Compatibility: EGA VGA
Expects: AX    1101H
BL    font block selector code (see below)
Returns: (none)
Info: This fn lets you activate any of the EGAs four font blocks or any
of the VGAs 8 font blocks.

It also makes it possible to create a 512-element character set.
Use it to identify which (previously-loaded) font block is to be
displayed when bit 3 of the Video Attribute is clear and which to
be displayed when bit 3 of the attribute is set.

Attribute bit 3 normally selects the intensity of the foreground
color.  After using this fn, you will have only 8 possible
foreground colors, but you can display up to 512 characters
simultaneously (for instance, you can display italic characters
along with upright characters).

BL is a set of bit-fields which identify which font block to use
when video attribute bit 3 is 1 or 0.

EGAFontBlockRec  (for EGAs)
 7 6 5 4 3 2 1 0 
 0 0 0 0 cs1 cs0 
                   bits mask
          0-1: 03H (0-3) font when attr bit 3=0
         2-3: 0cH (0-3) font when attr bit 3=1

VGAFontBlockRec  (for VGAs)
 7 6 5 4 3 2 1 0 
 0 0 a b cs1 cs0 
                   bits  mask
     -        0,1,4: 13H (0-7) font when attr bit 3=0
              2,3,5: 2cH (0-7) font when attr bit 3=1

In the VGA version, bits 4 and 5 are used in an upwardly-
compatible way to extend the possible selections to 8.

When both selections are the same (for instance, BL=00H selects
both characters sets to be font block 0), then BIOS disables the
512-character set handling and enables normal use of the video

For instance, to enable font block 2 only, use BL=82H.  You would
first need to prepare the font block via INT 10H 1101H (or 1102H
or 1104H).

Notes: When using a 512-character set, you need not give up high-
intensity characters.  Just use INT 10H 10H functions to program
the color palette.

- -

INT 10H 1103H: Activate Font Block