3C0h:  Attribute Controller: Address register
bit 0-4  Address of data register to write to port 3C0h or read from port 3C1h
         (Reads only on VGA).
      5  If set screen output is enabled and the palette can not be modified,
         if clear screen output is disabled and the palette can be modified.


Port 3C0h is special in that it is both address and data-write register.
Data reads happen from port 3C1h. An internal flip-flop remembers whether it
is currently acting as address or data register.
Accesses to the attribute controller must be separated by at least 250ns.
Reading port 3dAh will reset the flip-flop to address mode.


3C0h index 0-Fh  (r/W):  Attribute: Palette
bit   0  (EGA) Primary Blue
      1  (EGA) Primary Green
      2  (EGA) Primary Red
      3  (EGA) Secondary Blue
      4  (EGA) Secondary Green
      5  (EGA) Secondary Red
    0-5  (VGA) Index into the 256 color DAC table.
               May be modified by 3C0h index 10h and 14h.

3C0h index 10h (r/W):  Attribute: Mode Control Register
bit   0  Graphics mode if set, Alphanumeric mode else.
      1  Monochrome mode if set, color mode else.
      2  9-bit wide characters if set.
         The 9th bit of characters C0h-DFh will be the same as
         the 8th bit. Otherwise it will be the background color.
      3  If set Attribute bit 7 is blinking, else high intensity.
      5  (VGA Only) If set the PEL panning register (3C0h index 13h) is
         temporarily set to 0 from when the line compare causes a wrap around
         until the next vertical retrace when the register is automatically
         reloaded with the old value, else the PEL panning register ignores
         line compares.
      6  (VGA Only) If set pixels are 8 bits wide. Used in 256 color modes.
      7  (VGA Only) If set bit 4-5 of the index into the DAC table are taken
         from port 3C0h index 14h bit 0-1, else the bits in the palette
         register are used.

3C0h index 11h (r/W):  Attribute: Overscan Color Register.
bit 0-5  Color of screen border. Color is defined as in the palette registers.
Note: The EGA requires the Overscan color to be 0 in high resolution modes.

3C0h index 12h (r/W):  Attribute: Color Plane Enable Register
bit   0  Bit plane 0 is enabled if set.
      1  Bit plane 1 is enabled if set.
      2  Bit plane 2 is enabled if set.
      3  Bit plane 3 is enabled if set.
    4-5  Video Status MUX. Diagnostics use only.
         Two attribute bits appear on bits 4 and 5 of the Input
         Status Register 1 (3dAh).
         Value       EGA            VGA
           0      Red/Blue        Bit 2/Bit 0
           1      Blue'/Green     Bit 5/Bit 4
           2      Red'/Green'     Bit 3/Bit 1
           3                      Bit 7/Bit 6

3C0h index 13h (r/W):  Attribute: Horizontal PEL Panning Register
bit 0-3  Indicates number of pixels to shift the display left
         Value  9bit textmode   256color mode   Other modes
           0          1               0              0
           1          2              n/a             1
           2          3               1              2
           3          4              n/a             3
           4          5               2              4
           5          6              n/a             5
           6          7               3              6
           7          8              n/a             7
           8          0              n/a            n/a

3C0h index 14h (r/W):  Attribute: Color Select Register  (VGA Only)
bit 0-1  If 3C0h index 10h bit 7 is set these 2 bits are used as bits 4-5 of
         the index into the DAC table.
    2-3  These 2 bits are used as bit 6-7 of the index into the DAC table
         except in 256 color mode.
Note: this register does not affect 256 color modes.

3C2h (R):  Input Status #0 Register
bit   4  Status of the switch selected by the Miscellaneous Output
         Register 3C2h bit 2-3. Switch high if set.
      5  (EGA Only) Pin 19 of the Feature Connector (FEAT0) is high if set
      6  (EGA Only) Pin 17 of the Feature Connector (FEAT1) is high if set
      7  (EGA Only ??) If set IRQ 2 has happened due to Vertical Retrace.
         Should be cleared by IRQ 2 interrupt routine by clearing port 3d4h
         index 11h bit 4.

