Why IO devices cannot be connected directly to the system bus?

Ans: The IO devices can't be directly connected to the system bus because the following reason

A) The data transfer rate of IO devices is slower that of CPU.

B) The IO devices in computer system has different data formats and work lengths that of CPU. So it is essential to use a module between system bus and IO device which is called IO module or IO system

Write the factors considered in designing an I/O subsystem?

 Ans:

1. Data transfer: Amount, rate to or from device.

 2. Data Location: Device selection, address of data within device (track, sector etc)

3. Memory or between an I/O device and CPU.

4. Synchronization: Output only when device is ready, input only when

Described Direct Memory Access

Ans: A modest increase in hardware enables an IO device to transfer a block of information to /from memory without CPU intervention. This task needs the IO device to produce memory addresses and transfer data via the bus by using interface controllers.