Question

The sequence of steps performed by microprocessor to fetch the data byte from memory :
• [A.] The byte is placed in the instruction decoder of the microprocessor.
• [B.] The byte from the memory location is placed on data bus.
• [C.] The control unit sends the control signal RD to enable the memory chip.
• [D.] Bus microprocessor places the 16-bit memory address from program counter on the address bus. Choose the correct answer from the options given below :

• A. A, B, C, D
• B. D, C, B, A
• C. D, B, C, A
• D. A, C, B, D

Hint:

Memory read sequence: \[ \text{Address on bus} \rightarrow RD \text{ signal} \rightarrow \text{Memory places data on data bus} \rightarrow \text{Processor decodes instruction} \]

Answer 1

The Correct Option is B

Concept: During a memory read operation, the microprocessor follows a fixed sequence of operations. The processor must:
• First specify the memory location.
• Then activate memory read control.
• Then receive data from memory.
• Finally decode or process the instruction/data.

Step 1: Identify the first operation. Before reading memory, the processor must specify: \[ \text{Which memory location is to be accessed} \] This is done by placing the address on the address bus. Statement: \[ D \] says: \[ \text{Processor places 16-bit address from PC on address bus} \] Therefore: \[ D \text{ occurs first} \]

Step 2: Identify the second operation. After address placement, processor must enable memory read operation. This is done using: \[ RD \] control signal. Statement: \[ C \] therefore comes next. Thus: \[ C \text{ occurs second} \]

Step 3: Identify the third operation. Once memory chip is enabled:
• Memory places requested byte onto data bus. This corresponds to: \[ B \] Hence: \[ B \text{ occurs third} \]

Step 4: Identify the final operation. After data reaches processor:
• The instruction/data is transferred to instruction decoder. This corresponds to: \[ A \] Thus: \[ A \text{ occurs last} \]

Step 5: Write complete correct sequence. Therefore the proper order is: \[ D \rightarrow C \rightarrow B \rightarrow A \]

Step 6: Write the final answer. Hence the correct option is: \[ \boxed{(B)\ D,\ C,\ B,\ A} \]