3C2h (W):  Miscellaneous Output Register
bit   0  If set Color Emulation. Base Address=3Dxh else Mono Emulation. Base
         Address=3Bxh.
      1  Enable CPU Access to video memory if set
    2-3  Clock Select
          0: 14MHz(EGA)     25MHz(VGA)
          1: 16MHz(EGA)     28MHz(VGA)
          2: External(EGA)  Reserved(VGA)
      4  (EGA Only) Disable internal video drivers if set
      5  When in Odd/Even modes Select High 64k bank if set
      6  Horizontal Sync Polarity. Negative if set
      7  Vertical Sync Polarity. Negative if set
         Bit 6-7 indicates the number of lines on the display:
              0=200(EGA)  Reserved(VGA)
              1=          400(VGA)
              2=350(EGA)  350(VGA)
              3=          480(VGA).
Note: Set to all zero on a hardware reset.
Note: On the VGA this register can be read from port 3CCh.

3C3h (W):  Video Subsystem Enable Register
bit   0  Enables the VGA display if set

3C4h index  0  (r/W):  Sequencer: Reset
bit   0  (EGA) Asynchronous Reset if clear
      0  (VGA) Synchronous Reset just as bit 1
      1  Synchronous Reset if clear

3C4h index  1  (r/W):  Sequencer: Clocking Mode
bit   0  If set character clocks are 8 dots wide, else 9.
      1  (EGA Only) If set the CRTC uses 2/5 of the clock cycles, else 4/5.
      2  If set loads video serializers every other character
         clock cycle, else every one.
      3  If set the Dot Clock is Master Clock/2, else same as
         Master Clock (See 3C2h bit 2-3). (Doubles pixels).
      4  (VGA Only) If set loads video serializers every fourth character
         clock cycle, else every one.
      5  (VGA Only) if set turns off screen and gives all memory cycles to the
         CPU interface.

3C4h index  2  (r/W):  Sequencer: Map Mask Register
bit   0  Enable writes to plane 0 if set
      1  Enable writes to plane 1 if set
      2  Enable writes to plane 2 if set
      3  Enable writes to plane 3 if set

3C4h index  3  (r/W):  Sequencer: Character Map Select Register
bit 0-1  (EGA) Selects EGA Character Map (0..3) if bit 3 of the character
         attribute is clear.
    2-3  (EGA) Selects EGA Character Map (0..3) if bit 3 of the character
         attribute is set.
  0,1,4  (VGA) Selects VGA Character Map (0..7) if bit 3 of the character
         attribute is clear.
  2,3,5  (VGA) Selects VGA Character Map (0..7) if bit 3 of the character
         attribute is set.
Note: Character Maps are placed at:
      Map no. (EGA/VGA)       Map no. (VGA)
         0       0k              4      8k
         1      16k              5     24k
         2      32k              6     40k
         3      48k              7     56k

3C4h index  4  (r/W):  Sequencer: Memory Mode Register
bit   0  Set if in an alphanumeric mode, clear in graphics modes.
      1  Set if more than 64kbytes on the adapter.
      2  Enables Odd/Even addressing mode if set. Odd/Even mode places all odd
         bytes in plane 1&3, and all even bytes in plane 0&2.
      3  (VGA Only)  If set address bit 0-1 selects video memory planes
         (256 color mode), rather than the Map Mask and Read Map Select
         Registers.

3C4h index 7 (R/W):  Sequencer Horizontal Character Counter Reset Register.
                                                                   (VGA  Only)
Note: Undocumented by IBM. May not be available in all clones.
      A write to this register will cause the Horizontal Character Counter
      to be held reset (=0) until a write happens to any of the Sequencer
      registers index 0..6.
      The Vertical Line counter is clocked by a signal derived from the
      Horizontal Display Enable (which does not occur if the Horizontal
      Character Counter is held reset).
      Thus a write to index 7 during Vertical Retrace can stop the display
      timing and allow software to start the next frame reasonably
      synchronous to an external event.

3C6h (R/W):  PEL Mask                                               (VGA Only)
bit 0-7  This register is anded with the palette index sent for each dot.
         Should be set to FFh.

3C7h (R):  DAC State Register                                       (VGA Only)
bit 0-1  0 indicates the DAC is in Read Mode and 3 indicates write mode.

3C7h (W):  PEL Address Read Mode                                    (VGA Only)
bit 0-7  The PEL data register (0..255) to be read from 3C9h.
Note: After reading the 3 bytes at 3C9h this register will increment,
      pointing to the next data register.

