reads and writes by the memory [2] BearWindows (2008-09-01). "Video Hardware
system. Acceleration in DOS Environment". BearWindows
• Framebuffer - Linear Frame Buffer (LFB) and Banked Vault. http://www.bearwindows.boot-land.net/
VGA (BVGA) Frame Buffer CPU cache mechanisms. af.htm. Retrieved 2010-11-02.
Retrieved from "http://en.wikipedia.org/w/index.php?title=Write-combining&oldid=443805853"
Categories:
• Computer buses
• Computer memory
• Computer hardware stubs
This page was last modified on 9 August 2011 at 03:56. Text is available under the Creative Commons Attribution-
ShareAlike License; additional terms may apply. See Terms of use for details. Wikipedia® is a registered trademark of
the Wikimedia Foundation, Inc., a non-profit organization.Contact us
Privacy policy About Wikipedia Disclaimers Mobile view
1