INT 10H 1121H: Setup User-Defined Font for Graphics Mode

                                                      Compatibility: EGA VGA
 Expects: AX    1121H
          BL    screen rows code: 00H = user-specified (in DL)
                                  01H = 14 rows
                                  02H = 25 rows
                                  03H = 43 rows
          CX    bytes per character definition
          DL    (when BL=0) custom number of character rows on screen
          ES:BP address of font-definition information
 Returns: (none)
    Info: Sets up a custom character set to be used by the BIOS when
          displaying text while in graphics modes.  This sets the INT 43H
          vector and sets BIOS variables so it can use the custom font.

    ES:BP points to a table containing the character-definition codes.  It
          should be CX*256 bytes long (if you are defining all 256 ASCII
          characters).  The table is laid out as a series of groups with
          CX bytes in each group.

          See Video Font Definition for byte layout.

    Note: This fn requires that you modify BP.  When calling from high-
          level languages such as C, be very careful to save and restore
          BP, since it is used to keep track of the "stack frame".

See Also: INT 10H 11H: EGA/VGA Character Generator Functions
          INT 10H: Video Services
          EGA/VGA Data Areas