> -----Original Message-----
> From: Graham North [SMTP:graham.north@CIRRUSRESEARCH.CO.UK]
> Sent: Thursday, March 13, 2003 9:48 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Subtract?
>
> Hi everyone,
>
> I need to subtract 1 from a byte on the 18F6720, and I'm confused by the
> fact that there seem to be four instructions to subtract in the
> instruction
> set.
>
> There is:
>
> SUBFWB  Subtract f from W with borrow
>
> SUBLW           Subtract W from literal
>
> SUBWF           Subtract W from f
>
> SUBWFB  Subtract W from f with Borrow
>
> I have a byte called New_Low_Byte, and need to subtract 1 from it. I'm
> confused as to the fastest way to do it.
>
> Any help would be great.
>
> Thanks
>
> Graham
>
Subtracting 1 is a special case, the fastest way is to use the DECF
(decrement file) instruction:

decf    New_Low_Byte,F

You can also check for zero after the decrement using decfsz and decfsnz.

Regards

Mike


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