IOBYTE Assignments
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CON device (bits 1,0):
   00 - TTY uses 2SIO serial port A*
   01 - CRT uses 2SIO serial port B
   10 - BAT indirect through RDR logical device
   11 - UC1 uses 2SIO serial port A

RDR device (bits 3,2):
   00 - TTY uses 2SIO serial port A
   01 - PTR uses 2SIO serial port B*
   10 - UR1 uses 2SIO serial port A
   11 - UR2 uses 2SIO serial port B

PUN device (bits 5,4):
   00 - TTY uses 2SIO serial port A
   01 - PTP uses 2SIO serial port B*
   10 - UP1 uses 2SIO serial port A
   11 - UP2 uses 2SIO serial port B
 
LST device bits (7,6):
   00 - TTY uses 2SIO serial port A
   01 - CRT uses 2SIO serial port B
   10 - LPT uses 2SIO serial port B*
   11 - UL1 uses 2SIO serial port A

* Default setting