'else' control structure in assembly.

[pops]

What is the most efficient way to write the 'else' program structure in assembly?

The best I've come up with so far is:

Code: Select all

lda #$10    ; if ScrollY <= $10
cmp ScrollY
bcs smallScroll
    ; do code for 'if' = true
jmp endif
smallScroll:
    ; do code for 'if' = false
endif:

[Movax12]

That is basically it. Remember you can use a branch instruction rather than a JMP if you know a flag will always be in a known state. Also, if you change the flow of code in your true block, you don't need to have the else. Example:

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lda #$10    ; if ScrollY <= $10
cmp ScrollY
bcs smallScroll
    ; do code for 'if' = true
    rts      ; leave this subroutine
jmp endif  ; not needed 
smallScroll:
    ; do code for 'if' = false
endif:

; rewrite:

lda #$10    ; if ScrollY <= $10
cmp ScrollY
bcs smallScroll
    ; do code for 'if' = true
    rts      ; leave this subroutine
smallScroll:

; else, continue:

Also your code may be wrong, that code will effectively be the same as: If #$10 < Scrolly , or If Scrolly > #$10

[doppelganger]

Actually it would be if #$10 >= ScrollY. Remember the carry flag is set after a subtraction unless the second number (which in this case is ScrollY) is greater, then it will be clear for borrow.

[Movax12]

I was basing that last statement on OP's comments, especially the 'if'=true part.

[pops]

Actually it would be if #$10 >= ScrollY. Remember the carry flag is set after a subtraction unless the second number (which in this case is ScrollY) is greater, then it will be clear for borrow.

My bad. I'm still getting used to assembly and the 6502.

[blargg]

Also, if you change the flow of code in your true block, you don't need to have the else.

Your example has the else, it's just that the if and else blocks both end in an rts.

Often the else can be eliminated by doing the work of it before the if, then replacing it if the condition is true. e.g.

Code: Select all

        ldx #10 ; do this work all the time
        lda pos
        cmp #5
        bne skip
        ldx #20 ; overwrite
skip:

[Movax12]

Yes. I find that method interesting. One could code both ldx instructions inside an if-else-endif structure, but it's more efficient to load a default that may be 'wrong'.

[thefox]

Actually it would be if #$10 >= ScrollY. Remember the carry flag is set after a subtraction unless the second number (which in this case is ScrollY) is greater, then it will be clear for borrow.

My bad. I'm still getting used to assembly and the 6502.

You can use macros to create aliases for opcodes with easier to remember names, such as:

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.macro jl_unsigned dest ; jump if less
    bcc dest
.endmacro

.macro jge_unsigned dest ; jump if greater or equal
    bcs dest
.endmacro

[tepples]

In 65816 mode, the assembler automatically includes macros that define blt (branch less than) and bge (branch greater or equal) as synonyms for bcc and bcs.

[lidnariq]

If you're using ca65, use .macpack generic, which provides add, sub, bge, blt, bgt, and ble. I like including .macpack longbranch, too: it provides jeq, jne, jmi, jpl, jcs, jcc, jvs, and jvc.

[thefox]

I like including .macpack longbranch, too: it provides jeq, jne, jmi, jpl, jcs, jcc, jvs, and jvc.

FYI, in at least some version, these macros were buggy (they always generated a long jump).

Looks like bge/blt/bgt/ble macros of the "generic" macpack are only documented in the snapshot documentation, that's probably why I missed them.

[Dwedit]

What is the most efficient way to write the 'else' program structure in assembly?

The best I've come up with so far is:

Code: Select all

lda #$10    ; if ScrollY <= $10
cmp ScrollY
bcs smallScroll
    ; do code for 'if' = true
jmp endif
smallScroll:
    ; do code for 'if' = false
endif:

This is literally what a compiler does.