Online Computer Terms Dictionary - C

cache

<memory management> /kash/ A small fast memory holding recently accessed data, designed to speed up subsequent access to the same data. Most often applied to processor-memory access but also used for a local copy of data accessible over a network etc.

When data is read from, or written to, main memory a copy is also saved in the cache, along with the associated main memory address. The cache monitors addresses of subsequent reads to see if the required data is already in the cache. If it is (a cache hit) then it is returned immediately and the main memory read is aborted (or not started). If the data is not cached (a cache miss) then it is fetched from main memory and also saved in the cache.

The cache is built from faster memory chips than main memory so a cache hit takes much less time to complete than a normal memory access. The cache may be located on the same integrated circuit as the CPU, in order to further reduce the access time. In this case it is often known as primary cache since there may be a larger, slower secondary cache outside the CPU chip.

The most important characteristic of a cache is its hit rate - the fraction of all memory accesses which are satisfied from the cache. This in turn depends on the cache design but mostly on its size relative to the main memory. The size is limited by the cost of fast memory chips.

The hit rate also depends on the access pattern of the particular program being run (the sequence of addresses being read and written). Caches rely on two properties of the access patterns of most programs: temporal locality - if something is accessed once, it is likely to be accessed again soon, and spatial locality - if one memory location is accessed then nearby memory locations are also likely to be accessed. In order to exploit spatial locality, caches often operate on several words at a time, a "cache line" or "cache block". Main memory reads and writes are whole cache lines.

When the processor wants to write to main memory, the data is first written to the cache on the assumption that the processor will probably read it again soon. Various different policies are used. In a write-through cache, data is written to main memory at the same time as it is cached. In a write-back cache it is only written to main memory when it is forced out of the cache.

If all accesses were writes then, with a write-through policy, every write to the cache would necessitate a main memory write, thus slowing the system down to main memory speed. However, statistically, most accesses are reads and most of these will be satisfied from the cache. Write-through is simpler than write-back because an entry that is to be replaced can just be overwritten in the cache as it will already have been copied to main memory whereas write-back requires the cache to initiate a main memory write of the flushed entry followed (for a processor read) by a main memory read. However, write-back is more efficient because an entry may be written many times in the cache without a main memory access.

When the cache is full and it is desired to cache another line of data then a cache entry is selected to be written back to main memory or "flushed". The new line is then put in its place. Which entry is chosen to be flushed is determined by a "replacement algorithm".

Some processors have separate instruction and data caches. Both can be active at the same time, allowing an instruction fetch to overlap with a data read or write. This separation also avoids the possibility of bad cache conflict between say the instructions in a loop and some data in an array which is accessed by that loop.

See also direct mapped cache, fully associative cache, sector mapping, set associative cache.

(1997-06-25)

cache block

cache line

cache coherency

<storage> (Or "cache consistency") /kash koh-heer'n-see/ The synchronisation of data in multiple caches such that reading a memory location via any cache will return the most recent data written to that location via any (other) cache.

Some parallel processors do not cache accesses to shared memory to avoid the issue of cache coherency. If caches are used with shared memory then some system is required to detect when data in one processor's cache should be discarded or replaced because another processor has updated that memory location. Several such schemes have been devised.

(1998-11-10)

cache conflict

<storage> A sequence of accesses to memory repeatedly overwriting the same cache entry. This can happen if two blocks of data, which are mapped to the same set of cache locations, are needed simultaneously.

For example, in the case of a direct mapped cache, if arrays A, B, and C map to the same range of cache locations, thrashing will occur when the following loop is executed:

	for (i=1; i<n; i++)
		C[i] = A[i] + B[i];

See also ping-pong.

(1997-01-21)

cache consistency

cache coherency

cache hit

<storage> A request to read from memory which can satisfied from the cache without using the main memory.

Opposite: cache miss.

(1997-01-21)

cache line

<storage> (Or cache block) The smallest unit of memory than can be transferred between the main memory and the cache.

Rather than reading a single word or byte from main memory at a time, each cache entry is usually holds a certain number of words, known as a "cache line" or "cache block" and a whole line is read and cached at once. This takes advantage of the principle of locality of reference: if one location is read then nearby locations (particularly following locations) are likely to be read soon afterward. It can also take advantage of page-mode DRAM which allows faster access to consecutive locations.

(1997-01-21)

cache memory

cache

cache miss

<storage> A request to read from memory which cannot be satisfied from the cache, for which the main memory has to be consulted.

Opposite: cache hit.

(1997-01-21)

Cache On A STick

<architecture> (COAST) Intel Corporation attempt to's standardise the modular L2 cache subsystem in Pentium-based computers.

A COAST module should be about 4.35" wide by 1.14" high. According to earlier specifications from Motorola, a module between 4.33" and 4.36" wide, and between 1.12" and 1.16" high is within the COAST standard. Some module vendors, including some major motherboard suppliers, greatly violate the height specification.

Another COAST specification violated by many suppliers concerns clock distribution in synchronous modules. The specification requires that the clock tree to each synchronous chip be balanced, i.e. equal length from edge of the connector to individual chips. An unbalanced clock tree increases reflections and noise.

For a 256 kilobyte cache module the standard requires the same clock be used for both chips but some vendors use separate clocks to reduce loading on the clock driver and hence increase the clock speed. However, this creates unbalanced loading in other motherboard configurations, such as motherboards with soldered caches in the system.

(1996-06-10)

caching

cache