INT 10 - VIDEO - AX PC - REGISTER EXTERNAL CHARACTER
        AX = 5100h
        BH = character width in bits (10h)
        BL = character height (10h)
        DX = character code (DH = F0h-F3h, DL=40h-7Eh,80h-FCh)
        ES:BP -> character bitmap
Return: AL = status (00h successful, 01h failed)
SeeAlso: INT 1F