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10/15: Lecture Topics

Explore the types of I/O devices, communication methods with the processor and memory, I/O performance, and overcoming the I/O bottleneck. Learn about different devices such as mice, magnetic disks, and networks, and delve into topics like mouse motion detection, disk interfaces, networking protocols, and communicating with devices. Understand the impact of various I/O strategies on system performance and efficiency. Discover key concepts like memory-mapped I/O, direct memory access (DMA), and the importance of device-to-processor and device-to-memory communication.

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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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