Calling extern function in asm

[goku420]

In an attempt to wrap my mind around how I could execute asm instructions from my kernel, I did a test. Here's my code:

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extern puts

global test

test:
	mov ebx,msg
	mov eax,puts
	call eax
	ret
	
SECTION .data
	msg     db  'Hello, world!',0xa  

Then I put the call to test in my main function. However when I run it, I get this:



The 'default' and 'initializing' strings are supposed to be there.

I'm assuming that the 'mov ebx,msg' isn't really doing anything, but I don't know how I would pass the argument to the caller. What should I do?

[tiger717]

1. Just call puts by

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call puts

- no need to change any registers.

2. In x86 32bit programming, the cdecl calling convention (or however you want to call it) is used which means
passing arguments over the stack (last one first = right to left), you (the caller) are responsible for saving eax, ecx and edx on the stack and you get your return value over eax. You also have to clean the stack, i.e. remove the arguments you have pushed on it.

Example:

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; int some_function(char* a, int b);
extern some_function

test:
    mov eax, some_important_value   ; save some important value in eax
    push eax     ; secure eax for ourselves - no need if we don't care about eax
    push second_argument    ; you can actually pass a variable, no need to lea first
    push first_argument
    call some_function    ; call the function
    mov edx, eax   ; somehow use the return value if you need it
    add esp, 8  ; remove our arguments from the stack
    pop eax   ; and get eax back
    ret

PS: Sorry for the badly aligned comments.

Note that this only applies to x86 (32 bits). In AMD64 (or however you want to call it), the AMD64 ABI is used which is really different - if you want to support Long Mode one day.

[Antti]

@tiger717: If using cdecl, callee does not clean the stack.

[goku420]

Thanks.

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test:
	push msg
	call puts
	add esp, 4 ; clean stack, 4 for 32 bit
	ret

Seems to do the trick.

[tiger717]

@Antii and remyabel: Err, yes. Sorry, haven't really coded anything using cdecl. Former message edited.