Write combining (WC) is a

computer bus In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...

technique for allowing

data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted ...

to be combined and temporarily stored in a

buffer Buffer may refer to: Science * Buffer gas, an inert or nonflammable gas * Buffer solution, a solution used to prevent changes in pH * Buffering agent, the weak acid or base in a buffer solution * Lysis buffer, in cell biology * Metal ion 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 In mathematics, especially order theory, a weak ordering is a mathematical formalization of the intuitive notion of a ranking of a set, some of whose members may be tied with each other. Weak orders are a generalization of totally ordered set ...

. 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 A write buffer is a type of data buffer that can be used to hold data being written from the cache to main memory or to the next cache in the memory hierarchy to improve performance and reduce latency. It is used in certain CPU cache architectu ...

can be treated as a

fully associative A CPU cache is a memory which holds the recently utilized data by the processor. A block of memory cannot necessarily be placed randomly in the cache and may be restricted to a single CPU cache#Cache entries, cache line or a set of cache lines by ...

cache and added into the

memory hierarchy In computer architecture, the memory hierarchy separates computer storage into a hierarchy based on response time. Since response time, complexity, and capacity are related, the levels may also be distinguished by their performance and controlli ...

of the device in which it is implemented. Adding complexity slows down the

so this technique is often only used for memory which does not need

strong ordering Strong may refer to: Education * The Strong, an educational institution in Rochester, New York, United States * Strong Hall (Lawrence, Kansas), an administrative hall of the University of Kansas * Strong School, New Haven, Connecticut, United St ...

(always correct) like the frame buffers of

video cards A graphics card (also called a video card, display card, graphics adapter, VGA card/VGA, video adapter, display adapter, or mistakenly GPU) is an expansion card which generates a feed of output images to a display device, such as a computer moni ...

References

{{Reflist

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

CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...

s 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 Computer buses Computer memory

Technique

See also

References

External links