The K83 has its SFR registers located at the top of ram, and the common ones are in bank 63 ($3F00).

The MOVFF instruction only uses 12-bits of addressing, so the highest address it can reach is $0FFF.
To reach the upper registers past that you have to use the MOVFFL instruction instead. It's a 3-word instruction
(vs 2 for MOVFF) that can reach the entire ram space. I mention that since that could require changing any
jumps or gotos in the asm code as well.

This would effect any code like the following:
Code:
    movff POSTINC0, PRODL
    movff POSTINC0, PRODH
For the K83 (and K42) that would have to be:
Code:
    movffl POSTINC0, PRODL
    movffl POSTINC0, PRODH
You also have to be using a version of MPASM that supports this instruction (MPASM v5.71 or later)