Polling VS Interrupts

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Polling VS Interrupts

Polling

Polling is a method by which a host waits for controller’s response. It is a looping procedure, reading the status-register over and over till the busy bit of status register becomes clear. The controller uses or sets the busy bit when it is busy at work on a command and clears the busy bit when it is ready to take the next command. The host signals its demand via the command-ready bit in the command-register. The host sets the command-ready bit when a command is accessible for the controller to execute. In the given example, the host writes out-put through a port, synchronizing with the controller by hand-shaking

  • The host repetitively reads the busy bit till that bit turn into clear.
  • The host sets the writes a bite into the data-outregister and write-bit in the command-register.
  • The host sets the ready bit command.
  • When the controller gets noticed that the command-re ady bit is set then it sets the busy bit.
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  • The controller reads the command register and then gets the write command.
  • It reads the data-out register to acquire the byte, and does the I/O to the device.
  • The controller clears the command-ready bit and bit in the status register topoint out that the device I/O succeeded, and clears the busy bit to show that it is

Polling

Interrupts

A signal that acquires the attention of the CPU and is typically generated when I/O is required. For example, software interrupts are produced by a program demanding disk input or output.

Common Functions of interrupts

Hardware might trigger an interrupt at any time by sending a signal to the CPU, generally by
way-of the system bus.
■ Software can trigger an interrupt by executing a special operation called a system call also called a monitor call.
■ Whenever the CPU is interrupted, it stops what it is doing and instantly transfers execution to a static location which contains the starting address, where the service routine for the interrupt is situated.

■ The interrupt service routine executes; in the end, the CPU take up again the interrupted
computation.
■ The routine, in turn, would call the interrupt-specific handler.
■ A table for interrupt routine called as interrupt vector which has the addresses of all the
service routines
■ A exception/trap is a software-generated interrupt caused both by an error or a user request.

Interrupt Handling

The operating system reserves the state of the CPU by the program counter and storing registers
■ Defines which type of interrupt has occurred:
– polling
– vectored interrupt system
■ Separate segments of code decide what action must be taken for each type of interrupt.

 Interrupt time-line for a single process doing out-put is shown as:..

Interrupt Handling

Polling Vs Interrupts

Comparison Polling Interrupt
Basic CPU continuously checks device status whether it requires attention of CPU. Device notify CPU that it requires CPU’s attention.
Mechanism It is basically a protocol. It is basically a hardware mechanism.
CPU CPU has to wait and check whether a device requires servicing which waste a lot of CPU’s cycles.  When a device requires servicing the CPU is get disturbed which saves the CPU’s cycles.
Occurrence CPU poll the device at the consistent intervals. It can occurs at any instant of time.
Efficiency Polling becomes inefficient whenever CPU infrequently finds a device ready for service  It becomes inefficient whenever devices keep on interrupting frequently.

 

IO Hardware

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