> You said general purpose, which X, Y, and SP most certainly are not. You can't do much math ops with X and Y; SP is even more restricted. In that sense I wou

> You said general purpose, which X, Y, and SP most certainly are not. You can't do much math ops with X and Y; SP is even more restricted. In that sense I wouldn't say that the accumulator in 6502 is a general purpose register in the first place, since it's just as useless for addressing as the X and Y registers are for arithmetic and logic operations. Each register in the 6502 has a distinct (but often overlapping) set of available operations, and that you can do arithmetic with one of them doesn't mean that it's a general purpose register.