Q. What is Input - Output Instructions?

An I/O instruction is stored in memory of computer and is fetched as well as executed by processor producing an I/O-related command for I/O interface. With programmed I/O there is a close correspondence between I/O-related instructions and I/O commands that processor concerns to an I/O interface to execute instructions.

In systems with programmed I/O, I/O interface and the main memory as well as processors generally share system bus. So every I/O interface must interpret address lines to decide if command is for itself. There are two ways for doing so. These are known as isolated I/O and memory-mapped I/O. 

With memory-mapped I/O there is single address space for I/O devices and memory locations. Processor treats status and data registers of I/O interface like memory locations and uses similar machine instructions to access both I/O devices and memory. For a memory-mapped I/O only a single write and a single read line are required for memory or I/O interface write or read operations. These lines are triggered by processor for either I/O device access or memory access. Figure below displays memory-mapped I/O system structure.