What is Polling?

- 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.