3C8h (R/W):  PEL Address Write Mode                                 (VGA Only)
bit 0-7  The PEL data register (0..255) to be written to 3C9h.
Note: After writing the 3 bytes at 3C9h this register will increment, pointing
      to the next data register.

3C9h (R/W):  PEL Data Register                                      (VGA Only)
bit 0-5  Color value
Note:  Each read or write of this register will cycle through first the
       registers for Red, Blue and Green, then increment the appropriate
       address register, thus the entire palette can be loaded by writing 0 to
       the PEL Address Write Mode register 3C8h and then writing all 768 bytes
       of the palette to this register.

3CAh (R):  Feature Control Register                                 (VGA Only)
Bit   3  Vertical Sync Select. If set Vertical Sync to the monitor is the
         logical OR of the vertical sync and the vertical display enable.
Note: This register is written to port 3dAh and read from 3CAh.

3CAh (W):  Graphics 2 Position                                      (EGA Only)
bit 0-1  Select which bit planes should be controlled by Graphics Controller
         #2. Always set to 1.

3CCh (R):  Miscellaneous Output Register                            (VGA Only)
bit   0  If set Color Emulation. Base Address=3Dxh else Mono Emulation. Base
         Address=3Bxh.
      1  Enable CPU Access to video memory if set
    2-3  Clock Select. 0= 25MHz, 1= 28MHz, 2= Reserved
      5  When in Odd/Even modes Select High 64k bank if set
      6  Horizontal Sync Polarity. Negative if set
      7  Vertical Sync Polarity. Negative if set
         Bit 6-7 indicates the number of lines on the display:
            0=Reserved, 1=400, 2=350, 3=480.
Note: This register is written to port 3C2h and read from port 3CCh.

3CCh (W):  Graphics 1 Position                                      (EGA Only)
bit 0-1  Select which bit planes should be controlled by Graphics Controller
         #1. Always set to 0.

3CEh index  0  (r/W):  Graphics: Set/Reset Register
bit   0  If in Write Mode 0 and bit 0 of 3CEh index 1 is set a write to
         display memory will set all the bits in plane 0 of the byte to this
         bit, if the corresponding bit is set in the Map Mask Register (3CEh
         index 8).
      1  Same for plane 1 and bit 1 of 3CEh index 1.
      2  Same for plane 2 and bit 2 of 3CEh index 1.
      3  Same for plane 3 and bit 3 of 3CEh index 1.

3CEh index  1  (r/W):  Graphics: Enable Set/Reset Register
bit   0  If set enables Set/reset of plane 0 in Write Mode 0.
      1  Same for plane 1.
      2  Same for plane 2.
      3  Same for plane 3.

3CEh index  2  (r/W):  Graphics: Color Compare Register
bit 0-3  In Read Mode 1 each pixel at the address of the byte read is compared
         to this color and the corresponding bit in the output set to 1 if
         they match, 0 if not. The Color Don't Care Register (3CEh index 7)
         can exclude bitplanes from the comparison.

3CEh index  3  (r/W):  Graphics: Data Rotate
bit 0-2  Number of positions to rotate data right before it is written to
         display memory. Only active in Write Mode 0.
    3-4  In Write Mode 2 this field controls the relation between the data
         written from the CPU, the data latched from the previous read and the
         data written to display memory:
           0: CPU Data is written unmodified
           1: CPU data is ANDed with the latched data
           2: CPU data is ORed  with the latch data.
           3: CPU data is XORed with the latched data.

3CEh index  4  (r/W):  Graphics: Read Map Select Register
bit 0-1  Number of the plane Read Mode 0 will read from.

