microcoding a PC into a PDP-11 (was: RE: Classic Computers vs.

My 2 cents on this whole thing, A classic microcoded processor consists of vertical and horizontal unit addressing. Vertical addressing is the decoding of a bit field in a microinstruction to select one of N elements. Horizontal addressing consists of one microinstruction bit per device selected or code selections to more complicated devices, like the alu operation selection. The classical microprogrammed machine directly controls register <-> bus enables and alu source.destination, such things as incrementing a program counter and operating memory control units. The idea is that a collection of registers, alus and counters can be controlled / sequenced to get the data chip selects and r/w lines. The microprogrammed machine is just a bunch of registers and alu units with no order until microcode controls the symphony. Anything that decodes to multiple instructions internally IMHO, is not microcode. It can invoke other units ( memory controllers... ) which may be microprogrammed, implemented at state machines or even hardwired but sequence independently. So, That really cleared things up, bah! Decoding - a decoder, takes N bits in, has N^2 output lines, one enabled for any N input see 74154 4-16 decoder for really old example Jim Davis. Gary Oliver, you've done microcoded machines? whats your take? Tony Duell wrote: