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A comment on Microcode: the place where hardware and software meet. Parent comment

Re:Microcode: the place where hardware and software meet by cnvogel on Sunday March 10, 2013 @12:59PM (#5352545)
I think that the 6502 decode ROM which matches opcode bits and sequence numbers to internal CPU functions is a very good example. It can be seen as either just a rather complex demultiplexer or, with keeping the sequencing nature in mind as a minimal, hard coded, computer program.

Does microcode have to execute like traditional software to be called microcode?

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Re:Microcode: the place where hardware and software meet by kens on Sunday March 10, 2013 @03:53PM (#5353198)
The 6502 is an interesting case. The decode ROM (PLA) gets instructions about 1/3 of the way decoded, and then there's a whole pile of random control logic [1], [2] that generates the actual control lines [3]. The output from the decode ROM consists of semi-meaningless things such as ADC/SBC in state T0, or JSR in T5 [4], and after a whole bunch of gates these get turned into the actual control functions such as S bus to A register. [3]

[1] image of the 6502 at http://www.visual6502.org/images/6502/index.html

[2] schematic http://www.visual6502.org/wiki/index.php?title=650X_Schemati...

[3] block diagram http://www.weihenstephan.org/~michaste/pagetable/6502/6502.j...

[4] details of ROM http://visual6502.org/wiki/index.php?title=6507_Decode_ROM

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Re:Microcode: the place where hardware and software meet by grannyg00se on Sunday March 10, 2013 @04:26PM (#5353317)
"Random control logic" Oxymoron?

Is the control logic really random in some way? That sounds impossible. Or at least, very highly inefficient.

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Re:Microcode: the place where hardware and software meet by ColinWright on Sunday March 10, 2013 @04:55PM (#5353422)
No, it's obviously not random. It is carefully designed, but there is no regularity to it. The expression "random logic" is used to describe the circuitry that is "ad hoc" rather than a matrix, like RAM, registers, or an ALU.

Google the expression to learn more.

* http://www.google.co.uk/search?q=random+logic

* http://en.wikipedia.org/wiki/Random_logic

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