Author here. Yeah, I have a tendency to go into pretty big deep dives when I find stuff like this. It's so rewarding at the end, even if it does take a lot of time!
Finding a previously unknown undocumented instruction at this late date in a line of processors as prevalent and historically significant as the 68k is surprising. Congrats on your achievement! If someone does dive into fully characterizing the undocumented instruction so it can be properly supported in emulators (as you suggested), please post about it on HN. I suspect, like many undocumented CPU instructions, it was probably to help the original designers test or verify something during development but it would be interesting to know.
While obviously a subjective judgement, a lot of people who hand coded assembler on 68k processors regard the ISA as especially elegant, powerful and fun to develop for. In many ways I think of it as peak CISC, thanks to its orthogonal instruction set and wildly flexible addressing modes. And of course the platforms which used it are legendary, from consumer (Mac/Lisa, Amiga, Atari ST, Sinclair QL) to workstations (SUN, Apollo, Quantel) to gaming (Sega Genesis, Neo Geo, Capcom, Atari, Namco, Sega, Taito, Konami) to embedded (automation, print/network controllers, synthesizers, appliances). I'm certainly biased but to this day the 68k (and its 8-bit little brother the 6809) are the only CPUs I still enjoy writing assembler on.
I suspect, like many undocumented CPU instructions, it was probably to help the original designers test or verify something during development but it would be interesting to know.
Or simply be an emergent but unintended behaviour of the implementation, as is the case for most of the undocumented 6502, Z80, and x86 instructions I know of.
> a lot of people who hand coded assembler on 68k processors regard the ISA as especially elegant, powerful and fun to develop for. In many ways I think of it as peak CISC, thanks to its orthogonal instruction set and wildly flexible addressing modes.
I definitely agree...but I'd say Motorola really got carried away with those wildly flexible addressing modes. Which lead them into implementation, power draw, and gate-delay hells by the late 1980's and the 68040. The future was ever-rising transistor counts and clock speeds - and their 68k architecture just couldn't go there.
mrandish|1 year ago
While obviously a subjective judgement, a lot of people who hand coded assembler on 68k processors regard the ISA as especially elegant, powerful and fun to develop for. In many ways I think of it as peak CISC, thanks to its orthogonal instruction set and wildly flexible addressing modes. And of course the platforms which used it are legendary, from consumer (Mac/Lisa, Amiga, Atari ST, Sinclair QL) to workstations (SUN, Apollo, Quantel) to gaming (Sega Genesis, Neo Geo, Capcom, Atari, Namco, Sega, Taito, Konami) to embedded (automation, print/network controllers, synthesizers, appliances). I'm certainly biased but to this day the 68k (and its 8-bit little brother the 6809) are the only CPUs I still enjoy writing assembler on.
userbinator|1 year ago
Or simply be an emergent but unintended behaviour of the implementation, as is the case for most of the undocumented 6502, Z80, and x86 instructions I know of.
bell-cot|1 year ago
I definitely agree...but I'd say Motorola really got carried away with those wildly flexible addressing modes. Which lead them into implementation, power draw, and gate-delay hells by the late 1980's and the 68040. The future was ever-rising transistor counts and clock speeds - and their 68k architecture just couldn't go there.
dougg3|1 year ago