3CEh index  5  (r/W):  Graphics: Mode Register
bit 0-1  Write Mode: Controls how data from the CPU is transformed before
         being written to display memory:
           0: Mode 0 works as a Read-Modify-Write operation.
              First a read access loads the data latches of the EGA/VGA with
              the value in video memory at the addressed location. Then a
              write access will provide the destination address and the CPU
              data byte. The data written is modified by the function code in
              the Data Rotate register (3CEh index 3) as a function of the CPU
              data and the latches, then data is rotated as specified by the
              same register.
           1: Mode 1 is used for video to video transfers.
              A read access will load the data latches with the contents of
              the addressed byte of video memory. A write access will write
              the contents of the latches to the addressed byte. Thus a single
              MOVSB instruction can copy all pixels in the source address byte
              to the destination address.
           2: Mode 2 writes a color to all pixels in the addressed byte of
              video memory. Bit 0 of the CPU data is written to plane 0 et
              cetera. Individual bits can be enabled or disabled through the
              Bit Mask register (3CEh index 8).
           3: (VGA Only) Mode 3 can be used to fill an area with a color and
              pattern. The CPU data is rotated according to 3CEh index 3 bits
              0-2 and anded with the Bit Mask Register (3CEh index 8). For
              each bit in the result the corresponding pixel is set to the
              color in the Set/Reset Register (3CEh index 0 bits 0-3) if the
              bit is set and to the contents of the processor latch if the bit
              is clear.
      2  (EGA Only) Forces all outputs to a high impedance state if set.
      3  Read Mode
           0: Data is read from one of 4 bit planes depending on the Read Map
              Select Register (3CEh index 4).
           1: Data returned is a comparison between the 8 pixels occupying the
              read byte and the color in the Color Compare Register (3CEh
              index 2). A bit is set if the color of the corresponding pixel
              matches the register.
      4  Enables Odd/Even mode if set (See 3C4h index 4 bit 2).
      5  Enables CGA style 4 color pixels using even/odd bit pairs if set.
      6  (VGA Only) Enables 256 color mode if set.

3CEh index  6  (r/W):  Graphics: Miscellaneous Register
bit   0  Indicates Graphics Mode if set, Alphanumeric mode else.
      1  Enables Odd/Even mode if set.
    2-3  Memory Mapping:
           0: use A000h-BFFFh
           1: use A000h-AFFFh   EGA/VGA Graphics modes
           2: use B000h-B7FFh   Monochrome modes
           3: use B800h-BFFFh   CGA modes

3CEh index  7  (r/W):  Graphics: Color Don't Care Register
bit   0  Ignore bit plane 0 in Read mode 1 if clear.
      1  Ignore bit plane 1 in Read mode 1 if clear.
      2  Ignore bit plane 2 in Read mode 1 if clear.
      3  Ignore bit plane 3 in Read mode 1 if clear.

3CEh index  8  (r/W):  Graphics: Bit Mask Register
bit 0-7  Each bit if set enables writing to the corresponding bit of a byte in
         display memory.

3d4h index  0  (r/W):  CRTC: Horizontal Total Register
bit 0-7  (EGA) Horizontal Total Character Clocks-2
    0-7  (VGA) Horizontal Total Character Clocks-5

3d4h index  1  (r/W):  CRTC: Horizontal Display End Register
bit 0-7  Number of Character Clocks Displayed -1

3d4h index  2  (r/W):  CRTC: Start Horizontal Blanking Register
bit 0-7  The count at which Horizontal Blanking starts

3d4h index  3  (r/W):  CRTC: End Horizontal Blanking Register
bit 0-4  Horizontal Blanking ends when the last 5 (6 for VGA) bits of the
         character counter equals this field.
         (VGA) The sixth bit is found in port 3d4h index 5 bit 7.
    5-6  Number of character clocks to delay start of display after Horizontal
         Total has been reached.
      7  (VGA Only) Access to Vertical Retrace registers if set. If clear
         reads to 3d4h index 10h and 11h access the Lightpen read back
         registers ??

3d4h index  4  (r/W):  CRTC: Start Horizontal Retrace Register
bit 0-7  Horizontal Retrace starts when the Character Counter reaches this
         value.

3d4h index  5  (r/W):  CRTC: End Horizontal Retrace Register
bit 0-4  Horizontal Retrace ends when the last 5 bits of the character counter
         equals this value.
    5-6  Number of character clocks to delay start of display after Horizontal
         Retrace.
      7  (EGA) Provides Smooth Scrolling in Odd/Even mode. When set display
         starts from an odd byte.
      7  (VGA) bit 5 of the End Horizontal Blanking count (See 3d4h index 3
         bit 0-4).

3d4h index  6  (r/W):  CRTC: Vertical Total Register
bit 0-7  Lower 8 bits of the Vertical Total. Bit 8 is found in 3d4h index 7
         bit 0. (VGA) Bit 9 is found in 3d4h index 7 bit 5.
Note: For the VGA this value is the number of scan lines in the display -2.

