Cache On A STick

COASt, an acronym for cache on a stick, is a packaging standard for modules containing SRAM used as an L2 cache in a computer. COASt modules look like somewhat over-sized SIMM modules. These modules were somewhat popular in the early to mid-1990s, but with newer computers cache is built into either the CPU or the motherboard. COASt modules decoupled the motherboard from its cache, allowing varying configurations to be created. A low-cost system could run with no cache, while a more expensive system could come equipped with 512 KB or more cache. Later COASt modules were equipped with pipelined-burst SRAM.
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<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)