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".

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