- Polling
is often referred to as the polled operation.
- When the statuses of the external
devices are actively sampled by a client program just like a synchronous
activity is referred to as the polling.
- The common use of the polling is in the
input and output operations.
- In rare cases, polling is also called as the
software driven I/O or just simply as polled I/O.
- As and when required, polling
is also carried out with the busy waiting synonymous.
- Polling is then
referred to as the busy–wait polling.
- In this case whenever it is required to
carry out an input/ output operation, the system just checks the status of the
device required for fulfilling this operation until it is idle.
- When it becomes
idle it is accessed by the I/O operation.
- Such polling may also refer to a
state in which the status of the device is checked again and again for
accessing it if idle.
- If the device is occupied, the system is forced to return
to some other pending task.
- In this case the CPU time is wasted less when
compared to what happens in busy waiting.
- However, this is not a better
alternative to interrupt driven I/O polling.
- In single purpose systems that
are too simple, using busy-wait polling is perfectly fine if the system
cannot take any action until the I/O device has been accessed.
- But
traditionally, the polling was thought to be a consequence of the operating
systems and simple hardware that do not support multitasking.
- The polling
works intimately with the low level hardware usually.
- For example, a parallel
printer port can be polled for checking whether or not it is ready for printing
another character.
- This involves just the examination of a bit.
- The bit to be
examined represents the high or low voltage stage of the single wire in the
cable of the printer during the time of reading.
- The I/O instruction by which
this byte is read is also responsible for transferring the voltage state
directly to the eight flip flops or circuits.
- These 8 flip flops together
constitute one byte of a register of CPU.
Polling also has a number of
disadvantages.
- One is that there is limited time for servicing the I/O devices.
- Polling has to be done within this time period only.
- But in some cases there
are many devices to be checked which cause the polling time to exceed the given
limit.
- The host keeps on hitting the busy bit until the device becomes idle or
clear.
- When the device is idle, the state is written in to the command register
and also in the data out register.
- The command ready bit is set to 1.
- The
controller sets the busy bit once it knows that the command ready bit has
been set.
- After reading from the command
register, the controller carries out the required I/O operation on the device.
- On
the other hand, if the read bit has been set to one, the controller loads the
device data in to the data in register.
- This data is further read by the
host.
- Once the whole action has been completed, the command ready bit is
cleared by the controller.
- The error bit is also cleared for showing that the
operation has been completed successfully.
- At the end the busy bit is also set.
- Polling can be seen in the terms of master slave scenario where the master
sends inquiring about the working status slave devices i.e., whether they are
clear or engaged.
