rwessel (robertwessel.delete@this.yahoo.com) on April 8, 2017 9:33 pm wrote:
> wumpus (lost.delete@this.in-a.cave.net) on April 8, 2017 7:19 pm wrote:
> > You're missing a fundamental point. Microcode wasn't just
> > about "building a computer in a computer", it let
> > you build a computer *at* *all* much easier with it than
> > without it. But unlike later advances (such as verilog
> > and VHDL) it was explicitly tied to the clock. So during
> > CISC's hayday, if you were building a computer you
> > were almost certainly using microcode [I've heard there was quite a bit in the "hardwired" 6502].
>
>
> The 6502 was not microcoded.
>

That certainly is the received wisdom, but it clearly has a 21x130 "decoder ROM" that takes its input from not only the instruction being executed, but also for a "timing generation block" (which in turn depends on the output of the ROM). The whole thing looks suspiciously like microcode to me. The 130 control lines coming out certainly act the same as microcode, the main issue is if "timing generation block" is a FSM controlling the microcode (it certainly looks like it). Microcode or not, it doesn't interfere with the 6502's "simple instruction set computer" philosophy.

http://www.weihenstephan.org/~michaste/pagetable/6502/6502.jpg
http://visual6502.org/wiki/images/6/6a/Atari_6507_1B.jpg