Interesting question. Spec says 'a is always handled by the processor before b, and is the lower six bits.' For Non-basic opcodes, 'a' is actually the opcode,

Interesting question. Spec says 'a is always handled by the processor before b, and is the lower six bits.' For Non-basic opcodes, 'a' is actually the opcode, and b is the argument. This would imply JSR is evaluated before POP. What we want JSR POP to mean, of course, is 'jump to the last item on the stack, then push PC+1 to stack'. So I would guess that's how it actually works, and the spec needs a revision.