> Sorry guys I fat fingered my previous message.  Here's the rest of it.
>
> Guys, I have a value in W that's signed.  Adding this value to a set of
> two
> variables is easy enough:
> addwf     blahL,f
> clrf         wreg
> addwfc   blahH,f
>
> W contains a signed value.  If the value of W is a negative, doing the
> following doesn't work (to subtract it instead of adding it.)


This should work for both positive and negative values in both blah and w.
Are you sure the negative number is in two's complement form? For example,
an 8 bit negative 1 is 0xff, negative 2 is 0xfe, etc. When teaching twos
complement numbering, I tell my students to imagine a hexadecimal odometer
that counts forwards and backwards. Starting at zero and going forwared
(positive) we get 0x00, 0x01, 0x02, 0x03, etc. Starting from there and
going backwards, we get 0x03, 0x02, 0x01, 0x00, 0xff, 0xfe, 0xfd, etc. So,
0xfd is -3.

Another twos complement trick I read about MANY years ago in Byte magazine
is to change the sign of the "weight" of the most significant bit if the
byte (or word or whatever) is in twos complement form. Looking at the 0xfd
example of above, we convert it to binary to get:
1111 1101. Applying our new binary weights, starting at the msb, we find
this number is:

(1 * -128) + 1*64 + 1*32 + 1*16 + 1*8 + 1+4 + 0*2 + 1*1 = -3

Harold



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