In a typical microcomputer, what is true about the directionality of the address bus compared with the data bus?

Introduction / Context:
Computers connect the central processing unit to memory and peripheral devices using buses. Two main buses are the address bus and the data bus. Understanding how signals travel on these buses is important for analyzing how the processor reads from and writes to memory. This question focuses on the direction of signal flow on each bus.

Given Data / Assumptions:

• The CPU acts as the master that initiates memory and input or output transactions.
• The address bus carries addresses that select specific memory locations or device registers.
• The data bus carries actual data values that are read or written.

Concept / Approach:
For any memory or I or O operation, the CPU must specify which location or device it intends to access. It does this by placing an address on the address bus. The address always originates from the CPU and is sent outward to memory and devices. There is no need for memory to send addresses back to the CPU. Therefore, the address bus can be unidirectional from CPU to the rest of the system. The data bus, however, must support write operations, where data flows from CPU to memory, and read operations, where data flows from memory back to CPU. This requires the data bus to be bidirectional.

Step-by-Step Solution:
Step 1: Consider a memory write. The CPU puts both an address and a data value on the buses. The address goes to memory, and the data is written to that location.Step 2: In this case, both address and data originate from the CPU, but the address is never returned from memory.Step 3: Consider a memory read. The CPU still sends an address on the address bus to select the location.Step 4: Memory decodes the address and places the requested data on the data bus, which now flows in the opposite direction, from memory to CPU.Step 5: Conclude that the address bus operates in one direction only, while the data bus must handle signals in both directions.

Verification / Alternative check:
Circuit diagrams of microprocessor systems show address lines as outputs from the CPU connected directly to memory and peripheral chips. Data lines often connect through bidirectional buffers or transceivers, indicating that they can be driven by either the CPU or external devices at different times. This hardware arrangement confirms the unidirectional nature of the address bus and the bidirectional nature of the data bus.

Why Other Options Are Wrong:
Option B: States that both buses are unidirectional, which is incorrect because data must sometimes flow from memory to the CPU.Option C: Reverses the correct roles and suggests the address bus is bidirectional, which does not match real microprocessor design.Option D: Claims that neither bus is used for communication with memory, which contradicts the basic design of every microcomputer system.

Common Pitfalls:
Students sometimes confusingly think that because both memory and CPU communicate, all buses must be bidirectional. In reality, directionality depends on the type of information. Addresses are always generated by the CPU, while data is exchanged in both directions. Recognizing this distinction helps in understanding timing diagrams and bus arbitration.

Final Answer:
The correct answer is The address bus is unidirectional from the CPU to memory and input or output devices, while the data bus is bidirectional because data must flow both to and from the CPU.