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

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³ D0 ³ contains a pointer to the next RPL instruction in the current
ÀÄÄÄÄÙ program (like a hardware level program counter, but at an RPL
       level).

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³ D1 ³ points to the "top" of the stack (level 1).
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(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.)

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³ D ³ is the amount of free space between stack and heap.  If you push
ÀÄÄÄÙ or pop the stack be sure to update the value in D.

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³ B ³ points to top of heap.
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