29 Jul
2014
29 Jul
'14
1:46 p.m.
On Jul 29, 2014, at 4:15 PM, Chuck Guzis <cclist at sydex.com> wrote:
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
used a PLA for instruction decoding, but interestingly, the BCD
breaking wasn't done there; there were some linkages between the
main ALU and the BCD adjustment circuitry that were broken. I
don't know exactly why they would have done such a thing, other
than to restrict the target market of the eventual device. The
6502 portion of the NES' 2A03 CPU/sound generator/etc. chip is
otherwise a copy/paste job of the standalone version; perhaps
MOS offered a cheaper license for chips without BCD?
- Dave