Operating Systems

1
Operating Systems

• Input/Output Management

2
What is the I/O System

• A collection of devices that different
  sub-systems of a computer use to communicate with
  each other.
• Inputs are the signals received by the device,
  and outputs are the signals sent from it.
• Input Device keyboard, mouse.
• Output Device monitor, printer.

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The I/O Bus

• The CPU communicates with the I/O system by means
  of an I/O bus.
• The I/O bus is simply a common set of wires that
  connect all the I/O devices to the CPU. These
  wires are used to transmit data, housekeeping
  signals (such as clock pulses), addresses and
  instructions.
• The size or width of a bus is how many bits it
  carries in parallel.
• The speed of a bus is how fast it moves data
  along the path. This is usually measured in MHz.

4
I/O Bus Architectures

• The ISA (Industry Standard Architecture) bus was
  first introduced in1984. It had a 16-bit width
  and ran at a speed of 8 MHz.
• EISA (Extended ISA) was introduced in 1988 as an
  extension to the ISA standard. It had a 32-bit
  width but only ran at 8 MHz to be compatible with
  ISA devices.

5
I/O Bus Architectures

• PCI (Peripheral Component Interconnect) was
  created in 1993. PCI is available in both a 32
  bit version running at 33 MHz and a 64 bit
  version running at 66 MHz. PCI is currently the
  standard bus for PCs.
• AGP (Accelerated Graphics Port) was created in
  1997. The first version of AGP, now called AGP
  1.0 or AGP 1x, had a 32-bit width and operated at
  66 MHz. Newer versions of AGP increase the speed
  up to 266 MHz. AGP is used only for video
  controllers.

6
How I/O Devices Communicate

• The O/S sends commands or data to an I/O device
  by writing to its device registers.
• The O/S retrieves status or data from an I/O
  device by reading from its registers.
• Remember, registers are like memory storage
  spaces.

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How I/O Devices Communicate

• The address which the O/S uses to communicate
  with an I/O device is called the I/O address or
  the I/O port.
• I/O devices use interrupts (IRQs Interrupt
  ReQuests) to signal to the CPU that a task has
  been completed. Interrupts enable I/O devices to
  operate independently of, and at the same time
  with the CPU.
• On ISA buses, data can be sent directly from the
  I/O controller to memory without the involvement
  of the CPU.
• This is referred to as DMA (Direct Memory Access).

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I/O Addresses

• If more than one I/O device attempt to use the
  same I/O address an I/O conflict occurs. This
  can cause information to get mixed up and
  overwritten.
• I/O addresses vary in size, from 4 to 32 bytes.

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I/O Address Assignments

• Some I/O address assignments in Windows XP Device
  Manager.

10
Interrupts

• Device interrupts are fed to the processor using
  an interrupt controller.
• The interrupt controller has 8 input lines that
  take requests from one of 8 different devices.
  The controller then passes the request on to the
  processor, telling it which device issued the
  request.
• Modern PCs have 2 interrupt controllers. The 2nd
  controller is cascaded onto the 1st through input
  line 2 (IRQ2).

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Interrupts

• Interrupts 0, 1, 2, 8 and 13 are reserved for
  internal use, the remainder are used by I/O
  devices.
• On an ISA bus when more than one I/O device
  attempt to use the same interrupt at the same
  time an interrupt conflict occurs. The CPU is
  unable to determine which device raised the
  interrupt.
• Devices on a PCI bus can share interrupts.

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Interrupts
13
Typical Interrupt Usage
14
Typical Interrupt Usage

• Interrupt assignments in Windows XP Device
  Manager.

15
DMA

• DMA transfers are managed by a DMA controller. A
  DMA controller has 4 channels, numbered 0 to 3.
• Most PCs have 2 DMA controllers, with the 1st
  controller cascaded to the 2nd on channel 0.
  This leaves 7 usable DMA channels.
• A DMA conflict arises if two I/O devices try to
  use the same DMA channel at the same time.
• DMA channel 0 is reserved for system use.

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DMA
17
DMA Channel Assignment

• DMA channel assignment in Windows XP Device
  Manager.

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Objectives of the I/O System

• Efficiency
• I/O devices must do useful work at the maximum
  rate.
• Device independence
• programs can access any I/O device without
  specifying device in advance.
• Uniform naming
• name of I/O device is independent of how the
  device is manufactured.
• Error handling
• What to do if something goes wrong!
• Retransmitting data

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Structure of the I/O System

• Input-output control system (IOCS)
• The part of the O/S that deals with I/O activity.
• Performs initial processing and validation on the
  I/O request from the application and routes it to
  the appropriate device driver at the next stage.
• Device Driver
• A software module which manages the communication
  with, and control of, a specific I/O device.
• It converts requests from the application to
  specific commands to the I/O device.
• Device drivers are considered to be part of the
  O/S.
• Frequently written in assembly language

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Structure of the I/O System

• Device Controller
• Hardware device that is attached to the I/O bus
  and provides an interface between the computer
  and the I/O device.
• Responsible for sending data to the device in a
  way the device will understand

21
I/O Buffering

• A buffer is an intermediate main memory storage
  area under the control of the O/S.
• A buffer holds data in-transit between a process
  memory area and an I/O device.
• More than two buffers can be used to let the I/O
  activity keep up with the CPU processing.
• The buffers are organised into a circular queue
  with data being transferred into the queue at one
  end and being moved out of the queue at the other.