Never seen this before!!

[SteveB]

 
movlw   -97
addwf   TMR0

I have never come across any negative number getting loaded into a variable.
Is this the same as Loading 159-> (256-97=159) to Timer0 ?

It sure is.

You can look at the .LST file after you compile the program and find the line for "movlw -97". You will see that it will be something like:

00009E 0E9F 00232 movlw -97

00009E <-- Address of the the instruction

0E9F <-- Actual instruction code

If you look up the MOVLW code in the PIC datasheet, you will see the OP-CODE (most significant 8 bits of the 16 bit instruction) is $0E (%00001110). The literal is the 8 least significant bits, $9F in this case (which is 159).

[Megahertz]

Much appreciated your in-depth answer. Thanks Steve