Programmable logic array

This layout allows for many logic functions to be synthesized in the sum of products canonical forms.

PAL has programmable AND gates but fixed OR gates [clarification needed] In 1970, Texas Instruments developed a mask-programmable IC based on the IBM read-only associative memory or ROAM.

[1] PLA block diagram: The desired outputs for each combination of inputs could be programmed into a read-only memory, with the inputs being driven by the address bus and the outputs being read out as data.

However, that would require a separate memory location for every possible combination of inputs, including combinations that are never supposed to occur, and also duplicating data for "don't care" conditions (for example, logic like "if input A is 1, then, as far as output X is concerned, we don't care what input B is": in a ROM this would have to be written out twice, once for each possible value of B, and as more "don't care" inputs are added, the duplication grows exponentially); therefore, a programmable logic array can often implement a piece of logic using fewer transistors than the equivalent in read-only memory.

The earliest Commodore 64 home computers released in 1982 (into early 1983) initially used a programmed Signetics 82S100 PLA, but as the demand increased, MOS Technology / Commodore Semiconductor Group began producing a mask-programmed PLA, which bore part number 906114-01.