Input Output interface | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE) PDF Download

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. 

• 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

• 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