Input-Output Organization

Kanika Lakhani
Temporary Assistant Professor
Dept. of Computer Science
Faculty Of Technology & Engg.
Maharaja Sayajirao University
kanika.lakhani-cse@msubaroda.ac.in
Input - Output Interface
• 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:-
1. Peripherals are electromechnical and electromagnetic devices and CPU and
memory are electronic devices. Therefore, a conversion of signal values may be
needed.
2. The data transfer rate of peripherals is usually slower than the transfer rate of CPU
and consequently, a synchronization mechanism may be needed.
3. Data codes and formats in the peripherals differ from the word format in the CPU
and memory.
4. 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 BUS and Interface Module
• It defines the typical link between the processor and several peripherals.
• 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:
a. Control command- A control command is issued to activate the peripheral
and to inform it what to do.
b. Status command- A status command is used to test various status
conditions in the interface and the peripheral.
c. Data Output command- A data output command causes the interface to
respond by transferring data from the bus into one of its registers.
d. Data Input command- The data input command is the opposite of the data
output.

In this case the interface receives on 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:
a. Use two Separate buses , one for memory and other for I/O.
b. Use one common bus for both memory and I/O but separate control lines for
each.
c. Use one common bus for memory and I/O with common control lines.

I/O Processor:
• In the first method, the computer has independent sets of data, address and control
buses one for accessing memory and other for I/O.
• This is done in computers that provides a separate I/O processor (IOP).
• The purpose of IOP is to provide an independent pathway for the transfer of
information between external device and internal memory.
Asynchronous Data Transfer
• This Scheme is used when speed of I/O devices do not match with
microprocessor, and timing characteristics of I/O devices is not predictable.
• In this method, process initiates the device and check its status.
• As a result, CPU has to wait till I/O device is ready to transfer data.
• When device is ready CPU issues instruction for I/O transfer.
• In this method two types of techniques are used based on signals before data
transfer.
i. Strobe Control
ii. Handshaking
• In the block diagram fig. (a), the data bus carries the binary information
from source to destination unit.
• Typically, the bus has multiple lines to transfer an entire byte or word.
• The strobe is a single line that informs the destination unit when a valid data
word is available.

• The timing diagram fig. (b) the source unit first places the data on the data
bus.
• The information on the data bus and strobe signal remain in the active state
to allow the destination unit to receive the data.
Data Transfer Initiated by Destination Unit
• In this method, the destination unit activates the strobe pulse, to informing
the source to provide the data.
• The source will respond by placing the requested binary information on the
data bus.
• The data must be valid and remain in the bus long enough for the
destination unit to accept it.
• When accepted the destination unit then disables the strobe and the source
unit removes the data from the bus.
Disadvantage of Strobe Signal

• The disadvantage of the strobe method is that, the source unit initiates
the transfer has no way of knowing whether the destination unit has
actually received the data item that was places in the bus.

• Similarly, a destination unit that initiates the transfer has no way of

knowing whether the source unit has actually placed the data on bus.

• The Handshaking method solves this problem.

Handshaking
• The handshaking method solves the problem of strobe method by introducing a
second control signal that provides a reply to the unit that initiates the transfer.

Principle of Handshaking: The basic principle of the two-wire handshaking

method of data transfer is as follow:
• One control line is in the same direction as the data flows in the bus from the
source to destination.
• It is used by source unit to inform the destination unit whether there a valid data in
the bus.
• The other control line is in the other direction from the destination to the source.
• It is used by the destination unit to inform the source whether it can accept the
data.
• The sequence of control during the transfer depends on the unit that initiates the
transfer
Source Initiated Transfer using Handshaking

• The sequence of events shows four possible states that the system can
be at any given time.
• The source unit initiates the transfer by placing the data on the bus and
enabling its data valid signal.
• The data accepted signal is activated by the destination unit after it
accepts the data from the bus.
• The source unit then disables its data accepted signal and the system
goes into its initial state.
Destination Initiated Transfer Using Handshaking

• The name of the signal generated by the destination unit has been changed to
ready for data to reflects its new meaning.
• The source unit in this case does not place data on the bus until after it receives the
ready for data signal from the destination unit.
• From there on, the handshaking procedure follows the same pattern as in the
source initiated case.

The only difference between the Source Initiated and the Destination Initiated
transfer is in their choice of Initial sate.
Advantage of the Handshaking method
• The Handshaking scheme provides degree of flexibility and reliability
because the successful completion of data transfer relies on active
participation by both units.

• If any of one unit is faulty, the data transfer will not be completed.
Such an error can be detected by means of a Timeout mechanism
which provides an alarm if the data is not completed within time.