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The advantage of direct I/O is that the hardware circuit is usually simpler, since the number of I/O ports is limited. Partial decoding techniques can be used to design the address decoding circuit. When memory mapped I/O is used, full address decoding is needed, which results in a more complex hardware circuit.
The advantage of memory mapped I/O is that all instructions and addressing modes can be used for I/O access. This makes programming easier. When Direct I/O is supported, many microprocessors provide limited instructions and addressing modes for I/O access. These limitations make programming I/O access is more cumbersome.