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10/15: Lecture Topics

10/15: Lecture Topics. Input/Output Types of I/O Devices How devices communicate with the rest of the system communicating with the processor communicating with memory I/O Performance. Why I/O?. Without input, the only data available to programs: random numbers

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10/15: Lecture Topics

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  1. 10/15: Lecture Topics • Input/Output • Types of I/O Devices • How devices communicate with the rest of the system • communicating with the processor • communicating with memory • I/O Performance

  2. Why I/O? • Without input, the only data available to programs: random numbers • For that matter, where will the programs come from? • Without output, there’s no way to determine the result of computations

  3. Types of I/O • Behavior: • input only • output only • input and output • storage • Partner: human or machine • Data rate: from .01 KB/s or less to 60,000 KB/s or more

  4. Three Characteristic Devices • Mouse: input only; human; .01-.02 KB/s • Magnetic disks: storage; machine; 100-10,000 KB/s • Networks: input and output; machine; 500-6000 KB/s

  5. Detecting Mouse Motion • Mechanical • Two perpendictular wheels on the inside connected to potentiometers • Optical • LED and a photodetector on the bottom; requires a special gridded mouse pad • Optomechanical • The wheels have slots; an LED shines through them • Lots of others

  6. Mouse Interfaces • Pointing device must provide: • Status of each button • Rate of motion in X and Y directions • Software must interpret the input • Double clicks • Limits to motion, speed Screen position Device driver Rate of motion Mouse interface Grid lines

  7. Anatomy of a Magnetic Disk read/write heads spindle arm tracks sectors platters cylinder: all tracks at the same radius, two per platter

  8. Accessing Data from a Disk • First, the disk arm moves over the right cylinder: seek time • Next, wait for the data to rotate under the disk arm: rotational delay • Finally, read the data from the disk: transfer time • At this point, the data is in the disk controller and can be transferred to memory

  9. Disk Interface • Only two operations on a disk: • read block, write block • Hidden behind the interface: • block  sector mappings • Unknown to the disk: • contents of the blocks/sectors Files File system interface Blocks Disk interface Sectors

  10. Networks • The device that lets a system connect to a network: network interface card • Listens for data on the network important to this system • Bundles the bits into packets and signals the OS when a packet is complete • Also takes packets from OS and sends them as bits on the wire

  11. Networking Interfaces • OS puts extra packets in to define where stream begins and ends • NIC puts extra bits in to define where packets begin and end Streams Networking protocols Packets NIC interface Bits

  12. Communicating with Devices level 1 cache control main memory level 2 cache functional units registers PC input/ output the chip system bus

  13. System Bus • Lots of variations: • How many bits can be sent simultaneously (bus width)? • How will access to the bus be controlled? • Synchronous (clocked) vs. asynchronous (handshake protocol)

  14. Commands to I/O Devices • Memory-mapped I/O • A special region of memory is set aside for a device • Loads and stores to addresses in this region are interpreted as commands to the device • Provides easy access control via the memory system • Special I/O instructions

  15. Device to Processor Comm. • The device has some information for the processor. Two ways to convey it: • Issue an interrupt • Wait for the processor to ask for it - polling • Which is better, interrupt-driven I/O or polling? Depends on: • time sensitivity of data • whether data is expected

  16. Device to Memory Comm. • Direct Memory Access (DMA) allows devices and memory to communicate without involvement of processor • Processor sets up the transaction • Device and memory transfer the data • Device interrupts processor to signal completion • The processor gets a lot of other work done while transfer is happening

  17. Performance Issues in I/O • Processors double in speed every 18 months • Networks double in speed more slowly, perhaps every 3 years • Disks improve very slowly, because they are limited by mechanical factors

  18. The I/O Bottleneck • System A: processor speed = 100 MHz; disk transfer takes 10 ms • How many clock cycles elapse while disk transfer takes place? • System B: processor speed = 400 MHz; disk transfer still takes 10 ms • How many clock cycles now?

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