On Sep 29, 2010, at 6:34 PM, David Duffy (AVD) wrote:

> On 30/09/2010 10:47 AM, David Duffy (AVD) wrote:
>>  I have the need to load a 3 byte counter, then decrement in an
>> interrupt, detecting (set a flag) when the counter reaches 0x000000.

How about something like this?  It seems to work OK in sim, but I =20
can't say I've used it in real code.  It terminates when the count =20
reaches -1 instead of zero (I say that's more useful anyway!)

On the plus side, it'll work on 12bit cores, and is easily =20
extensible.  On
the minus side, it trashes W, (and the -1 vs 0 thing.)  It need not be =20
a subroutine

It does that elegant and sneaky thing where multi skip instructions =20
skip because they're skipping over the parts that would have changed =20
the bits so they wouldn't skip...

dec3:  ;; decrement a 3-byte number (b3:b2:b1).
       ;;  Return 0 in W when it reaches -1, 0 otherwise.
        movlw  -1
        addwf b1, f             ;decrement low byte.
        btfss STATUS,C          ; wrapped?
        addwf b2, f             ; W still -1; dec mid byte
        btfss STATUS,C
        addwf b3, f             ; W still -1; dec high byte
        btfss STATUS,C
        retlw 0                 ; result is -1
exit_nz: retlw -1


--=20
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