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Operating Systems. Input/Output Management. 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.

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operating systems

Operating Systems

Input/Output Management

what is the i o system
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.
the i o bus
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.
i o bus architectures
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.
i o bus architectures1
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.
how i o devices communicate
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.
how i o devices communicate1
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).
i o addresses
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.
i o address assignments
I/O Address Assignments
  • Some I/O address assignments in Windows XP Device Manager.
  • 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).
  • 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.
typical interrupt usage1
Typical Interrupt Usage
  • Interrupt assignments in Windows XP Device Manager.
  • 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.
dma channel assignment
DMA Channel Assignment
  • DMA channel assignment in Windows XP Device Manager.
objectives of the i o system
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
structure of the i o system
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
structure of the i o system1

Application Program

Input-output Control System (IOCS)

I/O Device

Device Driver

Device Controller


System Calls

Operating System

I/O Bus

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
i o buffering
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.