The art of I/O system design is to find a design that meets goals for cost, dependability and variety of devices while avoiding bottlenecks in I/O performance. Avoiding bottlenecks means that components must be balanced between main memory and I/O devices, because performance - and hence effective cost performance - can only be good at the weakest link in the I/O chain.
 

The architect must plan for expansion, both in numbers and types of I/O devices, so that customers can tailor the I/O to their applications. Finally, storage must be dependable, adding new constraints on proposed designs.

  Below is a series of steps to follow in designing an I/O system. The answers to each step may be dictated by market requirements or simply by cost, performance, and availability goals.

• List the different types of I/O devices to be connected to the machine, or list the standard buses that the machine will support
• List the physical requirements for each I/O device. Requirements include size, power, connectors, bus slots, expansion cabinets, and so on
• List the cost of each I/O device, including the portion of cost of any controller needed for this device.
• List the reliability of each I/O device.
• Record the CPU resource demands of each I/O device. This list should include
  • Clock cycles for instructions used to initiate an I/O, to support operation of an I/O device (such as handling interrupts), and to complete I/O
  • CPU stalls due to waiting for I/O to finish using the memory, bus, or cache
  • CPU clock cycles to recover from an I/O activity, such as cache flush
• List the memory and I/O bus resource demands of each I/O device. NOTE that even when the CPU is not using the memory, the bandwidth of main memory and the I/O bus is limited.
• Assess the performance and availability of the differnt ways to organize these I/O devices. Finally select the best organization.

The diagram below shows an example of I/O organization of a computer system.