Digital Equipment Corporation

Michael Leary

William Ralph Wheeler

Hamid Partovi

Steven W Butler

Rajesh Khanna

Elizabeth M Cooper

Michael A Case

A semiconductor memory device according to the invention includes a main memory array comprising a plurality of memory sub-arrays. Each of the memory sub-arrays comprises a plurality of columns and at least one redundant column. Each column of the memory sub-array also includes multiplexing means, coupled to the input and output path of the respective column and an input and output path of a neighboring column. In addition, the redundant column is coupled to the input and output path of a neighboring column. In the event that one of the columns of the memory sub-array is defective, the multiplexing means of each of the columns between the defective column and the redundant column acts to couple the input and output paths of that column to the input and output paths of the neighboring column. With such an arrangement, the defective column is bypassed and a memory device capable of operating without defects is provided.

Method and apparatus for providing high-speed column redundancy

A tag comparator and bank selector for a set-associative cache in a computer system operates in a minimum time so that a cache hit or miss signal is generated early in a memory cycle. The data memory of the cache has two (or more) banks, with a tag store for each bank, and the two banks are accessed separately and in parallel using the index (low order address bits) while the tag translation is in progress. Two bit-by-bit tag compares are performed, one for each tag store, producing two multibit match indications, one bit for each tag bit in each tag store. These two match indications are applied to two separate dynamic NOR gates, and the two outputs applied to a logic circuit to detect a hit and generate a bank-select output. There are four possible outcomes from the compare operation: both banks miss, left bank hits, right bank hits, and both banks hit. The later condition indicates a possible ambiguity, and neither data item should be used, so a miss is signalled. The comparator is in large part self-timed using a flow-through design, as distinguished from being enabled on clock edges. Delay elements in the bank select logic allow the banks to be timed against each other, and current limiters are employed to equalize the timing of miss signals, regardless of the number of match lines switching high (which is data dependent). An address producing 19-of-20 match bits will result in a NOR gate output of about the same timing as an address producing no match bits, even though the former will turn on only one transistor to discharge the precharged output node of the NOR gate, whereas the later will turn on twenty paths for discharge. Although a two-way set associative cache is shown herein as an example embodiment, one of the features of the invention is that higher levels of associativity, e.g., four-way and eight-way, are equally well accommodated.

Fast tag compare and bank select in set associative cache

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Jefferson, MA, United States of America

Michael Leary