Hello Clyde !

> On Fri, Jul 18, 1997 at 02:24:07PM +0300, KORKMAZ SAiT ULAS wrote:
> >   Is there a way to read the stack . I am designing a multitasking
>
> The short answer is "no".
>
> >   Does anybody know a technique to do this ?
>
> It's impossible on a PIC - the only thing you can do with the
> stack is use it for call/return. If you want to do true
> multi-tasking, the PIC is not for you.


The long answer is yes ! ;) But it will be software implemented stack.

Let we reserve some byte array STACK[] , about 4 bytes for example.
So we can store 2 procedure addresses - Proc_1 and Proc_2 .

STACK[0] = HIGH ( Proc_1 ) ; STACK[1] = LOW ( Proc_1 ) ;
STACK[2] = HIGH ( Proc_2 ) ; STACK[3] = LOW ( Proc_2 ) ;

And we have some piece of code (SEMAPHORE) with one byte pointer
(POINTER),
which point to position in STACK[] of next called address . If we wish
to call
Proc_1 than POINTER=0 , in case of Proc_2 POINTER=2 etc. Certainly in
multitasker
we should also have additional flags , but i don't discuss about it .
(For example
flags that indicate what task is active/down/... ) POINTER var is useful
to know
what last Proc_ was called .


Finally, to call Proc_1 we do foolowing :

; First we should initialize STACK[] array

MOVLW HIGH Proc_1
MOVWF STACK
MOVLW LOW  Proc_1
MOVWF STACK+1
....

; Main part - CALL Proc_1

        MOVLW 0
        CALL  SEMAPHORE ;CALL Proc_1

; CALL Proc_2

        MOVLW 2
        CALL  SEMAPHORE ;CALL Proc_2

SEMAPHORE:
        MOVWF POINTER
        ADDLW STACK
        MOVWF FSR

        MOVFW INDF
        MOVWF PCLATH

        INCF  FSR
        MOVFW INDF
        MOVWF PCL      ; It's hidden GOTO ;)

; There are ability to save PCLATH and any another registers if need
; before GOTO to called Proc_ .

Proc_1:
        ...
        ...
        RETURN

Proc_2:
        ...
        ...
        RETURN

That's all . I was thinking this idea to make multitasking with PIC ,
but I still haven't time to do it ;(((

WBR Dmitry.

P.S. I have interest to look any theoretical/practical ideas in this
area .