Editing Programmable logic array (section)

== Advantages over read-only memory ==
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. This is particularly valuable when it is part of a processing chip where transistors are scarce (for example, the original [[MOS Technology 6502|6502]] chip contained a PLA to direct various operations of the processor<ref>[http://www.pagetable.com/?p=39 How MOS 6502 Illegal Opcodes really work]</ref>).