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Forums > C64 Coding > JAM instruction
2013-08-18 02:58
Six

Registered: Apr 2002
Posts: 293
JAM instruction

Does anyone have a description of what happens internally in the CPU to make the JAM instructions lock up? I'm curious if anyone has looked into exactly WHY they behave like they do and thought it might be an interesting topic to research, but not sure where to start.
2013-08-18 04:03
Mixer

Registered: Apr 2008
Posts: 452
Could this be of some use to your research?

http://visual6502.org/JSSim/index.html
2013-08-18 07:21
Ninja

Registered: Jan 2002
Posts: 411
IIRC some C= Hacking magazine had an explanation. They are all on the web...
2013-08-18 08:35
MagerValp

Registered: Dec 2001
Posts: 1078
They figured it out with Visual6502 and Michael Steil has a good explanation on his blog:

http://www.pagetable.com/?p=39

TL;DR: the instruction decoder is a state machine, and some illegals get it to a state where it never finishes the current instruction.
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