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Direct I/O and memory-mapped I/O
Direct I/O (Intel)
- separate I/O and memory address spaces.
- A control line indicates whether address information is for memory or I/O.
- Intel microprocessors use direct I/O.
Memory-mapped I/O (Motorola)
- I/O and memory share the same address space.
- Motorola microprocessors use memory-mapped I/O.
There are two approaches to I/O mapping in microprocessor design. One approach is to use direct I/O, which separates the I/O address space from memory address space. In direct I/O, the instructions to access memory and I/O are different. The Intel 80x86 family of processors uses this approach. The connection of I/O components to computer buses is very similar to the connection of memory components to computer buses.
The other approach, called memory-mapped I/O, treats I/O ports just like memory locations, letting them share the same address space and using the same instructions to access them. The Motorola 68xxx family of processors uses memory-mapped I/O. Each approach has advantages and disadvantages.