This vector (0:007c) is a pointer to a table of Font Definition Data that
defines ASCII characters 128-255 (80H-ffH) that will be used to display
text (via INT 10H 0eH et al.) while in graphics mode.

The oldest CGA video systems do not provide definitions for these
characters.  On these systems, characters 128-255 (when in graphics modes)
are displayed as random dots unless you install your own table at the
INT 1fH vector.

Notes:   On CGAs, text-mode font data is inaccessible.
  EGA/VGA systems provide access to all font data.  They
automatically set INT 1fH to point to a table in ROM.

See Font Definition Data for a layout of this data.  The CGA BIOS expects
these character to be eight dots high (eight bytes per character).

See Also: INT 43H (EGA/VGA Graphics Character Font Pointer)
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INT 1fH: CGA Graphics Character Font Pointer