whats wrong with this code???

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rookie23

Joined: 19 Sep 2009
Posts: 4

rookie23 01 Oct 2009, 09:15

hi there
im trying to make my custom interrupt.. i searched for some code to start and this is what i have now.. but when i run the program it just shows symbols!!.. i dont get the problem.. can anyone tell me what´s wrong??
on the other hand.. what should i need to do in my NewInt9 to store in the keyboard buffer a different letter than the one i press??.. (if i press a "b" .. store a "k" for example)..
thanks!!

Code:

org 100h

start:
        mov ah,09h      ; mensaje2
        mov dx,msj2
        int 21h
        mov     al,09h        
        mov     ah,35h        
        int     21h

        mov     [oldint9],bx  
        mov     [oldint9+2],es


        mov ah,09h      ; mensaje3
        mov dx,msj3
        int 21h

        mov     al,09h  
        mov     dx,Newint9    ; DS:DX
        mov     ah,25h
        int     21h

        mov ah,09h      ; prueba 1
        mov dx,msj6
        int 21h

loop1:
        mov     ah,0  
        int     16h     
                       
        mov     dl,ah
        mov     ah,2
        int     21h

        mov ah,09h      ; prueba 2
        mov dx,msj7
        int 21h

endp:
        ;call    ReStoreInt9

        mov ah,09h      ; mensaje5
        mov dx,msj5
        int 21h

        xor ah,ah
        int 16h

        mov ah, 4Ch     ;finish
        int 21h

Newint9:
        pusha
                        ;code

        mov ah,09h      ; mensaje 1
        mov dx,msj1
        int 21h


        mov     al, 0x20        ; EOI al PIC1
        out     0x20, al
        ; jump exit.
        call    ReStoreInt9

        popa
        iret


ReStoreInt9:
        mov ah,09h      ; mensaje4
        mov dx,msj4
        int 21h

        mov     al,09h 
        mov     dx,[oldint9]
        mov     ds,[oldint9+2] ; pointer to DS:DX
        mov     ah,25h
        int     21h
        ret

oldint9 dw 0
msj1 db 13,10,"New Vector is Working!!",13,10,24h
msj2 db 13,10,"Getting old vector...",13,10,24h
msj3 db 13,10,"Setting new vector...",13,10,24h
msj4 db 13,10,"Restrogin vector...",13,10,24h
msj5 db 13,10,"End",13,10,24h
msj6 db 13,10,"prueba 1",13,10,24h
msj7 db 13,10,"prueba 2",13,10,24h

01 Oct 2009, 09:15

SeproMan

Joined: 11 Oct 2009
Posts: 70
Location: Belgium

SeproMan 11 Oct 2009, 13:39

Some thoughts that might get you on the way...
------------------------------------------------------
Why do you remove your Interrupt Handler from within itself?

You never loop back to the label Loop1. Why is that?

When writing an Interrupt Handler the only segment register that you can really trust is CS! So you could write :
pusha
push ds
push cs
pop ds
...
pop ds
popa
iret
Moreover, it's not a good idea to call DOS services from within this Interrupt Handler ( DOS not being re-entrant)

When retrieving the old Interrupt Vector is is better to preserve the ES register.

_________________
Real Address Mode.

11 Oct 2009, 13:39

rookie23

Joined: 19 Sep 2009
Posts: 4

rookie23 16 Oct 2009, 00:24

thnks for your answer seproman!!

- Why do you remove your Interrupt Handler from within itself?
mov al, 0x20 ; EOI al PIC1
out 0x20, al
is this what you mean?.. i saw these lines were necessary for the eoi.. after you do everything you want to.. so i just wrote them down..

- You never loop back to the label Loop1. Why is that?
i forgot to erase this.. loop1 label was created just for control purposes..

- When writing an Interrupt Handler the only segment register that you can really trust is CS! So you could write :

pusha ; start?
push ds ; start?
push cs ; start?

pop ds ;finish?
... ;finish?
pop ds ;finish?
popa ;finish?
iret
so, i need tu use this code when my interrupt handler starts and finishes??
does it matter if my program is a .com? or it is irrelevant?

