The basic reason for the stack pointer being in internal ram is due to access time and availability to micromachine registers for certain opcodes.

When you allow the stack to go external, you pay a big price in speed and the micromachine circuitry required on the processor chip.

The 8051 is a great "bit-banger" - Learn its forte.

If you need a Z-80, which is a lot of fun, you're on the wrong forum.

-duh