Write combining (WC) is a

computer bus In computer architecture, a bus (historically also called a data highway or databus) is a communication system that transfers data between components inside a computer or between computers. It encompasses both hardware (e.g., wires, optical ...

technique for allowing

data Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted for ...

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 * Lysis buffer, in cell biology * Metal ion buffer * Mineral redox buffer, in geology Technology and engineeri ...

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 communication. 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 represented as ...

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 (mathematics), set, some of whose members may be Tie (draw), tied with each other. Weak orders are a general ...

. 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 architecture ...

can be treated as a

fully associative Cache placement policies are policies that determine where a particular memory block can be placed when it goes into a CPU cache. A block of memory cannot necessarily be placed at an arbitrary location in the cache; it may be restricted to a parti ...

cache Cache, caching, or caché may refer to: Science and technology * Cache (computing), a technique used in computer storage for easier data access * Cache (biology) or hoarding, a food storing behavior of animals * Cache (archaeology), artifacts p ...

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 contr ...

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 (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.

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 primary processor in a given computer. Its electronic circuitry executes instructions of a computer program, such as arithmetic, log ...

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