This describes the I/O control ports for the Color/Graphics Adapter.  The
CGA is built around the Motorolla 6845 CRT controller chip.

  The CGA decodes ports 3d0H through 3dfH (actually using 3d2H-3dcH)

Note: These ports overlap the EGA I/O Ports for upward compatibility.
With some exceptions, the CGA information applies to the EGA.

An early Tech Ref indicated that the CGA could force hardware interrupt
level 2 (IRQ 2) upon vertical retrace.  However, the CGA has never had
this ability (but EGA/VGA supports this feature).

Note: BIOS stores "mirrors" of the current CRTC port values in the
BIOS Data Area.  To change a single bit, you should read the
appropriate variable, set the bit, perform the OUT, and store the
new value in the variable.

Port  Description
3d4H  Write: CRT Controller register select.  Use this port to select any of
the 18 CRTC registers.  Do an OUT 3d4H,reg_no followed by a read from
or write to port 3d5H.
3d5H  CRT Controller registers.  After selecting via port 3d4H, output or
input here to access the CRTC internal registers.

Write: See Video Initialization Table for a summary of the CRTC
registers and a way to look up standard BIOS settings for the
display modes.

Read:  Registers R12 through R17 can be read to determine the current
video page address, cursor address, and light pen information.
3d8H  Write: Mode Select Register (BIOS keeps current value at 40:0065)
 7 6 5 4 3 2 1 0 
     b r v c g w 
              0: width (text modes): 0=40 clms; 1=80 clms
              1: 1=enable graphics modes; 0=text modes
              2: 1=disable color burst on composite monitor
              3: 1=enable video signal; 0=disable (all black)
              4: dot resolutn: 0=320 2-bit dots; 1=640 1-bit dots
              5: enable blink: 1=text attribute bit 7 blinks fgnd
0=attr bit 7 is bkgd intensity
3d8H  Write: Color Select Register
 7 6 5 4 3 2 1 0 
     b i I R G B 
              0-3: color. Text modes: IRGB of border
                          Graphics modes: IRGB of background
              4: intensity. Text: bkgd intensity
                            Mid-res graphics: pixel intensity
              5: blue. Select grafx palette 0 or 1 (1 adds blue)
3daH  Read: Status Register
 7 6 5 4 3 2 1 0 
          0: retrace.  1=display in vert or horiz retrace.
          1: 1=light pen trigger is triggered; 0=armed
          2: 1=light pen switch is open; 0=closed
          3: 1=vertical sync pulse is occurring.  Display is
in vertical retrace--access won't cause "snow"
3dbH  Read/Write: Light Pen Latch Reset.  Any OUT clears light pen latch.
3dcH  Read/Write: Light Pen Latch Set.  Any OUT sets the light pen latch.
See Also: EGA I/O Ports
INT 10H (BIOS video services)
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Color/Graphics Adapter I/O Ports