a) Added sim_debug statements, formatted console messages.
b) Removed old debugging statements and redundant byte/word mask operations.
c) Added more device commands.
d) Added zx-200a and changed nulldev to return 0 vice 0xff.
e) Added commands and corrected RTYPE and RBYTE behavior so ISIS-II would boot.
f) Corrected device calls in SBC_reset, corrected get_mbyte and put_mbyte to
match control bits from ipc_cont.c.
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