JNTU B.Tech III COMPUTER ORGANIZATION AND OPERATING SYSTEMS Previous Question Paper (August - 2022 - R13)

A computer is a complex system composed of several interconnected functional units. These units work together to process data and execute instructions. The primary functional units are:

1. Central Processing Unit (CPU): This is the brain of the computer. It performs most of the processing inside the computer and controls all other units. The CPU itself is typically divided into:

• Arithmetic Logic Unit (ALU): Performs arithmetic (addition, subtraction, etc.) and logical (AND, OR, NOT) operations.
• Control Unit (CU): Directs all operations of the CPU and other units. It fetches instructions from memory, decodes them, and generates control signals to execute them.
• Registers: Small, high-speed storage locations within the CPU used to hold data, instructions, and addresses temporarily during processing. Key registers include the Program Counter (PC), Instruction Register (IR), Memory Address Register (MAR), Memory Data Register (MDR), and Accumulator.

2. Memory Unit: Stores programs and data. It is essential for the computer to function. There are two main types of memory:

• Primary Memory (Main Memory/RAM): Fast memory that holds data and instructions currently being used by the CPU. It is volatile, meaning its contents are lost when power is turned off.
• Secondary Memory (Storage): Slower but non-volatile memory used for long-term storage of programs and data (e.g., hard drives, SSDs).

3. Input Unit: Devices that allow data and instructions to be entered into the computer. Examples include keyboards, mice, scanners, and microphones.

4. Output Unit: Devices that display or present the results of the computer's processing to the user. Examples include monitors, printers, and speakers.

5. System Bus: A communication pathway that connects the various functional units of the computer, allowing them to transfer data, addresses, and control signals. It typically consists of three parts: Data Bus, Address Bus, and Control Bus.

Diagram:

(Imagine a block diagram here showing CPU, Memory, Input Unit, Output Unit, and System Bus connecting them. The CPU block would further show ALU, CU, and Registers. Arrows would indicate the flow of data, addresses, and control signals between these units.)

The basic computer instruction cycle, also known as the fetch-decode-execute cycle, is the fundamental sequence of operations that a CPU performs to process instructions. It consists of the following phases:

1. Fetch Phase: The control unit fetches the next instruction from memory. The address of this instruction is stored in the Program Counter (PC). The instruction is then loaded into the Instruction Register (IR). The PC is incremented to point to the next instruction.
2. Decode Phase: The control unit decodes the instruction in the IR to determine what operation needs to be performed and what operands (data) are involved. This involves interpreting the opcode and identifying the operands, which might be in registers or memory locations.
3. Execute Phase: The control unit directs the appropriate hardware components (like the ALU) to perform the operation specified by the instruction. This might involve fetching data from memory or registers, performing calculations, and storing the result back into a register or memory.
4. Memory Access Phase (Optional): If the instruction requires access to memory (e.g., to read data or store a result), this phase handles that memory operation. This phase is not present in all instructions (e.g., simple arithmetic operations might not need it).
5. Write-back Phase (Optional): The result of the execution is written back to a register or memory location as specified by the instruction. This phase is also not always present.

Instruction Cycle with Flowchart:

(Imagine a flowchart here. It starts with an initial state. An arrow leads to 'Fetch Instruction' which updates PC and loads IR. From 'Fetch Instruction', an arrow leads to 'Decode Instruction'. From 'Decode Instruction', arrows branch out based on the instruction type. For example, an arrow might lead to 'Fetch Operands' if memory access is needed, then to 'Execute Operation' (e.g., ALU operation). If the result needs to be stored, an arrow leads to 'Write Result' to register/memory. Finally, all paths lead back to the start state or to 'Fetch Instruction' to begin the cycle again. The flowchart should clearly show the sequence and decision points.)

Detailed Flowchart Description:

1. Start: The cycle begins.
2. Fetch Instruction: The address from the PC is sent to the memory via the MAR. The instruction at that memory address is read and placed into the MDR, then transferred to the IR. The PC is incremented.
3. Decode Instruction: The CU examines the opcode in the IR to determine the instruction type.
4. Decision Point (Instruction Type): Based on the decoded instruction, the flow branches.
• If memory access is required (e.g., LOAD, STORE):
• Fetch Operands/Address: The address for the memory access is calculated or fetched.
• Memory Access Phase: Data is read from or written to memory.
• Execute Operation: If the instruction involves computation (e.g., ADD), the ALU performs the operation using fetched operands.
• Write-back Phase: The result is stored in a register or memory.
• If no memory access is required (e.g., NOP, JUMP):
• Execute Operation: The operation is performed directly (e.g., updating PC for a jump).
5. End of Instruction: The instruction is completed.
6. Loop: The cycle returns to the 'Fetch Instruction' step to process the next instruction.