I/O ports on ISA bus

• Each device connected to the I/O bus has its own set of I/O addresses called I/O ports

• The I/O address space provides up to 65536 8-bit I/O ports

• Four special assembly language instructions called in, ins, out, and outs allow the CPU to read from and write to an I/O port

• The I/O ports of each device are structured into a set of specialized registers: control, status, input, and output