Operating Systems Lecture Notes

15 February 2012 • Peripheral Devices

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Outline

The Problem

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• What are these things hanging off the system?
  • And why are they there?
• How does the operating system deal with them?

Peripheral Devices

• Peripheral devices (or peripherals or devices) are information handlers.
• A device may be an information
  • source (keyboard), or
  • a sink (display),
  • a source and a sink (disks).
• A device may store information (tapes), or transmits it (networks).

Devices Differences

• Devices differ drastically in their characteristics.
  • Differ from each other, and from the host system.
• The differences include
  • Qualitative: input only (mouse), output only (audio), or input-output (disk).
  • Quantitative: transfer rates, capacity, reliability.
  • Ephemeral (networks) or not (printers).

Devices In A System

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Management Objectives

• Devices should be managed to provide:
  • Protection - devices are shared objects; they handle unshared information.
  • Convenience - devices are gnarly, varied, and always changing.
  • Efficiency - devices are bottle-necks; they are abstract.
• The central conflict - abstraction vs. control; convenience vs. efficiency

Device Management

• Provide efficient data transfer between device and system.
• I-O system organization - the device driver, the resource manager.
  • Provide interfaces - application, kernel, and device.
  • Operate the device.
  • Keep track of pending requests.
  • Enforce the resource policy.

Device Operation

• Establish data and parameters; start the operation; wait for completion; determine status; handle results; repeat
• Which does what?
  • CPU vs. device.
  • The trend is to move function from the CPU to the device.

CPU-Device Interaction

• The CPU directs the device, and possibly helps with some functions - error recovery.
• Several styles of CPU-device interaction.
  • Polling
  • Interrupts
  • Direct memory access.
  • Channel processors.

Device Polling

• CPU continually tests for completion.
• Doesn’t scale well, difficult to get right, wastes the CPU.

Device Interrupts

• Interrupt-driver driven I-O - efficiently waiting for completion.
• The device informs the CPU when completed.
• Scales well, relatively easy to program, efficient CPU use.

Direct Memory Access

• Direct memory access (DMA) moves data between the device and system.
• Devices may have buffers to store data.
• The CPU can do it - expensive.
• The device can do it - needs access to memory; capable devices.

Memory-Mapped I-O

• How do the CPU and devices communicate?
• Special cpu instructions.
• Writing locations memory - memory-mapped I-O.

Buffering

• Amortize operations over more data - n 1-byte operations vs 1 n-byte operation.
• Smooth out bursty data transmissions.
• Even out the speed disparity - double buffering.

Summary

References

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This page last modified on 2012 February 15.