COA Input Output Organisation

INPUT-OUTPUT ORGANIZATION

  • Asynchronous data transfer

  • Mode of Transfer

  • Priority Interrupt

  • Direct Memory Access

CPU Versus Peripherals

CPU

  • Electronic device

  • Fast data transfer rate

  • Data in the form of Words

Peripherals

  • Electromechanical or electromagnetic devices

  • Slow data transfer rate

  • Data in the form of codes

Input-Output Interface

  • Transfers information between internal storage (CPU & Memory) and external devices.

  • Synchronizes data flow between CPU and peripherals.

Input-Output Interface Communication

  • Typical communication link shows interaction between processor and peripherals.

Types of Commands for Interface

  1. Control Command: Activates peripheral and instructs tasks.

  2. Status Command: Tests status conditions in interface and peripheral.

  3. Data Output Command: Transfers data from bus into a register.

  4. Input Data Command: Receives data from peripheral and buffers it.

Example of I/O Interface

  • Includes:

    • Two data registers (ports)

    • Control register

    • Status register

    • Bus buffers

    • Timing and control circuits

  • Communicates with CPU via data bus.

Asynchronous Data Transfer

  • Internal operations synchronized by common clock pulses.

  • CPU and I/O Interface operate independently—often using their own clocks, resulting in asynchronous operation.

Modes of Asynchronous Data Transfer

Strobe Control

  1. Source Initiated Strobe

  2. Destination initiated Strobe

Handshaking

  1. Source-initiated transfer.

  2. Destination-initiated transfer.

Source-Initiated Strobe for Data Transfer

  • Data bus carries binary information from source to destination.

  • The strobe signals when data is valid on the bus.

Destination-Initiated Strobe for Data Transfer

  • Destination activates strobe, requesting data from the source.

Handshaking Overview

  • Resolves strobe method limitations by introducing a second control signal to confirm data transfer completion.

  • Ensures both units confirm data transfer participation.

Data Transfer Modes

  1. Programmed I/O: CPU monitors peripheral for transfer readiness—time-intensive.

  2. Interrupt Initiated I/O: CPU executes other tasks while waiting for an interrupt request from the interface.

  3. Direct Memory Access (DMA): Allows devices to transfer data without constant CPU monitoring.

Priority Interrupt

  • Establishes priority among multiple requests to determine servicing order.

  • High-speed devices (e.g., disks) prioritized over slower devices (e.g., keyboards).