INPUT OUTPUT INTERFACE

                                                                I/O INTERFACE

I/o Devices are another key element of the computer system as each module acts as an interface to the system bus or central switch and controls one or more peripheral devices. 

in other words,I/O module helps communication between the peripherals and the bus

 Why are the peripherals not connected to the system bus directly?

o    Peripherals have different ways of operations. It is not practical to ask the processor to control all different devices.

o    Data transfer rate in devices are much slower than the memory or processor. So it is not practical to link high speed bus directly to the devices.

o    Some devices might have higher speed than the bus, so it is not practical to have a direct link.

o    Peripherals may use different data formats and word lengths than the computer they are attached to.

Functions of a I/O module

I/O Operations

Three techniques are possible for I/O operation:

1.       Programmed I/O

o    Data are exchanged between the processor and the I/O module.

o    When the processor is executing a program and gets and instruction related to I/O, it issues a command to the respective I/O module.

o    The I/O module will perform the requested operation.

o    The I/O will not alert or interrupt the processor once the operation is done.

o    It is the responsibility of the processor to keep on checking the status of the I/O module to see if the operation is complete.

2.     Interrupt driven I/O

o    This method overcomes CPU waiting time.

o    The processor issues an I/O command to a module and then goes on to do some other useful work.

o    The I/O module will then interrupt the processor to request service when is ready to exchange data with the processor.

o    The processor then executes the data transfer and then resumes its processing.

3.       Direct Memory Access (DMA)

o    When large volumes of data are to be moved between memory and the peripherals, an efficient technique is Direct Memory Access (DMA).

DMA Function

o    DMA involves an additional module on the system bus.

o    The DMA module is capable of imitating the processor and taking over control of the system from the processor.

o    It needs to do this to transfer data to and from memory over the system bus.

External Interface

·         Used to connect devices to the I/O module.

·         A common characteristic of interface is whether it is serial or parallel.

·         Parallel Interface

o   Multiple lines connect the peripheral and the I/O module. So multiple bits are transferred at once.

o   Used in high speed peripherals such as hard disk, printer, scanner.

·         Serial Interface

o   There is only one line to transmit data and bits are transferred one at a time.

o   Used in slower devices such as mouse and keyboard.