[sdiy] IIR Shift Filter problem - help with bug spotting in PIC ASM
steve at bendentech.co.uk
Fri Mar 10 11:34:02 CET 2017
On 10/03/2017 09:59, Roman Sowa wrote:
> It struck me this morning when I looked at it again.
> You cannot xorwf the carry/borrow bit, that will never work.
> Only clrc or setc can do that.
> I'm talking about this part:
> ; Borrow is zero if result required borrow (negative result)
> ; Flip the state of the borrow flag
> movlw 0x01
> xorwf STATUS, f ; Borrow set if negative
I'm intrigued by this thread and, like Tom and much to my annoyance,
cannot yet see the problem with the "bad" code.
However, I don't think it can be that. From a typical PIC datasheet:
"The STATUS register can be the destination for any
instruction, like any other register. If the STATUS
register is the destination for an instruction that affects
the Z, DC or C bits, then the write to these three bits is
disabled. These bits are set or cleared according to the
Neither MOVLW nor XORWF affect the carry/borrow bit, so modifying it in
that way _should_ be fine.
Tom, you told us that the "bad" version isn't working, but in what way?
Can you see what is actually happening to the values?
I hope somebody works it out, because I really want to know the answer now!
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