One reason to consider them equivalent to registers is that the cycle time for zero page and register-based instructions is the same (2-3 cycles). The full 16-

One reason to consider them equivalent to registers is that the cycle time for zero page and register-based instructions is the same (2-3 cycles). The full 16-bit instructions ("absolute") take at least 1 extra cycle and often 2 if crossing a page. Of course there is the Atari 2600 where you only have 128 of those bytes with RAM (80-FF) and the stack is set to page 0 instead of 1 (not that you'll use the stack a lot in an Atari 2600 program).