INT 10 - VIDEO - PCjr - MANIPULATE CRT/CPU PAGE REGISTERS
        AH = 05h
        AL = subfunction
            80h read CRT and CPU page registers
                Return: BH = CRT page register
                        BL = CPU page register
            81h set CPU page register
                BL = CPU page
            82h set CRT page register
                BH = CRT page
            83h set both CPU and CRT page registers
                BL = CPU page
                BH = CRT page
Notes:  the CPU page determines which 16K block of the first 128K of physical
          memory will be mapped at B800h by the hardware
        the CRT page determines the start address of the memory used by the
          video controller

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