Write combining (WC) is a computer bus technique for allowing

data In the pursuit of knowledge, data (; ) is a collection of discrete Value_(semiotics), values that convey information, describing quantity, qualitative property, quality, fact, statistics, other basic units of meaning, or simply sequences of sy ...

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

bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...

s 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 ...

. 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 Cache, caching, or caché may refer to: Places United States * Cache, Idaho, an unincorporated community * Cache, Illinois, an unincorporated community * Cache, Oklahoma, a city in Comanche County * Cache, Utah, Cache County, Utah * Cache County ...

and added into the memory hierarchy of the device in which it is implemented. 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 buffer A framebuffer (frame buffer, or sometimes framestore) is a portion of random-access memory (RAM) containing a bitmap that drives a video display. It is a memory buffer containing data representing all the pixels in a complete video frame. Mode ...

s 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 mo ...

References

{{Reflist

External links

6x86opt, ctppro, CTU, DirectNT, FastVid, fstorion, K6Speed, MTRRLFBE, S3 Speed Up & Write Allocate Monitor
enable

LFB The London Fire Brigade (LFB) is the Fire department, fire and rescue service for London, the capital of the United Kingdom. It was formed by the Metropolitan Fire Brigade Act 1865, under the leadership of superintendent Eyre Massey Shaw. It ha ...

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 Computer buses Computer memory

Technique

See also

References

External links