Re: Multitasking by Dmitry Kiryashov .

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 .