Input/Output

INPUT and OUTPUT

Definition:

•A bus is a shared communication link, which uses one set of wires to connect multiple subsystems.

• I/O is very much architecture/system dependent

• I/O requires cooperation between

  – processor that issues I/O command (read, write etc.)

  -  buses that provide the interconnection between processor, memory      and I/O devices

  – I/O controllers that handle the specifics of control of each device and      interfacing

  – devices that store data or signal events


TYPE AND CHARACTERISTIC OF INPUT/OUTPUT:

Input devices

-Keyboard :  The data and instructions are input by typing on the keyboard. The          message typed on the keyboard reaches the memory unit of a computer.

-Mouse      :   It’s a pointing device. The mouse is rolled over the mouse pad, which in         turn controls the movement of the cursor in the screen.

• Output devices

-Speaker :    High definition, or low distortion, to create a dynamic impact.

-Printer :      Produces a representation of an electronic document on physical media


Example of Input/Output :

qInput devices              qOutput devices

keyboard:-                    Speaker:-

Mouse:-                     Printer:-

HANDLING I/O INTERRUPT:

• Interrupts: alternative to polling

  - I/O device generates interrupt when status changes, data   ready

  - OS handles interrupts just like exceptions (e.g., page faults)

  - Identity of interrupting I/O device recorded in ECR

• I/O interrupts are asynchronous

  - Not associated with any one

  - Don’t need to be handled immediately

• I/O interrupts are prioritized

  - Synchronous interrupts (e.g., page faults) have highest priority

  - High-bandwidth I/O devices have higher priority than low

INTERRUPT OVERHEAD:

• Parameters

  - 500 MHz CPU

  - Interrupt handler takes 400 cycles

  - Data transfer takes 100 cycles

  - 4 MB/s, 16 B interface disk transfers data only 5% of         time

• Data transfer (x) time

  0.05 * (4M B/s)/(16 B/xfer)*[(100 c/xfer)/(500M c/s)] =           0.25%

• Overhead for polling?

  (4M B/s)/(16 B/poll) * [(400 c/poll)/(500M c/s)] = 20%

• Overhead for interrupts?

  + 0.05 * (4M B/s)/(16 B/poll) * [(400 c/poll)/(500M c/s)] =    1%