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/* The Decode ROM (PLA) */
Since the PLA allows partial decoding, one entry can fire on different instructions. For example, all instructions loading the second byte as immediate share one single PLA entry (microcode line). In comparison with textbook microcode engines, this is equivalent to a kind of compressed microcode. [https://retrocomputing.stackexchange.com/questions/6656/how-was-microcode-implemented-in-retro-processors Source]
A PLA is much more efficient than a ROM because it can take advantage of "don't care" entries. Specifically, the 6502 has a 130x21 PLA = 2730 entries. A ROM would need 11 inputs (instruction + timing) and 130 outputs, so 130*2^11 = 266240 entries plus the decoding logic. (You could reduce the ROM size by using tricks such as multiple levels of ROM or partially decoded ROMs.)
Instruction sets are usually defined so groups of bits have meanings and can be decoded separately. This makes the PLA a good fit.
For a specific example, the 6502 PLA decodes instructions matching 100XX1XX to the control line STY (ignoring the timing bits for simplicity). This takes 1 row in the PLA, but it would take 16 entries in a ROM. [https://news.ycombinator.com/item?id=13128074 Source]
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