POINTERS.DOC: The list of Saturn register use in RPL.

┌────┐
│ D0 │ contains a pointer to the next RPL instruction in the current
└────┘ program (like a hardware level program counter, but at an RPL
       level).

┌────┐
│ D1 │ points to the "top" of the stack (level 1).
└────┘

(Note: The stack only contains pointers to objects, not the objects
themselves.  Thus D1+5 points to level 2; D1+10 to level 3, etc.)

┌───┐
│ D │ is the amount of free space between stack and heap.  If you push
└───┘ or pop the stack be sure to update the value in D.

┌───┐
│ B │ points to top of heap.
└───┘