3d4h index  7  (r/W):  CRTC: Overflow Register
bit   0  Bit 8 of Vertical Total (3d4h index 6)
      1  Bit 8 of Vertical Display End (3d4h index 12h)
      2  Bit 8 of Vertical Retrace Start (3d4h index 10h)
      3  Bit 8 of Start Vertical Blanking (3d4h index 15h)
      4  Bit 8 of Line Compare Register (3d4h index 18h)
      5  (VGA) Bit 9 of Vertical Total (3d4h index 6)
      6  (VGA) Bit 9 of Vertical Display End (3d4h index 12h)
      7  (VGA) Bit 9 of Vertical Retrace Start (3d4h index 10h)

3d4h index  8  (r/W):  CRTC: Preset Row Scan Register
bit 0-4  Number of lines we have scrolled down in the first character row.
         Provides Smooth Vertical Scrolling.
    5-6  (VGA Only) Number of bytes to skip at the start of scanline. Provides
         Smooth Horizontal Scrolling together with the Horizontal Panning
         Register (3C0h index 13h).

3d4h index  9  (r/W):  CRTC: Maximum Scan Line Register
bit 0-4  Number of scan lines in a character row -1. In graphics modes this is
         the number of times (-1) the line is displayed before passing on to
         the next line (0: normal, 1: double, 2: triple...).
         This is independent of bit 7, except in CGA modes which seems to
         require this field to be 1 and bit 7 to be set to work.
      5  (VGA) Bit 9 of Start Vertical Blanking
      6  (VGA) Bit 9 of Line Compare Register
      7  (VGA) Doubles each scan line if set.
         I.e. displays 200 lines on a 400 display.

3d4h index  Ah (r/W):  CRTC: Cursor Start Register
bit 0-4  First scanline of cursor within character.
      5  (VGA) Turns Cursor off if set

3d4h index  Bh (r/W):  CRTC: Cursor End Register
bit 0-4  Last scanline of cursor within character
    5-6  Delay of cursor data in character clocks.

3d4h index  Ch (r/W):  CRTC: Start Address High Register
bit 0-7  Upper 8 bits of the start address of the display buffer

3d4h index  Dh (r/W):  CRTC: Start Address Low Register
bit 0-7  Lower 8 bits of the start address of the display buffer

3d4h index  Eh (r/W):  CRTC: Cursor Location High Register
bit 0-7  Upper 8 bits of the address of the cursor

3d4h index  Fh (r/W):  CRTC: Cursor Location Low Register
bit 0-7  Lower 8 bits of the address of the cursor

3d4h index 10h (R):  CRTC: Light Pen High Register     (EGA Only)
bit 0-7  (EGA Only)  Upper 8 bits of the address of the lightpen position.

3d4h index 10h (r/W):  CRTC: Vertical Retrace Start Register
bit 0-7  Lower 8 bits of Vertical Retrace Start. Vertical Retrace starts when
         the line counter reaches this value. Bit 8 is found in 3d4h index 7
         bit 2. (VGA Only) Bit 9 is found in 3d4h index 7 bit 7.

3d4h index 11h (R):  CRTC: Light Pen Low Register                   (EGA Only)
bit 0-7  (EGA Only)  Lower 8 bits of the address of the lightpen position.

3d4h index 11h (r/W):  CRTC: Vertical Retrace End Register
bit 0-3  Vertical Retrace ends when the last 4 bits of the line counter equals
         this value.
      4  if clear Clears pending Vertical Interrupts.
      5  Vertical Interrupts (IRQ 2) disabled if set. Can usually be left
         disabled, but some systems (including PS/2) require it to be enabled.
      6  (VGA Only) If set selects 5 refresh cycles per scanline rather
         than 3.
      7  (VGA Only) Disables writing to registers 0-7 if set 3d4h index 7
         bit 4 is not affected by this bit.

3d4h index 12h (r/W):  CRTC: Vertical Display End Register
bit 0-7  Lower 8 bits of Vertical Display End. The display ends when the line
         counter reaches this value. Bit 8 is found in 3d4h index 7 bit 1.
         (VGA Only) Bit 9 is found in 3d4h index 7 bit 6.

