6502 increment PC control
(first cut - would like to include a circuit diagram)
m (Protected "6502 increment PC control" ([edit=autoconfirmed] (indefinite) [move=autoconfirmed] (indefinite)))
Revision as of 17:07, 19 February 2011
The 6502 Program Counter has a dedicated incrementer which has to be able to increment across 16 bits in a single cycle.
It always increments during an instruction fetch cycle, to fetch a possible operand, but in general the decision whether or not to increment is a complex one.
As it happens, some of the logic implementing that decision is absent from Balazs' schematic, probably because of a bad patch in the die photograph. It also happens to use some unusual NMOS logic techniques.
Here's the layout, as rendered by visual6502's JSSim:
The highlighted signal bottom centre is the negative-sense signal "dpc36_#IPC", and the highlighted signal near the middle is "short-circuit-branch-add"