Onspec Electronic, Inc.

Tetra Assoc. Inc.

Nick Antonopoulos

Larry Lawson Jones

Arockiyaswamy Venkidu

A parity generating circuit that can replace the parity bit DRAM on a 9-bit SIMM. The parity generating circuit includes a parity generating tree which outputs the resulting even parity from the 8 data bits on a read. A 9th data input from another parity generator on the system mother board is compared to the generator tree output when DRAM is written to. If a mismatch occurs, the type of parity generated by the generator tree is opposite to the type of parity that the mother board generates, and the parity tree output must be inverted on subsequent reads. A latch is provided to store the compare result, which also indicates the type of parity required, even or odd, on the particular system the SIMM is installed on. The latch is loaded when the DRAM is written to. The state of the latch is used to output the correct type of parity on a read from DRAM by inverting the output of the parity generating circuit if needed. This eliminates the cost of the 9th DRAM chip on the SIMM, yet allows for parity checking of the data paths on the system board traces and any older 9-bit memory present in the system.

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Nick Antonopoulos