X0 contains the 12-bit mask, 7777B (octal), used by the simulator to isolate
PDP-8 words in the larger CDC-6000 memory and registers. See other register
Unlike other X registers, X0 is not linked to memory by the CDC-6000
architecture. For this reason useful constants are often stored in X0.
© 1975, 1998, Ward Cunningham
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