COASt, an acronym for "cache on a stick", is a packaging standard for modules containing SRAM used as an

L2 cache A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, which ...

in a computer. COASt modules look like somewhat oversized

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modules. These modules were somewhat popular in the Apple and PC platforms during early to mid-1990s, but with newer computers cache is built into either the CPU or the

motherboard A motherboard (also called mainboard, main circuit board, mb, mboard, backplane board, base board, system board, logic board (only in Apple computers) or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expand ...

. 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. The standard was originally defined by

Motorola Motorola, Inc. () was an American Multinational corporation, multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent p ...

to be between 4.33 and 4.36 inches (110 and 111 mm) wide, and between 1.12 and 1.16 inches (28 and 29 mm) high. It could be found in many

Apple Macintosh The Mac (known as Macintosh until 1999) is a family of personal computers designed and marketed by Apple Inc. Macs are known for their ease of use and minimalist designs, and are popular among students, creative professionals, and software en ...

in the early-to-mid-90s, but disappeared as the Mac moved to the

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platform.

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...

also used the COASt standard for their

Pentium Pentium is a brand used for a series of x86 architecture-compatible microprocessors produced by Intel. The original Pentium processor from which the brand took its name was first released on March 22, 1993. After that, the Pentium II and Pe ...

systems, where it could be found as late as 1998 in Pentium MMX systems utilizing

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such as 430VX and 430TX. Later, Intel combined this architecture with the CPU and created the

Slot 1 Slot 1 refers to the physical and electrical specification for the connector used by some of Intel's microprocessors, including the Pentium Pro, Celeron, Pentium II and the Pentium III. Both single and dual processor configurations were impleme ...

CPU cartridge which contained both the CPU and separate cache chips. The slot that the COASt module plugged into was named "CELP", or "card edge low profile", referring to the small circuit board and the conductors on its edge. It had 80 contacts on each side of a circuit board (for a total of 160), spaced 0.050" apart, plus an identification notch between contacts 42 and 43.

Operation

COASt modules provided either 256K or 512K of direct-mapped cache, organized as 8192 or 16384 lines of 32 bytes. A 64-bit data bus allowed the cache line to be transferred in a 4-cycle burst. The modules contained 256K or 512K of fast pipeline burst SRAM, plus 8 or 11 bits of even faster static RAM per line to store the cache tags. (The module provides pins for 11 lines, but many motherboards and modules provided only 8.) Some variants (illustrated to the right) placed the tag RAM on the motherboard and only the main cache RAM was on the module. Consider the 256K module first. An 8-bit tag allows caching memory up to 256 times the cache size, or 64 MiB. An 11-bit tag supports up to 512 MiB. Each cache line also has a valid bit and a dirty bit, stored in the cache controller. (16 Kbits, or 2 Kbytes, total size.) A 512K module contains twice as many cache lines, and so requires one fewer tag bit to support the same cacheable memory size. The leftover tag bit is instead used to store the cache line dirty bit, and all 16 Kbits in the cache controller are used for valid bits.

References

COASt Modules
PCGuide, April 17, 2001.

PCGuide, April 17, 2001. *Renn, Brian

December 12, 1998. * Supports 11-bit tag. * Supports 8-bit tag only.

External links

{{Commons category, COAST Computer memory Cache (computing)