76543210               76543210               76543210 
          =00000000=00           =00000000=00           =00000000=00
          =00000000=00           =00000000=00           =00000000=00
          =00111000=38           =11111100=FC           =00001100=0C
          =01101100=6C           =01100110=66           =00001100=0C
          =11000110=C6           =01100110=66           =00000000=00
          =11000110=C6           =01100110=66           =00011100=1C
          =11000110=C6           =01111100=7C           =00001100=0C
          =11111110=FE           =01100110=66           =00001100=0C
          =11000110=C6           =01100110=66           =00001100=0C
          =11000110=C6           =01100110=66           =00001100=0C
          =11000110=C6           =01100110=66           =00001100=0C
          =11000110=C6           =11111100=FC           =11001100=CC
          =00000000=00           =00000000=00           =11001100=CC
          =00000000=00           =00000000=00           =01111000=78
          =00000000=00           =00000000=00           =00000000=00
          =00000000=00           =00000000=00           =00000000=00
   c000:n+(m*0)  00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00 ASCII  0=
   c000:n+(m*1)  00,00,7E,81,A5,81,81,BD,99,81,81,7E,00,00,00,00 ASCII  1= 
   c000:n+(m*2)  00,00,7C,FE,FE,D6,FE,FE,BA,C6,FE,7C,00,00,00,00 ASCII  2= 
   c000:n+(m*65) 00,00,38,6C,C6,C6,C6,FE,C6,C6,C6,C6,00,00,00,00 ASCII 65=A
   c000:n+(m*66) 00,00,FC,66,66,66,7C,66,66,66,66,FC,00,00,00,00 ASCII 66=B
In the 8x16 font above, each character of the ASCII table is represented
by 16 bytes.  In each of those 16 bytes, each bit represents one pixel,
with the high-order bit representing the leftmost of eight pixels.

On an EGA or VGA you may obtain the address of any ROM-defined font via
INT 10H 1130H.  The definition of a particular ASCII character, c, starts
c*m bytes from the returned address (where m is the height of the
character matrix and thus the bytes-per character--usually 8, 14, or 16).

On a CGA or MDA, the font data is internal to the (inaccessible) character
generator.  However, the data layout used in the INT 1fH vector (which
defines ASCII characters > 127 for CGA graphics modes) is laid out as an
8x8 font.

 76543210                76543210 
          =00111100=3C            =00000000=00
          =01100110=66            =11000110=C6
          =11000000=C0            =00000000=00
          =01100110=66            =11000110=C6
          =00111100=3C            =11000110=C6
          =00011000=18            =11001110=CE
          =11001100=CC            =01110110=76
          =01111000=78            =00000000=00
   [0:7c]+(8*0)   3C,66,C0,66,3C,18,CC,78  ASCII 128=А
   [0:7c]+(8*1)   00,C6,00,C6,C6,CE,76,00  ASCII 129=Б
On EGAs and VGAs, the INT 43H vector points to a table of character
definitions for graphics mode.  The BIOS refers to the word at 0040:85 in
the BIOS Data Area to determine the size of each character definition.

  VGA Implied 9th Bit  
When displaying in text mode, the VGA uses an internally-handled 720-
pixel-wide screen width (in graphics mode, 640 is the highest supported
width).  Thus, VGA characters are really 9 pixels wide.  The 9th pixel is
usually displayed as blank (0).

However, certain Box & Special Characters would look bad if the rightmost
dot were blank.  So characters c0H through dfH are given special treatment
by the VGA character generator.  On these characters, the 8th bit is
extended into the 9th position.  Examine the Character Set Matrix to
verify that all of these characters are box-and-line characters which must
connect on the right side.  Although you can turn this off this "9th-bit
extension" feature (via ATC register 10H, bit 2; see EGA I/O Ports), there
is no way to select a different range.

  Dynamic Character Redefinition  
You may be aware of a wave of semi-graphical text-mode applications (such
as Norton Utilities and Central Point tools).  These programs redefine
certain characters to look like checkboxes, round "radio buttons" and so
forth.  The most interesting idea is the "semi-graphical" mouse pointer.

This appears miraculous at first, but is technically not too difficult.
You can use INT 10H 1100H to modify the "bank 0" character definitions to
instantly change the definition of a character.  The trick in tracking a
"graphical mouse cursor" is to replace a 2x2 character matrix at the mouse
position with some little-used characters (say, d0H-d3H).  Then redefine
those characters so that they look a lot like the characters they replace,
except that a mouse pointer overlays them.

Actually, the BIOS font-redefinition interface may be a bit too slow on
some VGAs, so programmers use direct port I/O to make the character-
generator RAM accessible and programmable on-the-fly.

When choosing which characters to redefine, you must keep in mind the
"9th-bit extension" described above; if you just pick some arbitrary
characters, your mouse pointer will appear to have "seams" when displayed
on VGA screens.  Only characters c0H-dfH will fill the gaps.

Programming for Direct Access to Character Generator RAM
The following sequence sets up the EGA and VGA for accessing character-
generator memory.  See EGA I/O Ports for related information.

out 3c4H, 0402H   Mask reg; enable write to map 2
out 3c4H, 0704H   Memory Mode reg ; alpha, ext mem, non-interleaved
out 3ceH, 0005H   Graphics Mode reg; non-interleaved access
out 3ceH, 0406H   Graphics Misc reg; map char gen RAM to a000:0
out 3ceH, 0204H   Graphics ReadMapSelect reg; enable read chargen RAM

After these OUTs, the font data begins at a000:0 and the first byte of
font data for a character begins at the character's ASCII value * 32.
After reading or writing the font data, the following sequence restores
the EGA/VGA to normal operations:

out 3c4H, 0302H   Mask reg; disable write to map 2
out 3c4H, 0304H   Memory Mode reg; alpha, ext mem, interleaved
out 3ceH, 1005H   Graphics Mode reg; interleaved access
out 3ceH, 0e06H   Graphics Misc reg; regen buffer to b800:0
out 3ceH, 0004H   Graphics ReadMapSelect reg; disable read chargen RAM

See Also: EGA  VGA
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Video Font Definition