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Just can't get what's wrong..
Posted: Mon Jun 06, 2005 11:00 pm
by dXtr
hi. I'm trying out some things with DJGPP and NASM.
For the moment I'm experementing with compiling things into com files.
I've succeded compiling this into a com file:
Code: Select all
start.asm:
[BITS 32]
[global start]
[extern _main]
start:
call _main
ret
main.c
void main(void)
{
__asm__ (
"mov $'e', %%al\n\t"
"mov $0x0E, %%ah\n\t"
"xor %%bl, %%bl\n\t"
"int $0x10\n\t"
:
:
);
}
using this linkscript:
Code: Select all
OUTPUT_FORMAT("binary")
ENTRY(start)
SECTIONS
{
.text 0x100 :
{
code = .; _code = .; __code = .;
*(.text)
}
.data :
{
data = .; _data = .; __data = .;
*(.data)
}
.bss :
{
bss = .; _bss = .; __bss = .;
*(.bss)
}
end = .; _end = .; __end = .;
}
but now I wanted to try something a little more "advanced" with functions:
Code: Select all
still same start.asm
main.c:
void putch(char c)
{
__asm__ (
"mov %0, %%al\n\t"
"mov $0x0E, %%ah\n\t"
"xor %%bl, %%bl\n\t"
"int $0x10\n\t"
:
: "r" (c)
);
}
void main(void)
{
putch('H');
}
this dosen't work and I can't figure out why.. especially after seeing the the disassembly (made some changes so I can run it in nasm):
Code: Select all
[bits 32]
[org 0x100]
start: call _main
ret
; void putch(char c)
_putch: push ebp
mov ebp, esp
; Print a character
mov al, [ebp+0x8]
mov al, al
mov ah, 0x0E
xor bl, bl
int 0x10
pop ebp
ret
; void main(void)
_main: push ebp
; putch('H');
; Print a character
mov al, 0x48 ; H
mov ebp, esp
mov ah, 0x0E
xor bl, bl
int 0x10
pop ebp
ret
is there something I'm missing?
(sorry for the long message
)
edit:
I noticed if I remove the
from the putch function in the asm file. the H letter gets printed out but not else =/
Re: Just can't get what's wrong..
Posted: Tue Jun 07, 2005 11:00 pm
by digo_rp
go to
www.osdever.net/downlods and get kernel3.zip it is in gcc and nasm, then you need bootloader that load pmode image... try bootf02.zip, it start pm do paging and jump to pmode image.
Re: Just can't get what's wrong..
Posted: Tue Jun 07, 2005 11:00 pm
by dXtr
using hiew (a hex/disasm like program) I think I found the problem...
output from hiew:
Code: Select all
00000000: E81100 call 000000014 -------- (1)
00000003: 0000 add [bx][si],al
00000005: C3 retn
00000006: 55 push bp
00000007: 89E5 mov bp,sp
00000009: 8A4508 mov al,[di][00008]
0000000C: 88C0 mov al,al
0000000E: B40E mov ah,00E ;"?"
00000010: 30DB xor bl,bl
00000012: CD10 int 010
00000014: 5D pop bp
00000015: C3 retn
00000016: 55 push bp
00000017: B048 mov al,048 ;"H"
00000019: 89E5 mov bp,sp
0000001B: B40E mov ah,00E ;"?"
0000001D: 30DB xor bl,bl
0000001F: CD10 int 010
00000021: 5D pop bp
00000022: C3 retn
it treats the 32bit adress as a 16bit so the program calls 0x14 instead of 0x16 as it should... or something like that
I tried chaenging [bits 32] to [bits 16] and like magic I got a 'H' printed out. yay!
but why can't I use [bits 32] ?
Re: Just can't get what's wrong..
Posted: Tue Jun 07, 2005 11:00 pm
by dXtr
digo_rp wrote:go to
www.osdever.net/downlods and get kernel3.zip it is in gcc and nasm, then you need bootloader that load pmode image... try bootf02.zip, it start pm do paging and jump to pmode image.
so I have to do it this way to test it? I can't do as I've done now creating a com file and running it under windows? =/
Re: Just can't get what's wrong..
Posted: Tue Jun 07, 2005 11:00 pm
by digo_rp
I?m just start to learn pmode too, as I discover gcc is 32bits pmode image... you need some program that start pmode and load that image created by you...
don?t know! this is the bestway I found, I think alexei founze has a loader.exe that can load your image created . I?ll take a loot for you and send it the link to you
Re: Just can't get what's wrong..
Posted: Tue Jun 07, 2005 11:00 pm
by dXtr
thanks.
but it's not that important it was mostly so I could try out my own libc that I'm writting and also to try if it was possibly to compile a file into a com file. now I'm sitting and doing some experementing with mixing asm and c instead.
edit:
now for an other question
lets say I do this in C:
print("Hello");
print is an asm func. thats takes one argument pushed to it:
Code: Select all
_print:
push ebp
mov ebp, esp
mov eax, [ebp+8]
pop ebp
ret
the adress to the string is now in eax, but how do I move this adress to ds:si instead?
Never mind think.. I got it to work now
Re: Just can't get what's wrong..
Posted: Tue Jun 07, 2005 11:00 pm
by rexlunae
dXtr wrote:but why can't I use [bits 32] ?
The problem is a misunderstanding of the meaning of the bits 32 and bits 16 directive. You are correct that in a DOS *.com file, you must use bits 16, at least to start. This directive does not control how the program is loaded. It is informational for Nasm, so that it knows which mode your code is running in. If you tell Nasm bits 16, it assumes you are running in a 16-bit segment, which is the only way that DOS runs *.com files. However, if you use 32-bit instructions while in "bits 16" mode (such as using 32-bit registers), Nasm will automatically insert overrides so that the 32-bit instructions work.
DOS *.com files are basically raw binaries. They are loaded as images at an offset (0x100 maybe...don't remember for sure) within a 16-bit segment. There is no header stored on disk, so there is no way for DOS to allow *.com files to be loaded any other way.
Re: Just can't get what's wrong..
Posted: Wed Jun 08, 2005 11:00 pm
by dXtr
ok then I understand it better
Re: Just can't get what's wrong..
Posted: Wed Jun 08, 2005 11:00 pm
by digo_rp
if you are passing the address of "hello" you have to take care the "hello" adress size example may be word or dword in some cases the ebp in asm should be mov eax, [ebp +6] and not +8