INT 0x18 services

[St8ic]

I'm trying to set up some basic OS services for my OS...
This code compiles fine, but crashes at boot. Any ideas?

Code: Select all

xor bx,bx
xor ax,ax
mov ss,ax
mov ax,cs
cli
mov word [ss:(0x20*4)],Int20_handler
mov word [ss:(0x20*4+2)],ax
sti

jmp normal

Int20_handler:
cmp ah,0x00
je mep
ret

mep:
blahblahblah
IRET

normal:
...

[Karig]

Well, one idea is that you return from your interrupt handler with RET instead of IRET if AH != 0.

[Pype.Clicker]

saving/restoring registers would also be a good idea (but i'm sure you did it, but just skip it here to save keystrokes )

[St8ic]

Ya, I be saveing/restoring the ax register...any others?

[asmboozer]

I'm trying to set up some basic OS services for my OS...
This code compiles fine, but crashes at boot. Any ideas?

Code: Select all

xor bx,bx
xor ax,ax
mov ss,ax
mov ax,cs
cli
mov word [ss:(0x20*4)],Int20_handler
mov word [ss:(0x20*4+2)],ax
sti

jmp normal

Int20_handler:
cmp ah,0x00
je mep
ret

mep:
blahblahblah
IRET

normal:
...

btw which address mode the statement mov word [ss:(0x20*4)],Int20_handler belong to?

[St8ic]

80386 Real-Address Mode ;D ...

[Pype.Clicker]

what is the value for SP at that time ? zeroing the stack segment without altering the stack pointer looks like commiting suicide in real mode: as your stack grows it could destroy its interrupt handlers or the BIOS data area ...

If you need a 0-segment, rather use GS or FS or ES if DS is busy for other things, but using SS sounds really weird to me ...

[St8ic]

SP is at 0x1300...that may be the problem?

[Pype.Clicker]

well, if SS was different from 0 before entering the INT 20h setup function, it will definitiely cause trouble by the time you'll issue a RET instruction, as the return address will no longer be the same ...

avoid toying with the stack segment if you don't have to switch to another stack ...