Clay,

I looked at your MatLab code and I assume this is similar to what you 
wish to implement in PIC assembly. If so, then it appears that all 
divisions can be replaced with multiplications. For example, you write:

dr = 2^20/seed
Fref_act = FGHz/dr

Which can obviously be written as:

Fref_act = (FGHz * seed) / 2^20

And division by 2^20 is the same thing as shifting right 20 bit positions.

In another place you show a division by a constant. This one can be 
replaced with a multiplication of 1/constant.

Having said this, I don't have any routines to perform 64bit 
multiplication. However, if I were to write one I'd either start from 
first principles and implement a shift/add routine, or I'd use 
pre-existing 32-bit routines. E.g. you may've already noticed that if N 
and M are 64-bit numbers, they can be written as:

   N = a*2^32 + b
   M = c*2^32 + d

and N*M can be written as

  N*M = ac*2^64 + (ad+bc)*2^32 + bd

A 64-bit multiplication has been replaced with 4 32-bit multiplications. 
  This can actually be replaced with 3 32-bit multiplications:

  N*M = ac*2^64 + (ac + bd - (a - b)(c - d)) *2^32 + bd

However, the middle term is now signed.

Scott
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