ENTRY: Another pseudo-instruction from CALL corner cases

A project log for YGREC8

A byte-wide stripped-down version of the YGREC16 architecture

yann-guidon-ygdesYann Guidon / YGDES 10/12/2023 at 05:120 Comments

What if you CALL using PC as SRI and another register as SND ?

SND gets PC+1 but PC will get PC, which ... yeah it wil get stuck. So we can detect this special case.

The result is that the next instruction's address will go into any other register, which is useful when starting a loop for example. So the corresponding opcode would be ENTER or ENTRY, useful when you don't want to compute addresses and your loop body has more than 8 instructions.

set R1 42 ; some loop counter

entry R2 ; sets R2 to PC+1

 ; do something long and useful here

  add -1 R1
  set R2 PC NZ
; end of loop

This looks nicer than

set $+1 R2

Now this increases the pressure on the registers. You could put the address on the stack top for example, but the loop counter would be better suited for this location, as it can be implicitly saved when a call occurs.

The address can be retrieved at the last moment:

set R1 42 ; some loop counter

 ; do something long and stupid here

  set loopentry R2
  add -1 R1
  set R2 PC NZ
; end of loop

The total code length is the same but the loop is slower by one instruction. It's not the highest code density ever, but it works.