6502 increment PC control

From VisualChips

Revision as of 20:03, 20 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"

Here's a diagram of the final few logic stages, which react to page-crossing branches, taken branches, single-byte instructions, interrupt handing (D1x1) and stalled cycles (use of RDY):

Note that the exclusive OR is implemented as a modified multiplexor which includes the subsequent AND function. The modification ensures that the AND's pulldown doesn't affect the signal notALUcout (node 206) which is used elsewhere, by pulling it down through the multiplexor's pass transistors.

Here's a transistor level view covering most of the same circuit: