Thread: [Nasm-users] Handling timer interrupt

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Hi,
I am trying to handle timer interrupt in a OS like code. I have got a
bootloader whose code is given below and is working (I am able to see the
mesg at the start up):
%define bootseg 0
%define bootoff 7C00h

%define loadoff 7E00h

ORG 7c00h      ;Because BIOS loades the OS at 
                       ; address 0:7C00h so ORG 7C00h 
                       ; makes that the refrence to date 
                       ; are with the right offset (7c00h).

   ; CS = 0 / IP = 7C00h // SS = ? / SP = ?
   ; You are now at address 7c00.
jmp start   ;Here we start the, BIOS gave us now the control.

;///////////////////////////////////////////
;//Here goes all the data of the program.
;///////////////////////////////////////////

xCursor db 0
yCursor db 0

nSector db 0
nTrack  db 0
nSide   db 0
nDrive  db 0

nTrays  db 0

;
szReady                db 'Are You Ready to start Loading the OS...',0
szErrorReadingDrive    db 'Error Reading Drive, Press any Key to
reboot...',0
;//Done Reading a track.
szPlaceMarker          db  '~~~~',0
szDone                 db  'Done',0

pOS                    dw   loadoff
;//Points to where to download the Operating System. 

;/////////////////////////////////
;//Here the program starts.
;/////////////////////////////////

start:

    CLI     ;Clear Interupt Flag so while setting 
        ;up the stack any interrupt would not be fired.
;----------------STACK
        mov AX,7B0h    ;lets have the stack start at 7c00h-256 = 7B00h
        mov SS,ax      ;SS:SP = 7B0h:256 = 7B00h:256
        mov SP,256     ;Lets make the stack 256 bytes.

        XOR AX,AX      ;Makes AX=0.
        MOV ES,AX      ;Make ES=0
        mov DS,ax

    STI     ;Set Back the Interupt Flag after 
        ;we finished setting a stack frame.

        Call ClearScreen       ;ClearScreen()
        LEA AX,[szReady]         ;Get Address of szReady.
        CALL PrintMessage      ;Call PrintfMessage()
        CALL GetKey                    ;Call GetKey()

    mov bp, 1 ; sectors to load
        CALL DownloadOS
;        CALL GetKey                    ;Call GetKey()
        CALL GiveControlToOS  ;Give Control To OS.

;/////////////////////////////////////
;//Prints a message to the screen.
;/////////////////////////////////////
PrintMessage:

        mov DI,AX      ;AX holds the address of the string to Display.
        Mov byte [xCursor],1  ;Column.

ContinuPrinting:

        cmp byte [DI],0    ;Did we get to the End of String.
        JE EndPrintingMessage  ;if you get to the end of the string return.

        mov AH,2               ;Move Cursor
        mov DH,[yCursor]         ;row.
        mov DL,[xCursor]         ;column.
        mov BH,0               ;page number.
        INT 10h
        INC byte [xCursor]

        mov AH,0Ah             ;Display Character Function.
        mov AL,[DI]            ;character to display.
        mov BH,0               ;page number.
        mov CX,1               ;number of times to write character
        INT 10h

        INC DI                 ;Go to next character.

        JMP ContinuPrinting    ;go to Print Next Character.

EndPrintingMessage:

        Inc byte [yCursor]            ;So Next time the message would
                               ;be printed in the second line.

        cmp byte [yCursor],25
        JNE dontMoveCorsurToBegin 
        Mov byte [yCursor],0

dontMoveCorsurToBegin:
        ret

;PrintMessage EndP 
;//////////////////////////////////////
;//Waits for the user to press a key.
;//////////////////////////////////////
GetKey: ; PROC

        mov ah,0
        int 16h ;Wait for a key press.
        Ret

;///////////////////////////////////////////
;//Gives Control To Second Part Loader.
;///////////////////////////////////////////
GiveControlToOS:

        LEA AX,[szDone]
        Call PrintMessage
        CALL GetKey

        jmp 7E0h:40h

	mov ax, [loadoff + 18h]

	push 7E0h
	push ax
	retf

;///////////////////////////////////
;//Clear Screen.
;///////////////////////////////////
ClearScreen:

        mov ax,0600h   ;//Scroll All Screen UP to Clear Screen.
        mov bh,07
        mov cx,0
        mov dx,184fh   
        int 10h

        Mov byte [xCursor],0  ;//Set Cursor Position So next 
                        ;//write would start in 
                        ;//the beginning of screen.
        Mov byte [yCursor],0

        Ret

;/////////////////////////////////
;//PrintPlaceMarker.
;/////////////////////////////////
PrintPlaceMarker:

        LEA AX,[szPlaceMarker]
        CALL PrintMessage  ;Call PrintfMessage()
;        CALL GetKey        ;Call GetKey()
        ret

;///////////////////////////////////
;//DownloadOS
;///////////////////////////////////
DownloadOS:

        mov byte [nDrive],0
        mov byte [nSide],0
        mov byte [nTrack],0
        mov byte [nSector],1 ; desired sector - 1!

