OSDev.org

Hi!
I'm tryng to link ASM code to a C program for writing a somewhat kernel.

------[kernel.c start]------
extern void hi(void);
extern void quit(void);
                                                                                
int main(){
hi();
quit();
}
------[kernel.c stop]------

------[plug.asm start]------
[BITS 32]
                                                                                
GLOBAL hi
GLOBAL quit
                                                                                
SECTION .text
                                                                                
hi:     mov byte [es:0xb8f9c],'H'
        mov byte [es:0xb8f9e],'i'
        ret
                                                                                
quit:   mov esp,ebp
        pop ebp
        retf
------[plug.asm stop]------

i compile it like this:
gcc -ffreestanding -c -o kernel.o kernel.c
nasm -f aout -o plug.o plug.asm
ld -Ttext 0x100000 --oformat binary -o kernel.bin kernel.o plug.o

but it crashes when it trys to run on FreeDOS or when i try to boot it.
(important to say that i bassed the code from: http://www.osdev.org/developers/guide01/index.jsp)

and what i have discovered is that is must be from the linker (LD) because e dissasambled the program and the CALLs go 2 bytes back from what they should be:

00000000  55                push bp
00000001  89E5              mov bp,sp
00000003  83EC08            sub sp,byte +0x8
00000006  83E4F0            and sp,byte -0x10
00000009  B80000            mov ax,0x0
0000000C  0000              add [bx+si],al
0000000E  29C4              sub sp,ax
00000010  E80B00            call 0x1e
00000013  0000              add [bx+si],al
00000015  E81700            call 0x2f
00000018  0000              add [bx+si],al
0000001A  C9                leave
0000001B  C3                ret
0000001C  90                nop
0000001D  90                nop
0000001E  90                nop
0000001F  90                nop
00000020  26C6059C          mov byte [es:di],0x9c
00000024  8F                db 0x8F
00000025  0B00              or ax,[bx+si]
00000027  48                dec ax
00000028  26C6059E          mov byte [es:di],0x9e
0000002C  8F                db 0x8F
0000002D  0B00              or ax,[bx+si]
0000002F  6989C3EC          imul ax,bx,word 0xec89
00000033  5D                pop bp
00000034  CB                retf
00000035  90                nop
00000036  90                nop
00000037  90                nop

The first CALL goes to 0x1e when it should be 0x20 and the second call goes to 0x2f when it should (probably) go to 0x31.

Should the
[0000001A  C9                leave]
[0000001B  C3                ret]
really be there ?, when why so many NOPs ?
Is the problem from my linker (LD) ? or just me ? (probably the 2nd one)

Sorry about my bad english,
(really be there ?, when why so many NOPs ?)
-> really be there ?, and why so many NOPs ?

i had give more lots of errors for sure

damn my bad english...
"because e dissasambled" = "beacuse i dissasembled"
and just to add a few things if you dont understand what i'm tryng to say if i mispeled something. the program compiles fine, but when i try to run it on another machine width FreeDOS or add a booter (that havent giveme any problem with kernels writen intyrly in Assembly) it crashes or jumps out.

(in FreeDOS it gives a "Invalid Opcode at 0124 239B 0286 0113 FFFE 091E 239B 239B 0100 239B 091E 009C 20CD")

i have done some tests and the only thing that was comune and the only thing weird i have discovered is the CALLs being 2 bytes back from what it should be.

try using [global _hi]
_hi:

This could be wrong, I'm throwing this off the top of my head, plus i'm at work and cant verify this so I'll repost when I get home if its incorrect.

[code]
quit:
  mov esp, ebp
  pop ebp
  ret
[/code]

this will never work. Find the difference:
[code]
int quux()
{
  zioooom();
}
is
quux:
  push ebp
  mov ebp, esp

  call zioooom

  mov esp, ebp
  pop ebp
  ret
[/code] and

[code]
int quux()
{
  zioooom();
  quit();
}
is
quux:
  push ebp
  mov ebp, esp

  call zioooom
  call quit

  mov ebp, esp
  pop ebp
  ret
[/code]

The code in quit() should be inlined instead of being called.
Cheers,
Adrian.

That (32-bit) won't run unless you CPU is in protected mode. FreeDOS is a 16-bit environment, as is the BIOS that "bootstraps" the kernel.