Not microcode
per se, but I seem to remember that Rabbit
Semiconductor took the Z80 and eliminated quite a few instructions
for their MCUs.
Along those lines, the 6502 in the Nintendo Entertainment System
(apparently intentionally) broke the BCD functionality. The 6502
That reminds me of the Tektronix 4050 series.
The 4051 used a Motorola 6800.
The 4052 and later used a 16-bit microcoded processor based on the
AMD2900 series. The microcode emulated a 6800, but without BCD.
Instead, it added some memory management: separate instruction (F, I
think standing for Fetch) and data (D) spaces, for 128K bytes of
physical memory. One really nice feature was the FSWP instruction -
swap the fetch space. This set a flip-flop, but didn't actually toggle
the high bit of the fetch address until the next jump instruction (or
related - I think jumping to or returning from subroutines did also
implemented any pending FSWP.)
I think, but I am not sure, that the choice of opcodes that updated the
F bit was hard coded, not a microcode thing. On the other hand, the
machine was made more general purpose than you might expect by having a
large chunk of hard-coded instruction decode stored not in logic, but in
another ROM.
Philip.
