Input Output interface Computer Science Engineering (CSE) Notes | EduRev

Computer Architecture & Organisation (CAO)

Computer Science Engineering (CSE) : Input Output interface Computer Science Engineering (CSE) Notes | EduRev

The document Input Output interface Computer Science Engineering (CSE) Notes | EduRev is a part of the Computer Science Engineering (CSE) Course Computer Architecture & Organisation (CAO).
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Input-Output interface

  • Input-Output interface provides a method for transferring information between internal storage (such as memory and CPU registers) and external I/O devices.
  • Peripherals connected to a computer need special communication links for interfacing them with the central processing unit.
  • The communication link resolves the following differences between the computer and peripheral devices.
    • Devices and signals
      Peripherals - Electromechanical Devices
      CPU or Memory - Electronic Device
    • Data Transfer Rate
      Peripherals - Usually slower
      CPU or Memory - Usually faster than peripherals
      Some kinds of Synchronization mechanism may be needed
    • Unit of Information Peripherals - Byte CPU or Memory - Word
    • Operating Modes Peripherals - Autonomous, Asynchronous
      CPU or Memory – Synchronous
  • To resolve these differences, computer systems include special hardware components (Interfaces) between the CPU and peripherals to supervise and synchronize all input and output interfaces. 

Input Output interface Computer Science Engineering (CSE) Notes | EduRev

  • Interface performs the following:
    • Decodes the device address (device code)
    • Decodes the commands (operation)
    • Provides signals for the peripheral controller
    • Synchronizes the data flow and supervises   the transfer rate between peripheral and CPU or Memory
  • I/O commands that the interface may receive:
    • Control command: issued to activate the peripheral and to inform it what to do.
    • Status command: used to test various status conditions in the interface and the peripheral.
    • Output data: causes the interface to respond by transferring data from the bus into one of its registers.
    • Input data: is the opposite of the data output.

I/O versus Memory Bus

  • Computer buses can be used to communicate with memory and I/O in three ways:
    • Use two separate buses, one for memory and other for I/O. In this method, all data, address and control lines would be separate for memory and I/O.
    • Use one common bus for both memory and I/O but have separate control lines. There is a separate read and write lines; I/O read and I/O write for I/O and memory read and memory write for memory.
    • Use a common bus for memory and I/O with common control line. This I/O configuration is called memory mapped.

Isolated I/O versus Memory Mapped I/O 

  •  Isolated I/O
    • Separate I/O read/write control lines in addition to memory read/write control lines
    • Separate (isolated) memory and I/O address spaces
    • Distinct input and output instructions
  • Memory-mapped I/O
    • A single set of read/write control lines (no distinction between memory and I/O transfer)
    • Memory and I/O addresses share the common address space which reduces memory address range available
    • No specific input or output instruction so the same memory reference instructions can be used for I/O transfers
    • Considerable flexibility in handling I/O operations

 

Example of I/O Interface

Input Output interface Computer Science Engineering (CSE) Notes | EduRev

  • Information in each port can be assigned a meaning depending on the mode of operation of the I/O device
    • Port A = Data; Port B = Command;  Port C = Status
  • CPU initializes (loads) each port by transferring a byte to the Control Register
    • Allows CPU can define the mode of operation of each port
    • Programmable Port: By changing the bits in the control register, it is possible to change the interface characteristics
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