Microprocessor Memory Fetch Sequence: D, C, B, A

To understand how a microprocessor fetches a data byte from memory, let's analyze the sequence of steps involved:

Step 1: Address Placement (D)

The process starts with identifying the target memory location. The microprocessor uses the program counter (PC) to get the 16-bit memory address. This address is then placed onto the address bus, specifying where the data resides.

Step 2: Control Signal Activation (C)

Following address placement, the microprocessor's control unit initiates the read operation. It sends a specific control signal, the read signal $\\overline{\text{RD}}$, to the memory system. This signal enables the target memory chip and indicates that data needs to be read.

Step 3: Data Transfer (B)

With the correct address on the address bus and the read signal active, the memory chip responds. It fetches the requested byte from the specified location and places it onto the data bus. The data bus acts as the pathway for transferring this byte back to the microprocessor.

Step 4: Data Processing (A)

Finally, the data byte travels from the data bus into the microprocessor. It is then directed to the appropriate internal component, such as the instruction decoder, for further processing or execution according to the program's instructions.

The correct sequence combining these essential steps is D, C, B, A.