Final answer:
The data-in register of an I/O port is read by the host to get input, which allows the system to receive data from external devices through the I/O port.
Step-by-step explanation:
The data-in register of an I/O port serves as a buffer between the peripheral device and the system's processor. When considering the functions associated with the data-in register, the correct answer to the provided question is:
a) read by host to get input
The host computer reads from the data-in register to obtain data that has been input from an external device, such as a keyboard, mouse, or any other peripheral. This process is part of the input operation in computer systems, where data is transferred from an external device into the system via the I/O port's data-in register.
To clarify the remaining options, the controller does not read from the data-in register; rather, it writes to it. The host computer writes to the data-out register to send output, not to the data-in register. Starting a command generally involves writing to a control register, not the data-in register.