[caf]

It's the program counter of the state that was interrupted by the single-step trap that's of interest, and that program counter is generally pushed onto the stack by the trap.

For example, this is how the trace exception on the M68k works - the program counter of the next instruction to be executed can be read off the stack by the exception handler. The 6502 doesn't have built-in software single-stepping but the same effect was sometimes achieved by tying a short timer to the NMI - and when the NMI is asserted, the interrupted program counter is pushed onto the stack.