CS1104 – Computer Organization http://www.comp.nus.edu.sg/~cs1104 Aaron Tan Tuck Choy School of Computing National University of Singapore

Lecture 15: I/O Devices and Buses Anatomy of a Computer Device Examples and Speeds Magnetic Disks Disk Device Terminology Disk Device Performance Disk Transfer Data Rate Synchronizing Processor and I/O Devices Polling Cost of Polling Alternative to Polling Summary CS1104-15 Lecture 15: I/O Devices and Buses

Lecture 15: I/O Devices and Buses Introduction Advantages of Buses Disadvantages of Buses General Organization of a Bus Master versus Slave CS1104-15 Lecture 15: I/O Devices and Buses

I/O: Anatomy of a Computer Five components. Disk (where programs, data live when not running) Processor (active) Computer Control (“brain”) Datapath (“brawn”) Memory (passive) (where data live when running) Devices Input Output Keyboard, Mouse Display, Printer CS1104-15 I/O: Anatomy of a Computer

I/O: Device Examples and Speeds I/O Speed: bytes transferred per second (from mouse to display: million-to-1) Device Behavior Partner Data Rate (Kbytes/sec) Keyboard Input Human 0.01 Mouse Input Human 0.02 Line Printer Output Human 1.00 Floppy disk Storage Machine 50.00 Laser Printer Output Human 100.00 Optical Disk Storage Machine 500.00 Magnetic Disk Storage Machine 10,000.00 Network-LAN I or O Machine 10,000.00 Graphics Display Output Human 30,000.00 CS1104-15 I/O: Device Examples and Speeds

I/O: Magnetic Disks Purpose: Long-term, nonvolatile, inexpensive storage for files Large, inexpensive, slow level in the memory hierarchy Arm Head Actuator Platters CS1104-15 I/O: Magnetic Disks

I/O: Disk Device Terminology Several platters, with information recorded magnetically on both surfaces (usually). Bits recorded in tracks, which in turn divided into sectors (e.g., 512 Bytes) Outer Track Inner Sector Actuator moves head (end of arm,one/surface) over track (“seek”), select surface, wait for sector rotate under head, then read or write. Actuator Head Arm Platters Cylinder: all tracks under heads. CS1104-15 I/O: Disk Device Terminology

I/O: Disk Device Performance Access time = Seek Time + Rotation Time + Transfer Time + Controller Overhead Seek Time? Depends no. tracks move arm, seek speed of disk Rotation Time? Depends on speed disk rotates, how far sector is from head Transfer Time? Depends on data rate (bandwidth) of disk, size of request Platters Outer Track Inner Sector Actuator Head Arm CS1104-15 I/O: Disk Device Performance

I/O: Disk Transfer Data Rate To keep things simple, originally kept same number of sectors per track Since outer track longer, lower bits per inch As competition grew, decided to keep BPI (“constant bit density”) the same for all tracks More capacity per disk more of sectors per track towards edge Since disk spins at constant speed, outer track has faster data rate 1.5X outer track vs. inner track! CS1104-15 I/O: Disk Transfer Data Rate

I/O: Synchronizing Processor and I/O Devices 500 MHz microprocessor can execute a 500 million load or store instructions per second, or 200,000 KB/s data rate. I/O devices from 0.01 KB/s to 30,000 KB/s. Input device may not be ready to send data as fast as the processor loads it. Output device may not be ready to accept data as fast as processor stores it . What to do? CS1104-15 I/O: Synchronizing Processor and I/O Devices

I/O: Polling Processor checks status before acting. Path to device generally has 2 registers: 1 register says its OK to read/write (I/O ready), often called Control Register. 1 register to contain data, often called Data Register. Processor reads from Control Register in loop, waiting for device to set Ready bit in Control Register to say its OK. Processor then loads from (input) or writes to (output) data register. Load from device/Store into Data Register resets Ready bit of Control Register. CS1104-15 I/O: Polling

I/O: Cost of Polling Assume for a processor with a 500-MHz clock, it takes 400 clock cycles for a polling operation (call polling routine, accessing the device, and returning). Determine % of processor time for polling. Mouse: polled 30 times/sec so as not to miss user movement. Floppy disk: transfers data in 2-byte units and has a data rate of 50 KB/second. No data transfer can be missed. “Hard” disk: transfers data in 16-byte chunks and can transfer at 4 MB/second. Again, no transfer can be missed. CS1104-15 I/O: Cost of Polling

I/O: Cost of Polling Mouse Polling Clocks/sec = 30 * 400 = 12000 clocks/sec % Processor for polling: 12*103/500*106 = 0.002%  Polling mouse little impact on processor Times Polling Floppy/sec = 50 KB/s /2B = 25K polls/sec Floppy Polling Clocks/sec = 25K * 400 = 10,000,000 clocks/sec % Processor for polling: 10*106/500*106 = 2%  OK if not too many I/O devices CS1104-15 I/O: Cost of Polling

I/O: Cost of Polling Times Polling Disk/sec = 4 MB/s /16B = 250K polls/sec Disk Polling Clocks/sec = 250K * 400 = 100,000,000 clocks/sec % Processor for polling: 100*106/500*106 = 20%  Unacceptable CS1104-15 I/O: Cost of Polling

I/O: Alternative to Polling Wasteful to have processor spend most of its time “spin-waiting” for I/O to be ready. Wish we could have an unplanned procedure call that would be invoked only when I/O device is ready. Interrupt program when I/O ready, return when done with data transfer. CS1104-15 I/O: Alternative to Polling

I/O: Summary I/O gives computers their 5 senses. I/O speed range is million to one. Processor speed means must synchronize with I/O devices before use. Polling works, but expensive. Interrupts works, but more complex. CS1104-15 I/O: Summary

Bus: Introduction Transit Link bus? A bunch of wires... Shared communication link. Single set of wires used to connect multiple subsystems. A bus is also a fundamental tool for composing large, complex systems. Control Datapath Memory Processor Input Output CS1104-15 Bus: Introduction

Bus: Advantages of Buses Memory Processor I/O Device Versatility: New devices can be added easily Peripherals can be moved between computer systems that use the same bus standard Low Cost: A single set of wires is shared in multiple ways Manage complexity by partitioning the design CS1104-15 Bus: Advantages of Buses

Bus: Disadvantages of Buses It creates a communication bottleneck The bandwidth of that bus can limit the maximum I/O throughput The maximum bus speed is largely limited by: The width of the bus The number of devices on the bus The need to support a range of devices with: Widely varying latencies Widely varying data transfer rates CS1104-15 Bus: Disadvantages of Buses

Bus: General Organization of a Bus Data Lines Control Lines Control lines: Signal requests and acknowledgments Indicate what type of information is on the data lines Data lines carry information between the source and the destination: Data and Addresses CS1104-15 Bus: General Organization of a Bus

Bus: Master versus Slave Master issues command Data can go either way A bus transaction includes two parts: Issuing the command (and address) – request Transferring the data – action Master is the one who starts the bus transaction by: issuing the command (and address) Slave is the one who responds to the address by: Sending data to the master if the master ask for data Receiving data from the master if the master wants to send data CS1104-15 Bus: Master versus Slave

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