Never seen this before!!

[Mike, K8LH]

 
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.

That's not exactly correct. Notice that the constant value is being added to the free running TMR0 register. In this case it seems the author is using Timer 0 to generate precise 100 cycle intervals. Of course this only works if you're not using the TMR0 prescaler since any write to TMR0 will also clear the prescaler. The -97 value accounts for one cycle for TMR0 overflow and the inherent two cycle delay you get after writing TMR0 before TMR0 starts counting again.

Hope this information helps.

Cheerful regards, Mike

[SteveB]

Good point. I was focused on the fact that the movlw had a negative value. Your explaination takes it to the next step as to why the value would be there in the first place. Nicely done!