INT 10H 1103H: Activate Font Block

                                                      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
          attribute.

          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.

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