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

Basically, instruction operands are divided into 4 classes:

- G (global): the operand refers to the global memory space and contains an offset and size.

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

- I (immediate)

- T (temporary): this is where intermediate results from computations are stored. For example, the result of `a + b` from `(a + b) * c` would be stored in the T memory space. Upon the end of a statement in the HLL, all values in the temporary space are discarded.

Reference: https://github.com/bbu/quaint-lang/blob/master/src/exec.c