What is the Difference between memory mapped IO and peripheral mapped IO?

A memory mapped IO device is an IO device that responds to a specific address when IO/M- is low. A peripheral (or IO) mapped IO device is an IO device that responds to a specific address when IO/M- is high.

Many system designers ignore IO/M- in favor of memory mapped IO.

• This eliminates one term in the chip select logic for every device.
• This allows you to use all addressing modes and instructions when manipulating an IO device, as opposed to using only IN and OUT.
• This allows you to potentially have more than 256 different IO devices.

The downside is that you reduce the addressable main memory in the system, i.e. you cannot have all 64K available to you, but this is not generally a problem in most controller designs. You also must decode 16 address lines instead of just 8 when accessing the device.