Pass user input from C to Assembler

Question

I am new to ARM Assembler. Using qemu emulator.

This solution didn't work for me.

I have this C file md1_main.c:

#include <stdio.h>
#include <stdlib.h>
#include "md1.h"

    int main (void)
    {
            int n;
            scanf("%d", &n);
            printf("Result = %u\n", asum(n));
            return 0;
    }

and .h file contains function prototype unsigned int asum(unsigned int n);

I am really confused, how to pass n into the assembler code.

Assembler code is md1.s:

.text
.align  2
.global asum
.type   asum, %function

asum:
    mov r1, #0
    mov r2, #1

loop:
    cmp r2, #3 ; instead of 3 there should be my input
    bgt end
    add r1, r1, r2
    add r2, r2, #1
    b loop

end:
    mov r0, r1
    bx lr

Just can't get it.

What target are you compiling this C code for? Platform - Linux, Windows or something else? — Ajay Brahmakshatriya, Mar 18 '17 at 15:47
Okay, So we will have to follow the Linux calling convention. Now the target? What assembly is that? X86? X64? ARM? — Ajay Brahmakshatriya, Mar 18 '17 at 15:51
Are you sure? Because those registers don't look like x86_64 registers. Neither do the instructions. — Ajay Brahmakshatriya, Mar 18 '17 at 15:53
@AjayBrahmakshatriya yes, this was the output on 'arch' command. we tried something simplified on lectures. — alalambda, Mar 18 '17 at 15:55
@alalambda Your assembly looks like ARM assembly. Have you tried looking up the calling convention for ARM? — fuz, Mar 18 '17 at 16:01

score 4 · Accepted Answer · answered Mar 18 '17 at 16:10

OP has mentioned the architecture as ARM 64. So I will answer according the calling convention.

The first 4 arguments are passed in r0, r1, r2, r3.

That is what the compiler will also do for you when compiling the C file. So you can expect your parameter n in the r0 register and you can directly use it.

I also see that your function returns an unsigned value. That will be returned in the r0 register.

See this, for a more detailed description of the calling convention.

Pass user input from C to Assembler

1 Answers1