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Monochrome Display Adapter

        3B4h    6845 Index Register (Output)

        3B5h    6845 Data Registers

                Reg#    Meaning                Text Graphics   Unit
                  R0    Horizontal Total        97      53      Char
                  R1    Horizontal Displayed    80      45      Char
                  R2    H.Sync position         82      46      Char
                  R3    H.Sync width            15       8      Char
                  R4    Vertical Total          25      91      Rows
                  R5    Vertical Adjust          6       2      Lines
                  R6    Vertical Displayed      25      87      Char
                  R7    V.Sync Position         25      87      Rows
                  R8    Interlace Mode           2       2
                  R9    Max.ScanLine Address    13       3
                  R10   Horizontal Display      12       0      Lines
                  R11   Horizontal Display       0       0      Lines
                  R12   Screen Start Address (H) 0       0
                  R13   Screen Start Address (L) 0       0
                  R14   Cursor Address (H)       0       0
                  R15   Cursor Address (L)       0       0

        3B8h    Mode Register (Output)

               Bit 7 6 5 4 3 2 1 0
                   x x . . . . . .  not used
                   . . x . . . . .  Blinking
                   . . . x . . . .  640x200 B&W
                   . . . . x . . .  Enable Video
                   . . . . . x . .  B&W Mode
                   . . . . . . x .  320x200 Graphics
                   . . . . . . . x  80x25 Mode

        3B9h    Color Register (Output)

                   Bit
               Bit 7 6 5 4 3 2 1 0
                   x x . . . . . .  not used
                   . . x . . . . .  320x200 Color set
                   . . . x . . . .  Alt. Back Colors
                   . . . . x . . .  Intensify RGB
                   . . . . . x . .  Blue
                   . . . . . . x .  Green
                   . . . . . . . x  Red

        3BAh    Status Register (Input)

                  Bit 7 6 5 4 3 2 1 0
                   x x x x . . . . not used
                   . . . . x . . .  Alpha Data
                   . . . . . x . .  Pen Switch off
                   . . . . . . x .  Light Pen Triggered
                   . . . . . . . x  Video Sync

        3BCh    Printer Data port

                   Data 0 - 7   (pins 2 - 9)

        3BDh    Printer Status port (input)

                   Bit
                   7 6 5 4 3 2 1 0
                   x . . . . . . .  Busy         (pin 11)
                   . x . . . . . .  -Acknowlegde (pin 10)
                   . . x . . . . .  Paper end    (pin 12)
                   . . . x . . . .  Select       (pin 12)
                   . . . . x . . .  -Error       (pin 14)
                   . . . . . x x x  nc

        3BEh    Printer Control port

                  Bit 7 6 5 4 3 2 1 0
                   x x x . . . . .  nc
                   . . . x . . . .  IRQ enable
                   . . . . x . . .  -slct in     (pin 17)
                   . . . . . x . .  -init        (pin 16)
                   . . . . . . x .  -autofeed    (pin 14)
                   . . . . . . . x  -strobe      (pin  1)

        3BFh    Configuration Switches (Hercules)
                  Bit 0: +Enable graphics
                  Bit 1: +Enable 2nd. page, "Full mode"