0  CO0 -- Current Object, coordinate X1
1  CO1 -- Current Object, coordinate Y1
2  CO2 -- Current Object, coordinate X2
3  CO3 -- Current Object, coordiante Y2
4  CO4 -- Current Object, flags/color
5  OI  -- Object Index
6  NO  -- Number of Objects
7  FS  -- Field Size
8  ARG -- Function Argument register
9  TP  -- ALU temporary register
A  MA0 -- Memory Address X (low order byte)
B  MA1 -- Memory Address Y (high order byte)
C  DX  -- X2 - X1 (9 bit sign-magnitude)
D  DY  -- Y2 - Y1 (9 bit sign-magnitude)
E  E   -- Bresenham error term
F  I   -- Loop index