On Mon, Mar 05, 2007 at 03:11:02AM +0100, Rikard Bosnjakovic wrote:
> At first I thought I liked RISC, but now when I started to actually
> code something (instead of just reading the book) I found it pretty
> tricky.

It's not RISC, it's just the PIC.

> For example, in a mainloop I want a particular variable hold
> the value 0, increase it every loop run and as soon as it turns 6 it's
> supposed to reset. That is, I want it to keep the values 0-5.

Counting up. Got it.

> After 30 minutes I got it to work, but it's somehow backwards because
> the subwf-instruction was a real caveat emptor (lots of warnings for
> it in my book).

Because it operates backwards, subtracting F from W, not the other way around.

> Here's the code:
> 
> 
> reinit
> 		movlw	d'6'
> 		movwf	segment
> 
> mainloop
> 		; do lots of fancy code here
> 		movlw	d'42'
> 		movlw	d'19'
> 
> 		decfsz	segment
> 		 goto	mainloop
> 
> 		goto	reinit

The traditional countdown loop. What are the movlw's for?

> The variable segment will keep the active number of the led-segment
> which is (later on) supposed to be multiplexed into, so I guess it
> doesn't matter if it goes backward or not.

Not really. It easy to detect zero. So if the loop doesn't matter, simply
count down.

> But still, I did not managed to make a "forward" version of the code.
> Anyone cares to give me a hint of how you could do it?

Sure. Don't use subwf. The best instruction for comparing for equality (or
inequality) is xorwf. It produces a zero (and sets the zero flag) when the
values in W and F are equal. So something like

;----------------

reinit   clrf   segment

mainloop
; lots of fancy code as before

        incf    segment,F
        movlw   d'6'       ; I prefer hex personally.
        xorwf   segment,W  ; put the result in W so that segment is preserved
        btfss   STATUS,Z   ; skip next instruction if equal
        goto    mainloop   ; otherwise loop
        goto    reinit     ; start back at the beginning.
;----------------

This code is written off the top of my head and is untested. Note that it's 
longer than the countdown code.

Hope this helps,

BAJ
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