Re: [Nasm-users] calling 'C' functions

[Frank K. <fbk...@zy...>]

zak100 wrote:
> Hi,
> I am trying to call 'C" functions from my tiny kernel code. Can somebody plz
> help me with this?
> ; call c function
> extern [_K_main]
> ;--- test kernel.bin---
> 
> ;    push cs
> ;    push cs
> ;    pop ds
> ;    pop es
>     mov ax, 7E0H
>     mov ds,ax
>     mov es,ax
>     mov si, msg
> 
>     mov ah, 0Eh
>     mov bx, 7
> top:
>     lodsb
>     cmp al, 0
>     jz blackhole
>     int 10h
>     jmp short top
> 
> blackhole:
>     
>     mov cx,1000
>     Delay: Loop Delay
>     call _K_main; this is in main.c
>     hlt
>     jmp blackhole
> 
> msg db 'Welcome to the Kernel!', 0
> 
> times 0x200-($-$$) db 0
>         
> 
> I am getting following error mesg:
> D:\nasm prog>nasm -f bin -o boot.bin test_kernel2.asm
> test_kernel2.asm:5: error: label or instruction expected at start of line

Hi Zulfi,

Removing the brackets will at least give you correct syntax (extern 
_K_main), but binary format still doesn't know about external 
references. What format is your proposed _K_main in? OMF probably, for 
16-bit code(???).

Try assembling with "nasm -f obj test_kernel2.asm" this will produce 
"test_kernel2.obj", by default. If your linker won't handle long file 
names, you can shorten it with the "-o" switch, but your linker is 
probably going to be happier with ".obj" than ".bin" (maybe not?).

Then... your linker is probably going to want to produce an MZ 
executable by default. I don't think it's too hard to write a loader for 
MZ, but it might be easier to persuade your linker to produce a .com 
file. I believe ld (for 32-bit Linux - won't help you) can be coerced to 
produce a flat binary, explicitly specifying origins for the various 
sections (if you know how - I don't). If your linker will do that, 
you're in luck. Failing that... I dunno.

Make sure your compiler isn't producing any "dosisms" in your C code, of 
course.

The OS-development guys may have better advice on this.

Best,
Frank

P.S. Thanks for asking here, rather than nasm-devel, Zulfi! :)