Peripheral Devices

Peripheral Device is defined as the device which provides input/output functions for a computer and serves as an auxiliary computer device without computing-intensive functionality.

Generally peripheral devices, however, are not essential for the computer to perform its basic tasks, they can be thought of as an enhancement to the user’s experience. A peripheral device is a device that is connected to a computer system but is not part of the core computer system architecture. Generally, more people use the term peripheral more loosely to refer to a device external to the computer case.

Classification of Peripheral devices:

It is generally classified into 3 basic categories which are given below:

  1. Input Devices:
    The input devices is defined as it converts incoming data and instructions into a pattern of electrical signals in binary code that are comprehensible to a digital computer. Example: Keyboard, mouse, scanner, etc.
  2. Output Devices: An output device is generally reverse of the input process and generally translating the digitized signals into a form intelligible to the user. The output device is also performed for sending data from one computer system to another. For some time punched-card and paper-tape readers were extensively used for input, but these have now been supplanted by more efficient devices. Example: Monitors, Printers, etc.
  3. Storage Devices: Storage devices are used to store data in the system which is required for performing any operation in the system. The storage device is one of the most requirement devices and also provide better compatibility. Example: Hard Disk, Flash Memory, etc.

Advantage of Peripherals Devices:

Peripherals devices provides more feature due to this operation of the system is easy. These are given below:

  • It is helpful for taking input very easily.
  • It is also provided a specific output.
  • It has a storage device for storing information or data
  • It also improves the efficiency of the system.

Input-output subsystems

The Input/output organization of computer depends upon the size of computer and the peripherals connected to it. The I/O Subsystem of the computer provides an efficient mode of communication between the central system and the outside environment.

The most common input output devices are: Monitor, Keyboard, Mouse, Printer, Magnetic tapes Input Output Interface provides a method for transferring information between internal storage and external I/O devices. Peripherals connected to a computer need special communication links for interfacing them with the central processing unit. The purpose of communication link is to resolve the differences that exist between the central computer and each peripheral.

The Major Differences are:-
 Peripherals are electromechnical and electromagnetic devices and CPU and memory are electronic devices. Therefore, a conversion of signal values may be needed.
 The data transfer rate of peripherals is usually slower than the transfer rate of CPU and consequently, a synchronization mechanism may be needed.
 Data codes and formats in the peripherals differ from the word format in the CPU and memory.
 The operating modes of peripherals are different from each other and must be controlled so as not to disturb the operation of other peripherals connected to the CPU.

To resolve these differences, computer systems include special hardware components between the CPU and Peripherals to supervises and synchronizes all input and out transfers. These components are called Interface Units because they interface between the processor bus and the peripheral devices.

I/O Device Interface

The I/O Bus consists of data lines, address lines and control lines. The I/O bus from the processor is attached to all peripherals interface. To communicate with a particular device, the processor places a device address on address lines.

Each Interface decodes the address and control received from the I/O bus, interprets them for peripherals and provides signals for the peripheral controller. It is also synchronizes the data flow and supervises the transfer between peripheral and processor. Each peripheral has its own controller.

For example, the printer controller controls the paper motion, the print timing. The control lines are referred as I/O command. The commands are as following:

  • Control command- A control command is issued to activate the peripheral and to inform it what to do.
  • Status command- A status command is used to test various status conditions in the interface and the peripheral.
  • Data Output command- A data output command causes the interface to respond by transferring data from the bus into one of its registers.
  • Data Input command- The data input command is the opposite of the data output.

In this case the interface receives an item of data from the peripheral and places it in its buffer register.

To communicate with I/O, the processor must communicate with the memory unit. Like the I/O bus, the memory bus contains data, address and read/write control lines. There are 3 ways that computer buses can be used to communicate with memory and I/O:

  1. Use two Separate buses, one for memory and other for I/O.
  2. Use one common bus for both memory and I/O but separate control lines for each.
  3. Use one common bus for memory and I/O with common control lines.
I/O interface for an input device

Programmed I/O Mode

In this mode of data transfer the operations are the results in I/O instructions which is a part of computer program. Each data transfer is initiated by a instruction in the program. Normally the transfer is from a CPU register to peripheral device or vice-versa. Once the data is initiated the CPU starts monitoring the interface to see when next transfer can made. The instructions of the program keep close tabs on everything that takes place in the interface unit and the I/O devices.

The transfer of data requires three instructions:

  • Read the status register.
  • Check the status of the flag bit and branch to step 1 if not set or to step 3 if set.
  • Read the data register.

In this technique CPU is responsible for executing data from the memory for output and storing data in memory for executing of Programmed I/O as shown in Figure.

Drawback of the Programmed I/O: The main drawback of the Program Initiated I/O was that the CPU has to monitor the units all the times when the program is executing. Thus the CPU stays in a program loop until the I/O unit indicates that it is ready for data transfer. This is a time consuming process and the CPU time is wasted a lot in keeping an eye to the executing of program.

Interrupt-Initiated I/O

In this method an interrupt facility an interrupt command is used to inform the device about the start and end of transfer. In the meantime the CPU executes other program. When the interface determines that the device is ready for data transfer it generates an Interrupt Request and sends it to the computer.

When the CPU receives such an signal, it temporarily stops the execution of the program and branches to a service program to process the I/O transfer and after completing it returns back to task, what it was originally performing.

In this type of I/O, computer does not check the flag. It continues to perform its task. Whenever any device wants the attention, it sends the interrupt signal to the CPU. CPU then deviates from what it was doing, store the return address from PC and branch to the address of the subroutine.

There are two ways of choosing the branch address:

  • Vectored Interrupt: In vectored interrupt the source that interrupts the CPU provides the branch information. This information is called interrupt vectored.
  • Non-vectored Interrupt: In non-vectored interrupt, the branch address is assigned to the fixed address in the memory.

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