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So simple, yet freaking
Posted: Fri Aug 04, 2006 6:11 am
by joke
Hi all,
i'm just trying to print a string in real mode. Everythings fine when my kernel is only one sector. Now, i tried to do the same thing by loading a sector of disk into memory. Well, nothing happens, the string does not appear , only a flashing cursor
here's my code
kernelt.asm
Code: Select all
[BITS 16]
%include 'proto2.inc'
Main:
mov si,welcome ;print message
call print
halt:
hlt
proto2.inc
Code: Select all
welcome db 'Welcome to my OS!',0 ;welcome message
print:
lodsb ;load from si
cmp al,0 ;see if string has ended
je printend ;if yes?
mov ah,0Eh ;function params
mov bh, 0Fh
mov bl, 0
int 10h ;print!
jmp print
printend:
ret
the bootloader, boot.asm
Code: Select all
; I used Daniel's tutor at os resources as a template
[ORG 0]
start:
mov ax,0x7C00
mov ds,ax
mov ax,0x0000
mov ss,ax ; setting my stack
mov sp,0xFFFF
reset_drive:
mov ax, 0
mov dl, 0
int 13h
jc reset
read:
mov ax,1000h ; ES:BX = 1000:0000
mov es,ax
mov bx,0 ;
mov ah,2 ; Load disk data to ES:BX
mov al,5 ; Load 5 sectors
mov ch,0 ; Cylinder=0
mov cl,2 ; Sector=2
mov dh,0 ; Head=0
mov dl,0 ; Drive=0
int 13h ; Read!
jc read ; ERROR => Try again
mov ax,es
mov cs,ax ; i added this later, but it didn't solve my problem
jmp 1000h:0000 ; Jump to the program
times 510-($-$$) db 0
dw 0AA55h
and i use and image file to combine these files, image02.asm
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%include 'boot.asm'
%include 'kernelt2.asm'
i compile
nasm -f bin image02.asm -o image.bin
and
dd if=image.bin of=/dev/fd0
I'd really appreciate any help cause it's so simple and it's not working
Thank you
Posted: Fri Aug 04, 2006 8:10 am
by JAAman
first, the most obvious problems:
1) you should specify 'bits 16' (though i dont remember -- it might be default for binary)
2) your loading the wrong DS:
start:
mov ax,0x7C00
mov ds,ax
the sector is loaded to absolute 0x7C00 -- which is 0x07C0:0
this should be:
3) this is not valid code (should give an error, but might not):
mov ax,es
mov cs,ax ; i added this later, but it didn't solve my problem
you cannot manually alter the CS register -- only a JMP, CALL, or RET/IRET can alter CS -- and you do that with your FAR CALL
last comment:
i would recommend not combining the files together -- assemble them separately, and then write them to the disk separately (not familiar enough with dd to say how you would do that using that tool)
Posted: Sat Aug 05, 2006 7:19 am
by joke
I've changed my code, but still no results. Here's my new code
kernelt2.asm
Code: Select all
[BITS 16]
;i've merged proto and kernelt
Main:
mov si,welcome
call print
;call unixpromt
;jmp Main
halt:
hlt
welcome db 'Welcome to my OS!',0
print:
lodsb
cmp al,0
je printend
mov ah,0Eh
mov bh, 0Fh
mov bl, 0
int 10h
jmp print
printend:
ret
boot.asm
Code: Select all
; I used Daniel's tutor at os resources as a template
[BITS 16]
[ORG 0]
start:
mov ax,0x7C0
mov ds,ax
mov ax,0x0000
mov ss,ax
mov sp,0xFFFF
reset_drive:
mov ax, 0
mov dl, 0
int 13h
jc reset_drive
read:
mov ax,1000h ; ES:BX = 1000:0000
mov es,ax
mov bx,0 ;
mov ah,2 ; Load disk data to ES:BX
mov al,5 ; Load 5 sectors
mov ch,0 ; Cylinder=0
mov cl,2 ; Sector=2
mov dh,0 ; Head=0
mov dl,0 ; Drive=0
int 13h ; Read!
jc read ; ERROR => Try again
jmp 1000h:0000 ; Jump to the program
times 510-($-$$) db 0
dw 0AA55h
i assembled
nasm -f bin kernelt2.bin -o kernel.bin
nasm -f bin boot.asm -o boot.bin
and
dd if=boot.bin of=/dev/fd0
dd if=kernel.bin of=/dev/fd0 obs=512 seek=1 ( i retried with seek=2 but didn't work either)
Any opinions on how to make it work are welcome
best regards
Posted: Sat Aug 05, 2006 8:00 am
by Mikae
I think, that you have to reload segment register DS to value 0x100 before jump to loaded kernel, or use [org 0x1000] directive in your kernel code and load 0x0 to DS.
Also, I'm not sure, but I think that BIOS remains unchanged values of registers ax and bx after call -- consult with docs
-- so, may be, you don't need to load this registers everytime in cycle.
Posted: Sun Aug 06, 2006 6:57 am
by joke
The thing is that the intel manuals don't zoom too much on real mode operations , sadly
(i mean like loading your kernel with interrupts, cause the describe everything else, like FLAGS,cr0-4,ther regs etc). I'll try to load the ds with 0x100 and post the results. Btw, if anyone knows dd , is my seek synax correct(obs=512,seek=1 is to correct)? I mean i want to write in sector 2, not overwrite my bootsector
Posted: Sun Aug 06, 2006 7:19 am
by joke
i added a
mov ax,0x100
mov ds,ax
before the
jmp 1000h:0000
but still no success
Posted: Sun Aug 06, 2006 8:03 am
by Mikae
Well, this is very strange. Did you try seek=2? If yes -- I have no ideas for now... You can send me your binary code, I'll try to test it.
mail: -=mika0x65=--=gmail=--=com=- (without '-=' and '=-')
Posted: Sun Aug 06, 2006 12:15 pm
by joke
i sent the bins as specified. Let's hope this brings something...
Posted: Sun Aug 06, 2006 12:19 pm
by Mikae
Check your mail, I've sent you an answer already
.
Posted: Sun Aug 06, 2006 1:39 pm
by joke
Hell , it worked !!!!!!! Yeah, it was that ds reg that made me troubles. A thousand thank yous Mikae. (and JaaMan). You saved me!