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From:	"Rod Pemberton" <do_not_have AT nohavenot DOT cmm>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: gcc as a linker....
Date:	Mon, 14 Jan 2008 06:58:02 -0500
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"linus" <linus AT yahoo DOT com> wrote in message
news:478b3603$0$36444$4fafbaef AT reader5 DOT news DOT tin DOT it...
> I 've compiled with nasm the code below and linked with your gcc...
> What does it mean these errors ? (the environment is Win XP ).
> Sorry, I am a beginner...gcc can be used as a linker ?
> Thanks a lot.
>
> C:\> gcc -oEXE hello.obj

gcc -o hello.exe hello.obj

I'm assuming that the code below is hello.asm and you've assembled with NASM
like so:

nasm -f coff -o hello.obj hello.asm

> c:/djgpp/lib/crt0.o:crt0.s:(.data+0xc2): undefined reference to `_main'
> c:/djgpp/lib/libc.a(crt1.o):crt1.c:(.text+0x404): undefined reference to
> `_main'
>
> collect2: ld returned 1 exit status

This is because there is no "_main" label in the code below.

> SECTION .data ; data section
> msg: db "Hello World",10 ; the string to print, 10=cr
> len: equ $-msg ; "$" means "here"
> ; len is a value, not an address
>
> SECTION .text ; code section
>        ;  global main ; make label available to linker
> ;main: ; standard  gcc  entry point
>

You've commented out the two lines (semi-colon at start of lines) that
define 'main', but for assembly you want '_main' like so:

global _main ; make label available to linker
_main: ; standard  gcc  entry point

> mov edx,len ; arg3, length of string to print
> mov ecx,msg ; arg2, pointer to string
> mov ebx,1 ; arg1, where to write, screen
> mov eax,4 ; write sysout command to int 80 hex
> int 0x80 ; interrupt 80 hex, call kernel
>

"int 0x80" is a Linux 32-bit function call.  This won't work for DOS.  I'll
give a working example for DOS below.

> mov ebx,0 ; exit code, 0=normal
> mov eax,1 ; exit command to kernel
> int 0x80 ; interrupt 80 hex, call kernel

"int 0x80" is a Linux 32-bit function call.  This won't work for DOS.  I'll
give a working example for DOS below.

DOS is 16-bit, but DJGPP produces 32-bit DPMI executables for 16-bit DOS
using a DPMI host.  The 16-bit conversion of your program is easy and only
requires NASM.  A 32-bit DOS example which links using DJGPP's GCC requires
a bit more work...  Basically, your code - including the string - is above
1Mb, however, the string must be relocated below 1Mb using DOS and DPMI
calls, before it can be printed.  The proper method is much longer in
assembly.  I was hoping to find a shorter method.  This would be useful to
me, so I may get back to you on the 32-bit version.  Or, I may not...

; nasm -f bin -o hello.com hello.asm
BITS 16
ORG 0x100

SECTION .text ; code section

  push cs
  pop ds ; offset portion of pointer to string
  mov dx, msg ; segment portion of pointer to string
  mov ah,09h ; write string command to int 21 hex
  int 0x21 ; interrupt 21 hex, call DOS

  mov al,0 ; exit code, 0=normal
  mov ah,4ch ; exit command to kernel
  int 0x21 ; interrupt 21 hex, call DOS

msg: db "Hello World",13,10,'$' ; the string to print, 10=cr, 13=lf,
$=terminator


Another option might be Mike Gonta's aeBIOS.  You should be able to do a
32-bit version almost like the code above.

http://groups.google.com/group/aeBIOS
http://www.mikegonta.com/aeBIOS/
(2nd is a link to binaries - which was has been down for a few days...)


Rod Pemberton

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