- Moreover, it's not a good idea to call DOS services from within this Interrupt Handler ( DOS not being re-entrant)
so, if i need to use a DOS int while im inside my int handler, would be possible to call this DOS int from a routine outside my interrupt handler?
ex: Newint9:
pusha
...
call int_dos_routine
...
popa
iret

int_dos_routine:
mov ah, 9
mov dx, msg
int 16h
ret

and finally: im using a customized KB handler (it is supposed to Razz

).. but while im inside the routine, would be possible to another program (windows environment) to know what´s going on?.. or to intercept/modify this handler?

i would really appreciate your answer
thnks

16 Oct 2009, 00:24

SeproMan

Joined: 11 Oct 2009
Posts: 70
Location: Belgium

SeproMan 19 Oct 2009, 19:43

You're removing your Interrupt Handler from within itself by writing :

Code:

call    ReStoreInt9

The 2 lines

Code:

mov     al, 0x20        ; EOI al PIC1 
out     0x20, al

are necessary only when one services the Keyboard at the hardware level. A lot of effort. BIOS does this very well indeed.

This is skeleton code that preserves all general registers and makes sure that DS = CS. The type of program is irrelevant.
pusha ; Save all general registers
push ds ; Save DS
push cs ; Copy CS to ...
pop ds ; ... DS

... ; BODY of routine

pop ds ; Restore DS
popa ; Restore all general registers
iret

Calling the INT_DOS_ROUTINE the way you suggested doesn't change a bit!
It still executes within the Interrupt Handler.

I would not bother too much about Windows. Everything you do will always remain IN the DOSBOX environment. So no interaction/interference of say Notepad/Word etc.
In the DOS environment however any program that loads after yours could potentially intercept the same Interrupt. But than again such a program would have been loaded by you, wouldn't it?

I had a lot of fun writing this small demo for you. Hope it is instructive.
Sorry, but the code in the attached file needs some minor changes because I used my own Assembler to write it. I did correct the code in this Post.

Code:

; Modified Keyboard Entry - 13/10/2009
  ORG     256
 mov     ax,cs
       mov     ss,ax
       mov     sp,MyStack+64*2
     mov     ds,ax
       mov     es,ax
; Some arbitrary modification
      mov     byte [Table+"a"],"z"
    mov     byte [Table+"z"],"a"
; Save existing Keyboard Interrupt Vector
       push    es
  mov     ax,3509h
    int     21h     ;DOS 'Get Interrupt Vector'
       mov     [Old_09h],bx
        mov     [Old_09h+2],es
      pop     es
; Setup new Keyboard Interrupt Vector
 mov     dx,New_09h
  mov     ax,2509h
    int     21h     ;DOS 'Set Interrupt Vector'
; Prompt user action
       mov     dx,Msg1
     mov     ah,09h
      int     21h     ;DOS 'Display String'
MainLoop:        mov     ah,01h
      int     21h     ;DOS 'Keyboard Input & Echo'
  cmp     al,27   ;<ESC> ?
      jne     MainLoop
; Restore old Keyboard Interrupt Vector
 push    ds
  lds     dx,[Old_09h]
        mov     ax,2509h
    int     21h     ;DOS 'Set Interrupt Vector'
       pop     ds
; Terminate
   mov     dx,Msg2
     mov     ah,09h
      int     21h     ;DOS 'Display String'
     mov     ax,4C00h
    int     21h     ;DOS 'Terminate/ReturnCode'
; ----------------------------------------------
New_09h:       push    ax bx si ds
 mov     ax,40h  ;BIOS_DataSegment
   mov     ds,ax
       mov     si,[001Ch]  ;BIOS_KeyboardBufferTailPointer
; Let BIOS take care of the hardware
 pushf
       call far dword [cs:Old_09h]
     cmp     si,[001Ch]      ;BIOS_KeyboardBufferTailPointer
     je      .t1     ;No new key in buffer
       mov     al,[si] ;ASCII (SCAN in [si+1])
     mov     bx,Table
    xlat    [cs:bx]
 mov     [si],al ;Updated ASCII
.t1:      pop     ds si bx ax
 iret
; ----------------------------------------------
    ALIGN   4
Old_09h:       dd      0
Table: times 256 db %-1
MyStack:        dw      64 dup (0)
Msg1: db      'Watch <a> and <z> being swapped...',13,10
    db      'Please press some keys. <ESC> exits!',13,10,'$'
Msg2: db      "-- That's all folks",13,10,'$'
; -----------------------------------------------

_________________
Real Address Mode.

19 Oct 2009, 19:43

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