6502 took a differnt approach than had been taken by Intel, Motorola, or Zilog. It's well to note that Zilog was started by people who abandoned Intel after the decision was made to design the 8085 rather than something more powerful, which was how they saw the Z80, and the 650x was designed by some guys who left Motorola because Motorola was not following their chosen path toward the more efficient architecture.

The 650x, starting with the 6501, used two small (8-bit) index registers rather than the 16-bit index used by Motorola. They didn't use the "concatenated" accumulator, and the essentially, through the use of special addressing modes, blurred the distinction between external memory and internal registers, at least for the "zero-page." The MOS-Technology guys figured out that a small (8-bit) stack pointer was quite adequate for many tasks. The result was a very small chip, which made for a very small price, and that bought them lots of market share.

I was a 6502-promoter back in the day. Its speed came not from lots of features and elaborate instructions, but from its cleverly implemented instruction set. It did not make the best of use of the available memory bandwidth, and, with extensive testing, I was able to establish that the Z80 actually provided better performance if one based that performance metric on the memory bandwidth demand.

I can go into much more detail, but I don't think it's appropriate, as it's just history and it's not 805x-related.

RE