ContinueDownload:

        INC byte [nSector]            ;Read Next Sector.
        cmp byte [nSector],19         ;Did we get to end of track.
        JNE StayInTrack

        CALL PrintPlaceMarker  ;Print now '~~~~' so the user would 
                               ;know that we finished reading a track
        INC byte [nTrack]             ;If we get to end of track Move to
next track.
        mov byte [nSector],1          ;And Read Next Sector.
        CMP byte [nTrack],5           ;Read 5 Tracks (Modify this value 
	                           ;to how much Tracks you want to read).
        JE      EndDownloadingOS

StayInTrack:

        ;ReadSector();
        Call ReadSector
    dec bp
    jz EndDownloadingOS

        JMP     ContinueDownload
        ;If didn't yet finish Loading OS.

EndDownloadingOS:

        ret

;////////////////////////////////////////
;//Read Sector.
;////////////////////////////////////////
ReadSector:

        mov byte [nTrays],0

TryAgain:

        mov AH,2               ;//Read Function.
        mov AL,1               ;//1 Sector.
        mov CH,[nTrack]
        mov CL,[nSector]         ;//Remember: Sectors start with 1, not 0.
        mov DH,[nSide]
        mov DL,[nDrive]
        Mov BX,[pOS]             ;//ES:BX points to the address 
                               ;to were to store the sector.
        INT 13h

    jnc EndReadSector

        mov AH,0               ;Else Reset Drive . And Try Again...
        INT 13h
        cmp byte [nTrays],3           ;Check if you tryed reading 
                               ;more then 3 times.

        JE DisplayError        ; if tryed 3 Times Display Error.

        INC byte [nTrays]

        jmp TryAgain       ;Try Reading again.

DisplayError:
        LEA AX,[szErrorReadingDrive]
        Call PrintMessage
        Call GetKey
        mov AH,0                       ;Reboot Computer.
        INT 19h

EndReadSector:
        ADD WORD [pOS],512  ;//Move the pointer 
                               ;(ES:BX = ES:pOS = 0:pOS) 512 bytes.
                               ;//Here you set the varible 
                               ;pOS (pOS points to were BIOS 
                               ;//Would load the Next Sector).
        Ret

;////////////////////////////////////
;//
;////////////////////////////////////

times 510 - ($ - $$) db 0
db 55h, 0AAh
;-------------------

An then I have a kernel code which includes a file also:

%include "lib.inc"
    mov ax, 7E0H
    mov ds,ax
    mov es,ax
    mov si, msg

    mov ah, 0Eh
    mov bx, 7
top:
    lodsb
    cmp al, 0
    jz blackhole1
    int 10h
    jmp short top

blackhole1:install 6

blackhole:   
    hlt
    jmp blackhole

msg db 'Welcome to the Kernel!', 0
msgA db 'Total minutes elapsed since Kernel start is', 0AH, 0DH
clkcounter db 0
secs db 0
mins db 0

times 0x200-($-$$) db 0

The include file is given below:

%macro install 1
  push ax
  mov al,%1
  mov ah, 0eh
  int 10h
  pop ax
  ;push ds
  ;mov ax, 0
  ;mov word[%1*4],isr_add
  ;mov word[%1*4+2],cs
  ;pop ds
%endmacro

isr_add:
  cli
  inc byte [byte cs:clkcounter]
  cmp byte [byte cs:clkcounter],18
  jz handle_secs
  sti
  iret
handle_secs:
  mov byte [byte cs:clkcounter],0
  inc byte [byte cs:secs]
  cmp byte [byte cs:secs],60
  jz handle_mins
  sti
  iret
handle_mins:
  mov byte [byte cs:secs],0
  inc byte [byte cs:mins]
  push ds
  push es

  xor ax,ax
  mov ds,ax
  mov es,ax
  ; should probably set up a sane stack here, too.
  mov si,msgA    ; "our dear string"

    ;mov bx,7
  mov ah,0Eh
msgloop:
  lodsb
  or al,al
  jz end44
  int 10h       ; since we're still in RM, we can use bios.
  jmp msgloop
 end44: pop es
  pop ds
  sti
  iret
  sti
  iret

For compilation I am using following commands:
nasm -f bin bootmz.asm -o boot.bin

nasm -f obj test_k.asm

alink -oEXE test_k.obj 

copy /b boot.bin+test_k.exe image.bin
partcopy image.bin 0 323 -f0

Can somebody plz help me with this?

Zulfi.
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