Re: [Nasm-users] Handling timer interrupt

[Frank K. <fbk...@zy...>]

zak100 wrote:
> Hi,
> I have changed the code but still I am getting garbage at kernel startup. I
> have got the macro definition at the beginning but the iret code at the end.
> I cant understand what ou mean by PIC.

What do you say to a student who hasn't done the assigned reading, Zulfi?

<http://www.arl.wustl.edu/~lockwood/class/cs306/books/artofasm/Chapter_17/CH17-3.html>

explains what I mean by PIC - Programmable Interrupt Controller - the 
8259 (or equivalent - I think modern hardware emulates this).

Might as well go to the original of this:

<http://homepage.mac.com/randyhyde/webster.cs.ucr.edu/www.artofasm.com/DOS/AoADosIndex.html>

You may want to read the whole thing.

> The new code is given below:
> ;Printing test mesg at kernel startup
> ;If I move macro at end , i get syntax error.
> %macro install 1 
>   push ax
>   mov ax, 0
>   mov word[%1*4],isr_add
>   mov word[%1*4+2],cs
>   pop ax
>  %endmacro

What's this supposed to do? It appears to overwrite your code. Why have 
you tried to make it a macro, when it loads the same "isr_add", no 
matter what the parameter?

>     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

Are you seeing this message? If you aren't getting this far, it's 
useless to try to continue!

> blackhole1:install 01ch; calling macro

Okay... suppose we've got int 1Ch patched...

>      xor ax,ax; print the mesgA using int10h
>      mov ds,ax
>      mov es,ax

Why????????

>     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

I suppose you're going to print the time here? It'll still be 00:00, 
presumably...


>  end44: jmp end44
>  
> 
>         
> blackhole:   
>     hlt
>     jmp blackhole
> 
> msg db 'Welcome to the Kernel!', 0
> msgA db 'Total minutes elapsed since Kernel start is', 0AH, 0DH

Is this supposed to be zero-terminated?

> clkcounter db 0
> secs db 0
> mins db 0
> 
> 
> isr_add:
>   cli
>   inc byte [byte cs:clkcounter]
>   cmp byte [byte cs:clkcounter],18; if clkcounter is 18, it means 1 sec
>   jz handle_secs
>   sti
>   iret
> handle_secs:
>   mov byte [byte cs:clkcounter],0
>   inc byte [byte cs:secs]
>   cmp byte [byte cs:secs],60;if secs is 60, it means 1 min
>   jz handle_mins
>   sti
>   iret
> handle_mins:
>   mov byte [byte cs:secs],0
>   inc byte [byte cs:mins]; we are done, now print the number of minutes
> elapsed
>   sti 
>   iret
> 
> times 0x200-($-$$) db 0

18.2 ticks/second != 18 ticks/second - close enough to see if your isr 
is working, I guess, but your clock is going to be wildly fast!

>  Kindly somebody plz help me with the above code.

I'd have to "test" anything I'd suggest. Might take a while...

Best,
Frank