Parameter passing convention for Function call from ARM Assembly to C

Question

I have a C code that calls a function defined in ARM Assembly. Two Parameters have to be passed.

If the function call looks like this:

functionName(a, b)

the registers x0 and x1 hold these values in which order? Is it x0 holds a and x1 holds b or the other way round?

http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf — BitBank, Jan 08 '16 at 06:40
thank you. Where does it talk about the left-right or right-left convention? — user2524261, Jan 08 '16 at 06:44
Possible duplicate of [ARM to C calling convention, registers to save](http://stackoverflow.com/questions/261419/arm-to-c-calling-convention-registers-to-save) — auselen, Jan 08 '16 at 07:24
This is one of the most frequent questions on this tag with many answers including. http://stackoverflow.com/questions/261419/arm-to-c-calling-convention-registers-to-save — auselen, Jan 08 '16 at 07:25

old_timer · Answer 1 · 2016-01-08T15:01:21.640

4

It took longer to ask the question than to just try it.

extern void bar ( unsigned int, unsigned int );

void foo ( void )
{
    bar(5,7);
}

compile then disassemble

traditional arm

00000000 <foo>:
   0:   e3a00005    mov r0, #5
   4:   e3a01007    mov r1, #7
   8:   eafffffe    b   0 <bar>

aarch64

0000000000000000 <foo>:
   0:   528000e1    mov w1, #0x7                    // #7
   4:   528000a0    mov w0, #0x5                    // #5
   8:   14000000    b   0 <bar>
   c:   d503201f    nop

msp430

00000000 <foo>:
   0:   3e 40 07 00     mov #7, r14 ;#0x0007
   4:   3f 40 05 00     mov #5, r15 ;#0x0005
   8:   b0 12 00 00     call    #0x0000 
   c:   30 41           ret

pdp-11

00000000 <_foo>:
   0:   1166            mov r5, -(sp)
   2:   1185            mov sp, r5
   4:   15e6 0007       mov $7, -(sp)
   8:   15e6 0005       mov $5, -(sp)
   c:   09f7 fff0       jsr pc, 0 <_foo>
  10:   65c6 0004       add $4, sp
  14:   1585            mov (sp)+, r5
  16:   0087            rts pc

edited Jan 08 '16 at 15:01

answered Jan 08 '16 at 06:46

old_timer

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Question looks like arm64? Nothing wrong with your answer though. – auselen Jan 08 '16 at 07:34
3

doesnt matter what processor, arm, aarch64, mips, x86, pdp-11. if you have a compiler for it and want to know the calling convention for two simple parameters. shouldnt take but around 40 to 50 seconds to find out. – old_timer Jan 08 '16 at 14:49
For future readers: https://godbolt.org/ has ARM, AArch64, MSP430, MIPS, and various other gcc and clang versions (including x86-64), and MSVC. – Peter Cordes Mar 10 '20 at 04:59

Parameter passing convention for Function call from ARM Assembly to C

1 Answers1