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//              featured, is designed to control up to sixteen general purpose

//      input and sixteen general purpose output lines of a module from a

//      single address on a 32-bit wishbone bus.

//

//      Input GPIO values are contained in the top 16-bits.  Any change in

//      input values will generate an interrupt.

//

//      Output GPIO values are contained in the bottom 16-bits.  To change an

//      output GPIO value, writes to this port must also set a bit in the

//      upper sixteen bits.  Hence, to set GPIO output zero, one would write

//      a 0x010001 to the port, whereas a 0x010000 would clear the bit.  This

//      interface makes it possible to change only the bit of interest, without

//      needing to capture and maintain the prior bit values--something that

//      might be difficult from a interrupt context within a CPU.

//