## Heap and Pointers ##
The first 16 heap addresses (`0-15`) are reserved when using the stdlib.
-Within that reservation, heap[0] is used by `random` and heap[15] is used
-by `spewreg`. The remaining 14 locations, heap[1]-heap[14] are used as
-general-purpose registers.
+Within that reservation, heap[0] is used by `random` and the block
+heap[2]-heap[15] by the stack rotation subroutines which time-share these
+pseudo-registers between the various stdlib subroutines.
By convention, functions which return a pointer will use the address `0` to
represent a `NULL` pointer.
1001 ----- print number from stack (stdio.pvvs)
1010 ----- stackrotate (stack.pvvs)
1011 ----- stackrotatereverse (stack.pvvs)
+ 1100 ----- deepdup (stack.pvvs)
010xxx - math functions
10000 ----- random (math.pvvs)
10001 ----- absolute value (math.pvvs)
111xxx - debug functions
111000 ----- dump heap (debug.pvvs)
111001 ----- dump stack (debug.pvvs)
+ 111010 ----- print sign (debug.pvvs)
+ 111011 ----- print magnitude (debug.pvvs)
+ 111100 ----- print string (debug.pvvs)
1xxxxxx - reserved for less common entry points
- 1000000 ----- slurp registers (heap.pvvs)
- 1000001 ----- spew registers (heap.pvvs)
+ 1000000 ----- lowbitand (logic.pvvs)
+ 1000001 ----- isnegative (math.pvvs)
1000010 ----- print sign of number (stdio.pvvs)
1000011 ----- print magnitude of number (stdio.pvvs)
1000100 ----- print string from stack (stdio.pvvs)