4 ms·

I've studied the 6502 in detail and I agree with you that the 6502 is not microcoded [*], for the reasons you mention. The 6502 has "random" control logic imple

by kens 4y ago

I've studied the 6502 in detail and I agree with you that the 6502 is not microcoded [*], for the reasons you mention. The 6502 has "random" control logic implemented with a PLA. (Since the PLA has multiple lines active at once, it's technically not a ROM.) You can't define a micro-instruction format for the 6502. There's a lot of detail on the 6502's PLA at this link: https://www.pagetable.com/?p=39 https://www.pagetable.com/?p=39 [*] For standard definitions of microcode.

colejohnson66 4y ago
Fair enough, but where would you draw the line between a PLA and horizontal microcode?
- kens 4y ago
  I would say that the difference is not the outputs, but how the information is stored. Microcode is "software" that is stored in a ROM, so you have one row of the ROM producing the outputs. You can point to one line of the microcode and say "here's the micro-instruction", and change one bit of it easily. (The definition of a ROM having one row active at a time while a PLA can have multiple rows active is from "The Architecture of Microprocessors".) You could use a PLA to produce exactly the same outputs from the same inputs, but it wouldn't be microcode. The PLA is "just" an optimization. But with a PLA, you can't change a micro-instruction without ripping out some of the logic and replacing it. You have a hardware implementation rather than a software implementation, which is the hallmark of microcode. In other words, changing a control signal value in the 8086's microcode is trivial, but changing a control signal in the 6502 is considerably harder. As a concrete example, the design of the Apollo Guidance Computer is clearly microcoded, with specific microinstructions. However, the implementation is gate logic that completely obscures the microcode design. (Not a ROM implemented with NOR gates, but highly-optimized logic.) So I consider the implementation of the AGC to not be microcode. (This is kind of splitting hairs, and I'm not really into subtle linguistic arguments.)

by kens 4y ago

colejohnson66 4y ago

Fair enough, but where would you draw the line between a PLA and horizontal microcode?

kens 4y ago
I would say that the difference is not the outputs, but how the information is stored. Microcode is "software" that is stored in a ROM, so you have one row of the ROM producing the outputs. You can point to one line of the microcode and say "here's the micro-instruction", and change one bit of it easily. (The definition of a ROM having one row active at a time while a PLA can have multiple rows active is from "The Architecture of Microprocessors".) You could use a PLA to produce exactly the same outputs from the same inputs, but it wouldn't be microcode. The PLA is "just" an optimization. But with a PLA, you can't change a micro-instruction without ripping out some of the logic and replacing it. You have a hardware implementation rather than a software implementation, which is the hallmark of microcode. In other words, changing a control signal value in the 8086's microcode is trivial, but changing a control signal in the 6502 is considerably harder. As a concrete example, the design of the Apollo Guidance Computer is clearly microcoded, with specific microinstructions. However, the implementation is gate logic that completely obscures the microcode design. (Not a ROM implemented with NOR gates, but highly-optimized logic.) So I consider the implementation of the AGC to not be microcode. (This is kind of splitting hairs, and I'm not really into subtle linguistic arguments.)