What exactly is used for the instruction decoding in the 650

[LBSC]

2. I have looked at 6502 schematics and it looks like a one-cold, but everyone else calls it a PLA or a decode ROM or a decode ROM PLA or whatever so, what exactly decodes the instructions?

[BigEd]

Logic gates decode the instructions! Seriously: the so-called PLA or ROM is just a nice organised way to build a lot of NOR gates. There is logic upstream of the PLA and there is logic downstream of it. Among the inputs are signals representing the progress of the cycles which make up each instruction. These signals are nearly a one-cold encoding, but not precisely.

There are various resources online, here's a very small selection:
http://www.visual6502.org/wiki/index.ph ... te_Machine
http://www.visual6502.org/wiki/index.ph ... ing_States
viewtopic.php?f=4&t=4186
http://visual6502.org/wiki/index.php?ti ... ed_Opcodes

[LBSC]

BigEd wrote:

Logic gates decode the instructions! Seriously: the so-called PLA or ROM is just a nice organised way to build a lot of NOR gates. There is logic upstream of the PLA and there is logic downstream of it. Among the inputs are signals representing the progress of the cycles which make up each instruction. These signals are nearly a one-cold encoding, but not precisely.

There are various resources online, here's a very small selection:
http://www.visual6502.org/wiki/index.ph ... te_Machine
http://www.visual6502.org/wiki/index.ph ... ing_States
viewtopic.php?f=4&t=4186
http://visual6502.org/wiki/index.php?ti ... ed_Opcodes

Thank you!