JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III COMPUTER ARCHITECTURE Previous Question Paper (February - 2022 - R18)

CISC (Complex Instruction Set Computer) and RISC (Reduced Instruction Set Computer) represent two distinct design philosophies in computer architecture:

CISC aims to reduce the number of instructions per program by providing complex instructions that perform multiple operations, whereas RISC focuses on simplifying instructions to increase the clock speed and optimize pipeline performance.

The Control Unit (CU) acts as the 'brain' of the CPU, directing the flow of data and instructions between the processor and other components. Its operation follows a systematic cycle:

1. Instruction Fetch: The CU retrieves the next instruction from memory using the Program Counter (PC) and stores it in the Instruction Register (IR).
2. Instruction Decode: The CU interprets the opcode in the IR to determine the required operation and identifies the necessary operands.
3. Effective Address Calculation: If the instruction involves memory access, the CU calculates the address of the data.
4. Operand Fetch: The CU triggers the necessary control signals to fetch data from memory or registers.
5. Execution: The CU sends specific control signals to the Arithmetic Logic Unit (ALU) or other functional units to perform the operation.
6. Write Back: The CU manages the storage of the result back into a register or memory location.

The CU operates either via hardwired logic (using combinational circuits for high speed) or microprogramming (using a control memory to store microinstructions for flexibility).