Introduction to I/O Devices and Buses: Overview, Types, and Performance Equations

1. CDA 3101 Spring 2016Introduction to Computer Organization I/O Devices and Buses 31 March, 5 April 2016

2. Overview of I/O and Buses • Movement of data and instructions • Parallel or Serial buses • Different I/O protocols • I/O devices vary widely • Bus Performance Equations • Types of I/Os • Polled: Device-specific queries • Interrupt-Driven: On-demand transfer • DMA: Fast transfer, block I/O

3. Bus Configuration • Transmitter/Receiver: Source/Sink for data • Channel: Path for data or instructions • Controller: Master control for bus actions Register File BUS Memory sw Base Reg Offset lw

4. I/O Types • Polled • Check each I/O device in turn, schedule I/O work on appropriate idle devices • Ask for device status (busy, wait, idle, dead…) • Interrupt-Driven • On-demand request of I/O services • Needs queue to save waiting I/O requests • Direct Memory Access (DMA) • Direct I/O Device to Memory Transfer • Very fast, used for large amounts of data (e.g., video, imagery, audio)

5. Physical vs. Logical I/O Buses • Physical Buses • Installed in Computer: ISA bus for graphics card • Limited by size and power supply of processor • Performance limited by bus width, controller speed • Logical I/O (reconfigurable buses) • Have physical buses available that can be configured differently • Useful for maximizing bus utilization • Make the CPU believe it has variable buses • Reconfigure buses to meet current I/O demand • Requires complex bus controller and scheduling software

6. Physical Buses N • Parallel Bus • N wires in parallel: • Advantage: Fast Disadvantage: Complexity • Serial Bus • One transmission path • Usually twisted pair (for alternating current) • Can be shielded (coaxial cable or Ethernet) • Simplex communication: Unidirectional • Duplex communication: Bi-directional • Problems: Collision (duplex), Errors, Faults • Advantage: Simplicity Disadvantage: Slow

7. Bus Performance Parameters • Bandwidth or Throughput • How much data can be transmitted through the bus per unit time (Units = Bits per Second) • Failure Rate and Cost • Rate: Probability of Bus failing • Cost: How much overhead required to restart • Need to flush bus (cycles of bus x bits per cycle) • Resend corrupted packets (bits to be resent) • Error Rate and Cost • Similar to Failure Rate

8. Bus Performance Equations • Assumptions • Bandwidth (B), Bit Error Rate (BER) • Failure Rate (FR), Failure Cost (FC) in bits/sec • Computation of Actual Bandwidth (B’) B’ = B x (1-BER) - FC/(1-FR) Error Effect Failure Penalty

9. Bus Performance Example #1 • Assumptions • Nominal Bandwidth: 32 MHz, 32 bits parallel • Failure Rate = 10-4 ; Failure Cost = 0.2 Mbps • Bit Error Rate = 10-6 • Computation of Actual Bandwidth (B’) B’ = B x (1-BER) - FC/(1-FR) = 32bits(32 x 106 Hz) (0.999999) - (0.2 x 106 bps/ 0.9999) = 1.023999 Gbps – 0.20002 Mbps = 1023.799 Gbits/sec = 0.02% penalty

10. Bus Performance Example #2 • Assumptions • Nominal Bandwidth: 32 MHz, 32 bits parallel • Failure Rate = 10-1 ; Failure Cost = 0.2 Mbps • Bit Error Rate = 10-6 • Computation of Actual Bandwidth (B’) B’ = B x (1-BER) - FC/(1-FR) = 32bits(32 x 106 Hz) (0.999999) - (0.2 x 106 bps/ 0.9) = 1.023999 Gbps – 0.2222 Mbps = 1023.7768 Gbits/sec = 0.03% penalty

11. Bus Performance Example #3 • Assumptions • Nominal Bandwidth: 32 MHz, 32 bits parallel • Failure Rate =10-1 ; Failure Cost = 0.2 Mbps • Bit Error Rate = 10-3 • Computation of Actual Bandwidth (B’) B’ = B x (1-BER) - FC/(1-FR) = 32bits(32 x 106 Hz) (0.999) - (0.2 x 106 bps/ 0.9) = 1.022976 Gbps – 0.2222 Mbps = 1023.7538 Gbits/sec = 0.23% penalty

12. Bus Performance Reality • Problems • Bus collision: Packets try to use same HW • Simplex: Less Collision, Duplex: More collision • Some practical test results PCI: 32 bits parallel at 32 MHz (128MB/s) Nominal BW = 1K MHz ~ 1GHz Simplex: 70-80% of BW Duplex: 20-40% of BW e.g., Duplex => 25 to 50 MB/s

13. Bus: Application => Requirements • Applications • Imaging: MxN pixels per frame, K bits per pixel • Video: F frames per second • Complexity = O(MNKF) bits per second Real: M,N = 1024, K = 24 bpp, F = 30 fps MNKF = 1M (720) = 720 Mbits/sec Simplex: 720 Mbps / 0.7 => B = 1.03 Gbps Duplex: 720 Mbps / 0.3 => B = 2.4 Gbps Lesson: Fast buses are required for imaging

14. Bus Technologies • Current • Copper wires, Traces on circuit board (2-8 GHz) • Coaxial, Fiber optic Internet (100Mbps to 2 Gbps) • Emerging • Free space optical: 20+ Gbps • BW limited by transmitter/receiver bandwidth • Problems with atmospheric scattering & absorption • “All optical” • Isn’t so – need some electronics and electro-optics • Speed limited by bandwidth of electronics • Foreseeable Future: Fiber Optic, Wireless

15. Conclusions • I/O effects data transfer by: • Dividing data stream into blocks or packets • Sending data packets serially along a bus • Keeping the I/O bus almost always full (occupied) to maximize I/O system throughput • Reconfigurable Buses are useful because: • Different I/O intensive applications have different I/O requirements • Bus configuration can adapt to meet requirements Next: Multiprocessors

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