3d4h index 13h (r/W):  CRTC: Offset register
bit 0-7  Number of bytes in a scanline / K. Where K is 2 for byte mode, 4 for
         word mode and 8 for Double Word mode.

3d4h index 14h (r/W):  CRTC: Underline Location Register
bit 0-4  Position of underline within Character cell.
      5  (VGA Only) If set memory address is only changed every fourth
         character clock.
      6  (VGA Only) Double Word mode addressing if set

3d4h index 15h (r/W):  CRTC: Start Vertical Blank Register
bit 0-7  Lower 8 bits of Vertical Blank Start. Vertical blanking starts when
         the line counter reaches this value. Bit 8 is found in 3d4h index 7
         bit 3.

3d4h index 16h (r/W):  CRTC: End Vertical Blank Register
bit 0-4  (EGA) Vertical blanking stops when the lower 5 bits of the line
         counter equals this field.
    0-6  (VGA) Vertical blanking stops when the lower 7 bits of the line
         counter equals this field.

3d4h index 17h (r/W):  CRTC: Mode Control Register
bit   0  If clear use CGA compatible memory addressing system
         by substituting character row scan counter bit 0 for address bit 13,
         thus creating 2 banks for even and odd scan lines.
      1  If clear use Hercules compatible memory addressing system by
         substituting character row scan counter bit 1 for address bit 14,
         thus creating 4 banks.
      2  If set increase scan line counter only every second line.
      3  If set increase memory address counter only every other character
         clock.
      4  (EGA Only) If set disable the EGA output drivers. This bit is used
         for other purposes in some Super VGA chips.
      5  When in Word Mode bit 15 is rotated to bit 0 if this bit is set else
         bit 13 is rotated into bit 0.
      6  If clear system is in word mode. Addresses are rotated 1 position up
         bringing either bit 13 or 15 into bit 0.
      7  Clearing this bit will reset the display system until the bit is set
         again.

3d4h index 18h (r/W):  CRTC: Line Compare Register
bit 0-7  Lower 8 bits of the Line Compare. When the Line counter reaches this
         value, the display address wraps to 0. Provides Split Screen
         facilities. Bit 8 is found in 3d4h index 7 bit 4.
         (VGA Only) Bit 9 is found in 3d4h index 9 bit 6.

3d4h index 22h (R):  Memory Latch Register                       (VGA - Undoc)
bit 0-7  Reads the contents of the Graphics Controller Memory Data Latch for
         the plane selected by 3C0h index 4 bit 0-1 (Read Map Select).
Note: This register is not documented by IBM and may not be available on all
      clones.

3d4h index 24h (R):  Attribute Controller Toggle Register.       (VGA - Undoc)
bit 0-4  Attribute Controller Index.
         The current value of the Attribute Index Register.
      5  Palette Address Source. Same as 3C0h bit 5.
      7  If set next read or write to 3C0h will access the data register.
Note: This register is not documented by IBM and may not be available on all
      clones.

3d4h index 30h-3Fh (W):  Clear Vertical Display Enable.          (VGA - Undoc)
bit   0  Setting this bit will clear the Vertical Display Enable thus blanking
         the display for the rest of the frame and giving the CPU total access
         to display memory until the start of the next frame.
Note: This register is not documented by IBM and may not be available on all
      clones.

3dAh (R):  Input Status #1 Register
bit   0  Either Vertical or Horizontal Retrace active if set
      1  (EGA Only) Light Pen has triggered if set
      2  (EGA Only) Light Pen switch is open if set
      3  Vertical Retrace in progress if set
    4-5  (EGA Only) Shows two of the 6 color outputs, depending on 3C0h index
         12h bit 4-5:
          Attr: Bit 4-5:   Out bit 4  Out bit 5
                   0          Blue       Red
                   1        I Blue       Green
                   2        I Red      I Green

3dAh (W):  Feature Control Register
bit   0  (EGA Only) Output to pin 21 of the Feature Connector.
      1  (EGA Only) Output to pin 20 of the Feature Connector.
      3  (VGA Only) Vertical Sync Select. If set Vertical Sync to the monitor
         is the logical OR of the vertical sync and the vertical display
         enable.
Note: On the VGA this register can be read from port 3CAh.

