Discuss the different modes of data transfer.

Programmed I/O

The CPU controls the whole transfer by a program. It checks the status flag of the device in a loop. When the flag shows ready, the CPU reads or writes the data register. After that it goes back to checking.

This is simple and needs little hardware. But the CPU is kept busy waiting. This is called busy waiting, and it wastes CPU time, especially when the device is slow.

Interrupt-initiated I/O

Here the CPU does not wait. The device sends an interrupt signal when it is ready. The CPU stops its program, saves its state, and runs the service routine that moves the data. Then it returns to the old program.

The CPU can therefore run other programs while the device is busy. The overhead is the time to save and restore the state for every word. So this mode is good for slow or medium speed devices such as keyboards, but not for large blocks of data.

Direct Memory Access (DMA)

The DMA controller moves blocks of data between the device and memory by itself. The CPU only sets up the start address and word count, and the controller interrupts the CPU at the end. Data goes straight over the system bus, so the CPU is free during the transfer.

DMA is the fastest mode and is used for disks, tapes and network cards. It needs extra hardware, and the controller and the CPU compete for the memory bus.

Comparison

A simple illustration

Think of a student waiting for a parcel. In programmed I/O, the student stands at the gate and keeps looking out. In interrupt-initiated I/O, the student studies and goes to the gate only when the doorbell rings. In DMA, a helper collects all the parcels and places them in the room, and tells the student only when the last one is in.

Choosing a mode

If the device is slow and the system is simple, programmed I/O is enough. If the CPU has other jobs to do, interrupts are better. If large amounts of data must move quickly, DMA is the best choice.