Polling

Polling is a process by which a host waits for controller response. It is a looping process, reading the status register over and over until the busy bit of status register becomes clear. The controller uses/sets the busy bit when it is busy working on a command, and clears the busy bit when it is ready to accept the next command. The host signals its wish via the command-ready bit in the command register. The host sets the command-ready bit when a command is available for the controller to execute.

In the following example, the host writes output through a port, coordinating with the controller by handshaking

• The host repeatedly reads the busy bit until that bit becomes clear.
• The host sets the write bit in the command register and writes a byte into the data-out register.
• The host sets the command-ready bit.
• When the controller notices that the command-ready bit is set, it sets the busy bit.
• The controller reads the command register and sees the write command.
• It reads the data-out register to get the byte, and does the I/O to the device.
• The controller clears the command-ready bit, clears the error bit in the status register to indicate that the device I/O succeeded, and clears the busy bit to indicate that it is finished.