Bruce Tomlin wrote:
In article <d4CdnR_y__CUmfXZnZ2dnUVZ_s-dnZ2d@comcast.com>, "Michael J. Mahon" <mjmahon@aol.com> wrote:True, but the decoding in the 6502 is handled by a kind of PLA, so it would likely not be very expensive (in real estate) to trap or NOP the invalid combinations.Nope, it's handled by good old random logic. A PLA is designed to be a programmable generic replacement for random logic, and is way too inefficient for high-volume VLSI.
My mistake--from this chip photo, I incorrectly identified either the registers or the ALU as a PLA: http://micro.magnet.fsu.edu/chipshots/mos/6502large.html Still, given the relatively ordered distribution of 6502 undefined ops, I think their detection would have been relatively simple. Of course, this is moot for at least two reasons: it wasn't done, and most designers of the time wouldn't have done it anyway. -michael Music synthesis for 8-bit Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it is seriously underused."