Under CP/M 2, input and output device selection are performed by the BIOS. Some BIOSes may implement a feature called the IOBYTE which allows the user to change which device the screen, printer etc. output go to.
The IOBYTE lives at address 3 (in the Zero Page) and should be changed using BDOS calls 7 and 8 (get/set IOBYTE). The value is bitmapped:
CP/M pages
Bits Bits 6,7 Bits 4,5 Bits 2,3 Bits 0,1
Device LIST PUNCH READER CONSOLE
Value
00 TTY: TTY: TTY: TTY:
01 CRT: PTP: PTR: CRT:
10 LPT: UP1: UR1: BAT:
11 UL1: UP2: UR2: UC1:
BAT = batch mode. Use the current Reader for console input, and
he current List (printer) device as the console output.
CRT = Standard console (keyboard and terminal screen).
LPT = Standard line printer.
PTP = Standard Paper Tape Punch.
PTR = Standard Paper Tape Reader.
TTY = Teletype device, eg a serial port.
UC1 = User defined (ie implementation dependent) console device.
UL1 = User defined (ie implementation dependent) printer device.
UPn = User defined (ie implementation dependent) output device.
URn = User defined (ie implementation dependent) input device.