Eric Smith wrote:
"Michael J. Mahon" <mjmahon@aol.com> writes:The 6502 was already a much less expensive processor than its competitors, because of a mask retouching technique MOS Technology developed that saved mask iterations.I saw that claim on Wikipedia. Is there any published reference?
I saw it in an interview published on the web, but I apparently didn't bookmark it. I'll let you know if I find it...
Stephen Harris wrote:With the 6502 essentially not having a microcode level, this optimisation is exposed to the programmer.The PLA is essentially equivalent to microcode. The difference is that it is addressed by the instruction register and a few state bits, rather than a micro-PC.
Exactly. I once worked on a machine that had an unusual "distributed PLA" approach: the opcode register and state counter were bussed along the backplane of the machine, and each opcode (or small family of similar ops) was decoded on the plugin card(s) that implemented it. It was the only machine I ever saw where individual ops could be removed from the machine by unplugging their cards! -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."