> - A (automatic): the operand refers to the current stack frame. The offset is relative to the current base pointer.

Did you mean stack pointer ? base pointer gets set once where as stack pointer changes between every function call.

> Here is how I implemented a small instruction set for the VM of my experimental language, without using GPRs.

stack pointer is still necessery unless some memory region is predefined as stack pointer.

'A class' can be made more space efficient by dividing stack into 16 chunks and then instruction would only need 4bit to address.