Odd Assembly Bug

[Langle]

Ok, I am creating a simple bootloader, and I found a weird bug. The buggy code:

Code: Select all

ReadDiskCHS:

	push ax
	push bx
	xor bl,bl
	mov al,'1'
	Call PrintChar
	pop bx
	pop ax

	mov AH,2
	int 13h		;Call BIOS

	push ax
	push bx
	xor bl,bl
	mov al,'2'
	Call PrintChar
	pop bx
	pop ax


	jc ReadDiskCHS	;If there was an error restart
	ret		;Else return

When I use the code like this, It doesn't hang, but when i take out the line

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	push ax

	push bx
	xor bl,bl
	mov al,'1'
	Call PrintChar
	pop bx
	pop ax

It just stops a int 13H
Here is the code for PrintChar:

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PrintChar:

	xor bh,bh
	mov ah,0eh
	int 10h 
	ret

Thanks for any help, The numbers were originally paced there for simple debugging purposes

[01000101]

You're not filling in all of the parameters for Int 0x13 AH=2. For instance, do you know what CX, DX, and BX are... are even what ES is? A description of what is required of this particular BIOS interrupt can be found here.

[xenos]

Code: Select all

xor bl,bl
...
xor bh,bh

This changes the carry flag, so if you insert this piece of code, the conditional jump will behave differently. You should also pushf / popf if you want to preserve the flag status and perform a conditional jump that relies on the previous flag state.

[Langle]

This changes the carry flag, so if you insert this piece of code, the conditional jump will behave differently. You should also pushf / popf if you want to preserve the flag status and perform a conditional jump that relies on the previous flag state.

Forgot about that, the code other registers is before this, the code i posted is a procedure.

[tantrikwizard]

You don't provide enough information to know what is going on at run-time. int 0x13 function 0x02 requires distinct values in certain registers before issuing the command. You don't supply that info here so there's no way to know what is going on. Read up on the following tutorials:

http://wiki.osdev.org/Bootloader
http://wiki.osdev.org/Rolling_Your_Own_Bootloader

You would also benefit by learning assembler and BIOS calls:
http://wiki.osdev.org/ATA_in_x86_RealMode_(BIOS)

When all else fails, read a book or two from the suggested readings:
http://forum.osdev.org/viewtopic.php?f=1&t=6989