WIP: 16C firmware hack for more memory

[RPNerd]

This is a 9-month-old thread, I know, but I'm shamelessly piggy-backing on it because I have a couple of questions about the NUT instruction set on which the participants in this thread might be able to shed some light.

As I understand it:

• instruction $3CC (CHK KB or ?KEY depending on the syntax you're using, I'll stick with the HP syntax here, so CHK KB) checks the CPU's KB register and if a key is pressed, sets the carry flag.
• Instruction $3C8 (RST KB) clears the KB register, although if a key is pressed when this is executed, the KB register will still contain the code of the key pressed.
• Instruction $220 (C=KEY) grabs the content of the KB register and sticks it in nybbles 3 & 4 of the C register.
  

On a Voyager unit, what exactly is the value of the KB register for each key pressed? Is it a scancode of sorts that's common to all Voyager units? Is there a resource online somewhere that can explain how this works in detail? I've not found anything yet.

Thanks in advance.