Question

I am trying to understand control flow in 6502 assembly.

Say I have the following code:

    ControlFlow:
      lda mem
      cmp #1
      bne .sub_one

      cmp #2
      bne .sub_two

      .sub_one:
        ; sub routine one goes here
        jmp .done ; <-------------- without this jmp, .sub_two will execute

      .sub_two:
       ; sub routine two goes here

      .done:

      rts

Personally, I would love a switch statement or some other control flow structure.
The JMP above concerns me as well. It seems like there is a better way to handle
multiple cases without this type of spaghetti code.

Answer 1

There is no better way actually but there might be an improvement like calling the FlowControl as a subroutine and returning with RTS.

This is the main flow.

  jsr ControlFlow
  ; main routine continues here

Here is the sub routine.

ControlFlow:
  lda mem
  cmp #1
  bne .sub_one
  cmp #2
  bne .sub_two
  cmp #3
  bne .sub_three
    ; case else here
  rts

  .sub_one:
    ; sub routine one goes here
  rts

  .sub_two:
   ; sub routine two goes here
  rts

  .sub_three:
   ; sub routine three goes here
  rts

if sub routines are too long, you need to use JMPs as mentioned before.

.jump_to_sub_one
  jmp .sub_one
.jump_to_sub_two
  jmp .sub_two
.jump_to_sub_three
  jmp .sub_three

ControlFlow:
  lda mem
  cmp #1
  bne .jump_to_sub_one
  cmp #2
  bne .jump_to_sub_two
  cmp #3
  bne .jump_to_sub_three
    ; case else here
  rts

  .sub_one:
    ; sub routine one goes here
  rts

  .sub_two:
   ; sub routine two goes here
  rts

  .sub_three:
   ; sub routine three goes here
  rts

That’s how it is done and unfortunately, there is no better way. This applies to many assembly languages if not all.