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         Programming the Registers

         Each of the video components has an address register and a
         number of data registers. The data registers have addresses
         common to all registers for that component. The individual
         registers are selected by a pointer (index) in its Address
         register. To write to a data register, the address register is
         loaded with the index of the desired data register, then the
         data register is loaded by writing to the common I/O address.

         The general registers do not share a common address; they each
         have their own I/O address.

         For compatibility with IBM Enhanced Graphics Adapter (EGA), the
         internal video subsystem palette is programmed the same as the
         EGA. Using BIOS to program the palette will produce a color
         compatible to that produced by the EGA. Mode 13h (256-color
         mode) is  programmed so that the first 16 locations in the DAC
         produce compatible colors.

         When BIOS is used to load the color palette for a color mode and
         monochrome display is attached, the color palette is changed.
         The colors are summed to produce shades of gray that allow color
         applications to produce a readable screen.

         Modifying the following bits must be done while the sequencer is
         held in a synchronous reset through its Reset register. The bits
         are:

         .  Bits 3 and 0 of the Clocking Mode register.

         .  Bits 3 and 2 of the Miscellaneous Output register.