Programmed I/O

• Simplest and least expensive scheme
• Device controller
• Bus
  • Address bus To select a memory location or I/O device
  • Data bus To transfer data
• Hardware buffer
• Handshaking protocol
• Disadvantages
  • Poor resource utilization
  • Only one device active at a time
  • Gross mismatch between the speeds of CPU and I/O devices

CPU and I/O Overlap

• Hardware flag -- Blocks CPU if device is busy
• Polling is done by test-device flag
• Interrupt driven I/O