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Asked: 2026-05-27T08:45:11+00:00

I defined a function called multiply and it takes arguments R0 and R1 and

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I defined a function called multiply and it takes arguments R0 and R1 and saves the result in R3. This sucks because whenever I call multiply I have to put the operands in R0 and R1 and move what was in R0, R1 and R3 somewhere else. Is there a way to make the function take the form multiply R4, R5, R6 similar to how add R1, R2, R3

ldr R0, =snakes
ldr R1, [R0], #4
mov R2, #15
mov R3, #6

If I want to multiply R2 and R3 this would require extra work. I call multiply often and am wondering if there’s a better way?

Multiply:
    stmfd   sp!,{r0-r2, lr}
    mov R2, #1
    mov R3, #0 
    repeat:
        add R3, R1, R3  
        add R2, R2, #1
        cmp R2, R0
        ble repeat
    mov R2, R3
    LDMFD   sp!,{r0-r1, pc}
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1 Answer

  1. Editorial Team

    You can use macros:

    .macro multiply A B C
{body}
.endm

    Values of parameters in the body are avaliable by \A, \B, \C

    there is a little mistake in your code:

    Multiply:
    stmfd   sp!,{r0-r2, lr}
    mov R2, #1
    mov R3, #0 
    repeat:
        add R3, R1, R3  
        add R2, R2, #1
        cmp R2, R0
        ble repeat
    mov R2, R3
    LDMFD   sp!,{r0-r1, pc}

    When R0 is 0, the result would be R1 instead of 0. So you have to check it before it enters the loop. The correct code will be:

    stmfd   sp!,{r0-r2, lr}
mov R2, #0
mov R3, #0 
repeat:
    cmp R2, R0
    be exit_loop
    add R3, R1, R3  
    add R2, R2, #1
    b repeat
exit_loop:
mov R2, R3
LDMFD   sp!,{r0-r1, pc}

    But this is not the most optimal method because there are 5 instructions in loop’s body instead of 4, and 2 branches instead of 1, so it will be difficult to predict them both. That’s why we going to do the following:

    stmfd   sp!,{r0-r2, lr}
mov R2, #-1
rsb R3, R1, #0 
repeat:
    add R3, R1, R3  
    add R2, R2, #1
    cmp R2, R0
    bne repeat
mov R2, R3
LDMFD   sp!,{r0-r1, pc}

    And finally, with macros it will look like this:

    .macro multiply C B A
stmfd   sp!,{\A, \B, \C, r2, lr}
mov R2, #-1
rsb \C, \B, #0 
repeat:
    add \C, \B, \C  
    add R2, R2, #1
    cmp R2, \A
    bne repeat
LDMFD   sp!,{\A, \B, \C, r2, pc}
.endm

multiply R3 R2 R1 will save the result into R3

    But I don’t know why you want to write your own multiply macro, I hope you know, that there is a set of multiply instructions…

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