On Tue, 7 Sep 1999 10:16:30 -0400 felix centeno <fcenteno@IAMNET.COM>
writes:
> Hi, Piclisters,
>
> What's do the following sentence:
>
> addlw  0 - 1

This adds -1 to W.  -1 in two's complement notation is 0xFF, so you could
use "addlw h'0ff'" instead.  I've had some (I think very old) versions of
MPASM not process 'addlw -1' correctly.  So using the 0 - 1 construction
is safer.

After the instruction executes, the value in W will be 1 less than it was
before.  Consider it a "decrement W" operation.  It will also roll under
from 00 to 0xFF.  The Z flag will be set if the new value of W is zero.
The C flag will be set if the old value of W was not zero.  Or, you could
say that the C flag will be clear only if the new value of W is 0xFF.
The DC flag is also affected.




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