#302 Stack dump for int6 (int0) handler loses int10 function code

[Aleksei Tarlykov]

All interrupt handlers that dump stack (13 words of stack) to screen could corrupt output because AX value could be not preserved during int10 call. So function code (ah=0eh) is lost and it's next value (and function) depends on bios implementation. In my case it was ah=3 "read cursor position" but could be "set mode", for example.
The current code works fine with some emulators as they preserve AX value, but looking at the original IBM PC XT BIOS from 1982/86, the following comment is placed in int10 header:
CS,SS,DS,ES,BX,CX,DX PRESERVED DURING CALL
ALL OTHERS DESTROYED

entry.asm:167

                 mov bp, sp              
                xor si, si         ; print 13 words of stack for debugging LUDIV etc.
stack_loop:             
                mov dx, [bp+si]
                call print_hex
                mov al, ' '
                int 10h
                                            <<--- AX value is lost here
                inc si
                inc si
                cmp si, byte 13*2
                jb stack_loop
                mov al, 0dh
                int 10h           <<- and here
                mov al, 0ah
                int 10h           <<- and here

[tom ehlert]

2 lines above you have

mov al, ' '

which REALLY destroys AX. no need to go to the IBM BIOS

btw: this is an abort program handler.

close this bug

[Paul Dufresne]

First, the file entry.asm, can be seen at:
https://github.com/PerditionC/fdkernel/blob/master/kernel/entry.asm

I disagree with Tom, the reported error is the code make the assumption that... at least AX, and more precisely AH... will not by changed by Int 10h, and indeed I don't see why we can make this assumption.

So at first glance, in this code most:
int 10h

should be replaced by:
push ax
int 10h
pop ax

Still a bit unsure if I miss an other register... but preserve AH value is essential for this code to work.