6502 increment PC control

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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"

Here's a diagram of the final few logic stages:


Note that the pulldown on the short-circuit-branch-add signal should be understood to override the output of the exclusive or. The exclusive OR is implemented as a modified multiplexor - the modification ensures that the pulldown doesn't affect the signal notALUcout (node 206) which is used elsewhere, by pulling it down through the multiplexor's pass transitors.

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