External Devices

• šHuman readable - to communicate with the  computer users.

  Example : Screen, keyboard, printer

• Machine readable - što communicate with the  equipment.

  Example : magnetic disk, sensors and actuators

• šCommunication - for communicating with remote devices, a machine readable device or another computer.

  Example : Network Interface Card in Modem

I/O Module Function

• Control and timing - to coordinate flow of traffic between external devices and internal resources

• šProcessor Communication - command decoding in I/O module, data exchange through data bus, status reporting about peripherals, and CPU assigning unique address for each I/O module.

• šDevice Communication - involves commands, status information and data

• šData Buffering - the data are buffered in the I/O module and then sent to the peripheral device at its data rate.

• šError Detection - for reporting errors to the processor 

I/O Mapping

I/O Techniques

A) Programmed I/O

- Data are exchanged between the processor and the I/O module

- 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.

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

- 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.


I/O Commands

• šControl – used to activate a peripheral and tell it what to do.
• šTest – Used to test various status conditions associated with an I/O module and its peripherals.
• šRead – Causes the I/O module to obtain an item of data from the peripheral and place it in an internal buffer
• Write – Causes the I/O module to take an item of data form the data bus and subsequently transmit that data item to the peripheral

Advantage

• šSimple method and requires small amount of special I/O hardware.

Disadvantage

• šThis technique is it is a time consuming process that keeps the processor busy needlessly.

 B) Interrupt driven I/O

• This method overcomes CPU waiting time.

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

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

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

4 general categories of techniques

• Multiple interrupt lines
• Software poll
• Daisy chain
• Bus arbitration

C) Direct Memory Access (DMA)

When large volumes of data are to be moved between memory and the peripherals, an efficient technique is DMA.

Function

• an additional module on the system bus.

• capable of imitating the processor and taking over control of the system from the processor.

• transfer data to and from memory over the system bus.

• get hold of the bus only when the processor does not need it

• the number of words to be read or written communicated via the data lines and stored in the data count register. 

External Interface

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

Parallel & Serial Interface

 - Parallel Interface

• Multiple lines connect the peripheral and the I/O module. So multiple bits are transferred at once.
• Used in high speed peripherals such as hard disk, printer, scanner.

- Serial Interface

• There is only one line to transmit data and bits are transferred one at a time.
• Used in slower devices such as mouse and keyboard.