Turbo C and Assembly Won't Work!

[pcmattman]

Hello all!

With Turbo C I can compile and link my kernel into a 16-bit exe (which my OS supports). However, the system hangs on the entry point (NASM code):

Code: Select all

extern _main

section _TEXT code

..start:
	mov ax,0x9000               ; put stack at 0x98000
	mov ss,ax
	mov sp,0x8000

	mov ax,seg _main	; segment of _main
	mov es,ax		; => es
	call [es:_main]		; go there now

	hangloop:
	pause
	jmp $

The program hangs on the call to es:_main, but without this contol just falls through to the hang loop.

Also, any ASM function I try to call from within my code don't execute at all - they are pretty much just skipped over without executing instructions.

[Combuster]

i doubt the call [es:_main] is what you want:

this is assembled as:
make a call to the address in memory location main, using prefix ES

and executed as
target = memory(es * 16 + main)
push ip
mov ip, target

the architecture has no direct opcode for call es:_main (without [ ] ), so either:

- You use a direct far call and either force the same segment (or fix the instruction using self-modyfing code, yuck)
- You create a far pointer in memory and use call far [structure]
- You push everything into place and use RETF

[bubach]

Why would you need a entry point for a 16-bit EXE program? You'r probably wrecking the EXE header and/or confusing the EXE bootloader.

[pcmattman]

I've since been able to boot into the kernel successfully - and use my assembly functions - but the new problem is how to get a byte argument from Turbo C? I look at the disassembly and there is nothing at all pushed onto the stack before my assembly functions run. Why is this happening?

[AJ]

Hi,

Is it possible to see the code for that part - I don't see any pushes in your code.

If you mean that you would like to pass an argument to '_main', you will need to manually push your arguments before caling it. I use GCC which pushes from right to left, but I guess all C compilers are the same in this respect

If you are expecting arguments from a previous boot loader, you need to retrieve them *before* you move the stack pointer!

Cheers,
Adam

[Solar]

I use GCC which pushes from right to left, but I guess all C compilers are the same in this respect

They need not be. Obviously there is the convention to use whatever the system libraries expect, but since we're talking about OS development, calling conventions could be left-to-right, unpadded, register passing or whatever...

[pcmattman]

Sorry, the code above is not the code I was referencing! I've looked at other code to be linked with 'tlink' and they all use the calling convention I would normally use. Here's my assembly:

Code: Select all

global _kputc

section _TEXT

_kputc:

	push bp
	mov bp,sp

	mov al,[ss:bp+4]		; first argument
	mov ah,0eh
	mov bx,07h
	int 10h
	
	pop bp

	xor ax,ax				; return 0
	retn

Called like so:

Code: Select all

extern kputc( char );
kputc( 'D' );

But I've solved it - the argument to kputc can't be 'char' - it must be 'int'.

[pcmattman]

New problem: functions can only be called once before the linker doesn't work properly anymore!! It says 'fixup overflow' and none of my functions work anymore. Any ideas?