concatenating 4 bytes into one pseudo 32 bit counter?

[HankMcSpank]

Wow Henrik...what can I say (thank heavens for folks like you & others on here)

I won't pretend to understand it all (once numbers start going above 256 values "woah, there goes scary stuff!").

re this bit...

Init:
  LSW = 0 
  MSW = 0
AddL = 500
AddH = 2000   ' 50*256+500=768500

Not understanding the bolded bits - can you please let me know what you're doing there?

I'm trying to see how/where I would enter what in DDS terms is called the tuning word (essentially the number that gets added to the accumulator each interrupt?

Also, I was hoping (eventually) that the tuning word would arrive serially (either being calculated manually or sent from another pic)....the maths are going to be a bit troubling...to glean the tuning word to set the required output frequency, it's

required frequency/interrupt rate * accumulator size

so for a wanted frequency of 4971Hz involving a 32 bit accumulator & and say an interupt rate of 20,000Hz

(4971/20000) * 4294967296

...that's gonna be a challenge in an 8 bit PIC?