Write down the difference between isolated I/O and memory mapped I/O. Also discuss advantages and disadvantages of isolated I/O and memory mapped I/O.

Isolated I/O keeps a separate address space for I/O. The CPU uses special instructions (IN and OUT) and a control signal such as IO/M to tell whether the address belongs to memory or to an I/O port.

Memory mapped I/O gives the I/O registers addresses from the same space as memory. The CPU uses the ordinary load and store instructions to talk to devices. No separate control signal is needed.

Difference between the two

Advantages of isolated I/O

• The whole memory address space is free for memory.
• I/O instructions stand out clearly in the program, so the code is easy to read.
• Port address decoding is simple because the number of ports is small.

Disadvantages of isolated I/O

• Only a few instructions (IN and OUT) can work with I/O, and they usually move data only through the accumulator.
• Extra control signals and extra instructions make the CPU more complex.
• Arithmetic or logic on device data cannot be done directly.

Advantages of memory mapped I/O

• All memory instructions work on I/O registers, so one can add, compare, shift or test bits of a device register directly.
• No special instructions or control pins are needed, so the CPU design is simpler.
• Many more I/O devices can be addressed.

Disadvantages of memory mapped I/O

• Device addresses take away a part of the memory space. This matters for a small address space.
• Address decoding is more complex, because the full address must be decoded for each device.
• It is harder to spot I/O operations in a program, since they look like normal memory accesses.

Which one to choose

A small processor with a limited address space and few devices often prefers isolated I/O, so that no memory addresses are lost. A modern processor with a large address space (32 or 64 bits) prefers memory mapped I/O, because the lost addresses hardly matter and the programming is far more flexible.