Cache memory using unique burst counter circuitry and asynchronous interleaved RAM banks for zero wait state operation

摘要

A cache memory system utilizing asynchronous/synchronous burst counter circuitry which lessens the need for expensive, high speed data SRAM to achieve zero wait-state operation. The burst counter circuitry takes advantage of the fact that a read address is present on the address bus approximately one-halfway through the initial bus cycle (T1) during a burst read. Unlike synchronous prior art burst counters, burst counter circuitry according to the invention is configured to forward the address to asynchronous address decoders as soon as it is present, rather than waiting for the next rising edge of the processor clock. For accesses to the first cache line, the timing budget therefore includes the first complete clock cycle of a burst read (T2) plus an extra half- clock cycle from T1. The extra time is utilized to retrieve data from the data SRAM core for provision to the processor data bus at the end of the bus cycle T2. Subsequent accesses are controlled by the burst counter in a synchronous fashion that corresponds to a processor specific burst ordering scheme. Due in part to the interleaved nature of the data SRAM, subsequent burst accesses are allotted almost 2 full clock cycles per data access. Thus, the shortest time in which the data SRAM must respond to an access request is the initial one and one-half clock cycles. Slower and less expensive data SRAMs can therefore be used to provide a cache memory capable of zero wait state operation.