CP/M 2 Input/Output mapping

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:

     Bits      Bits 6,7    Bits 4,5    Bits 2,3    Bits 0,1
     Device    LIST        PUNCH       READER      CONSOLE

       00      TTY:        TTY:        TTY:        TTY:
       01      CRT:        PTP:        PTR:        CRT:
       10      LPT:        UP1:        UR1:        BAT:
       11      UL1:        UP2:        UR2:        UC1:

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