Q.What is the primary role of system bus? Why is data bus is bidirectional while address bus is unidirectional?

Picture a large office where files must constantly move between the manager's cabin, the records room, and the reception desk. Instead of building a private corridor between every pair of rooms, the architect builds one main corridor that every room opens onto. Anything travelling between rooms uses this shared corridor. Inside a computer, that shared corridor is called a bus — a common pathway of electrical lines over which the processor, the memory, and the input/output devices exchange information.

A single transfer needs three different kinds of information to travel, and so the system bus is organised as three cooperating buses:

• Data bus — carries the actual content being moved: the number being stored, the instruction being fetched, the character being sent to a device. It is bidirectional, since data flows both into and out of the processor.
• Address bus — carries the location the transfer concerns, the way an envelope carries a house address. When the CPU wants a particular byte from memory, it places that byte's address on the address bus. This bus is one-directional: addresses originate from the processor.
• Control bus — carries the coordination signals that say what is happening and when: read or write, ready or busy, and timing signals that keep everyone in step. Without it, data and addresses would arrive with no instructions on what to do with them.

A helpful way to remember the trio: the data bus says what, the address bus says where, and the control bus says how and when. …