Computer Architecture and CPU: ALU, Control Unit, Registers and Instruction Cycle कंप्यूटर आर्किटेक्चर और CPU: ALU, कंट्रोल यूनिट, रजिस्टर और इंस्ट्रक्शन साइकिल

Computer Architecture and CPU

A computer uses different components to accept input, execute instructions, hold data and present results. This chapter explains how those components relate to one another, the main parts of the CPU and the basic instruction-execution process.

1. Main Functional Parts of a Computer

Example: A letter file is saved on an SSD. While it is being edited, relevant data is available in RAM, the CPU executes instructions and the monitor displays the results.

2. What Is a CPU?

CPU stands for Central Processing Unit. It executes program instructions and is commonly called the processor.

Three essential parts to understand are:

• ALU: Performs arithmetic and logical operations.
• Control Unit: Directs and coordinates instruction execution.
• Registers: Small, fast locations inside the CPU for holding data, instructions and addresses.

3. ALU versus Control Unit

Memory aid: Calculation and logic — ALU; direction and coordination — CU.

4. Important CPU Registers

Registers are very small, fast storage locations inside the CPU. The following names belong to a common educational model; actual processors can use different names and arrangements.

The Program Counter is not simply a counter of the total instructions executed. A branch or jump can change the address from which the next instruction is fetched.

5. The Fetch–Decode–Execute Cycle

The basic process of executing an instruction is called the Fetch–Decode–Execute Cycle.

Writing a result to a register or memory can also form part of an instruction. Some textbooks describe this separately as a Store or Write-back stage.

The opcode identifies the operation to perform. An operand relates to the data, register or location on which the operation acts.

6. System Buses

A bus is a communication pathway for signals between computer components. Introductory models describe three main types.

In a simple CPU-centred model, the address bus is described as outgoing from the CPU, while the data bus transfers in both directions. Control-signal direction depends on the particular signal.

Example: During a memory read, the location is associated with the address bus, the read command with the control bus and the returned data with the data bus.

7. Clock Speed and GHz

Clock speed describes the number of clock cycles per second. Its unit is hertz — Hz.

• 1 MHz: One million clock cycles per second.
• 1 GHz: One billion clock cycles per second.
• 3 GHz: Three billion clock cycles per second.

A higher GHz value alone does not guarantee better performance in every situation. Processor design, workload, cores and the memory system also matter.

8. Processor Cores

A core is a processing unit within a CPU that executes instructions.

• Dual-core: Two cores.
• Quad-core: Four cores.
• Octa-core: Eight cores.

Multiple cores can help with parallel work, but the benefit depends on how effectively software can divide the task.

9. Registers, Cache and RAM

Registers are small locations used for immediate CPU operations. Cache holds copies of useful data and instructions to reduce repeated access to slower main memory. RAM provides a larger working area for active programs and data.

In the usual introductory speed comparison, the order is registers → cache → RAM, with registers generally fastest.

These are not substitutes for long-term file storage on an SSD or HDD. Memory will be covered in greater detail in its own chapter.

10. The Stored-Program Concept

Under the stored-program concept, program instructions are held in memory from which the processor can retrieve them.

• Von Neumann architecture: The basic model uses shared memory and communication paths for instructions and data.
• Harvard architecture: The basic model uses separate memories and paths for instructions and data.

Modern processors can combine aspects of these approaches. The basic models should not be treated as complete descriptions of every modern machine.

11. Motherboard and Cooling

The motherboard is the main circuit board connecting the CPU, RAM and other components.

A heat sink helps remove heat from a processor. A fan can assist cooling through airflow. Different systems use different cooling arrangements.

A heat sink does not store data or perform calculations.

12. Quick Revision

• CPU — Central Processing Unit.
• ALU — arithmetic and logical operations.
• CU — control and coordination.
• PC — next instruction’s address; IR — current instruction.
• MAR — memory address; MDR — data transferred to or from memory.
• Instruction cycle — Fetch, Decode, Execute.
• Address bus — addresses; data bus — data; control bus — control signals.
• GHz measures clock frequency, not storage capacity.
• Quad-core means four cores.
• The CPU, motherboard and cabinet are different.

13. Practice Questions

These are original practice questions, not verified previous-year questions. Each question has one correct answer.

Question 1. Which unit compares two values to determine whether the first is greater than the second?

A. Control Unit
B. ALU
C. Heat sink
D. SSD

Question 2. Where is the address of the next instruction to be fetched generally held?

A. Accumulator
B. Instruction Register
C. Memory Data Register
D. Program Counter

Question 3. Which pair holds a memory address and the data retrieved from memory, respectively?

A. MAR and MDR
B. MDR and MAR
C. IR and PC
D. ACC and CU

Question 4. What is the correct basic instruction-cycle sequence?

A. Execute → Fetch → Decode
B. Decode → Execute → Fetch
C. Fetch → Decode → Execute
D. Fetch → Execute → Decode

Question 5. Memory Read and Write signals are mainly associated with the:

A. Data Bus
B. Control Bus
C. Address Bus
D. External network cable only

Question 6. What does a clock frequency of 2 GHz mean?

A. Two billion clock cycles per second
B. Necessarily two billion instructions per second
C. A storage capacity of 2 GB
D. Two processor cores

Question 7. Which statement about a quad-core processor is correct?

A. It necessarily has four separate motherboards.
B. Its clock speed is always 4 GHz.
C. It has four processing cores.
D. Its RAM capacity is always 4 GB.

Question 8. Which register holds the current instruction?

A. Program Counter
B. Memory Address Register
C. Accumulator only
D. Instruction Register

Question 9. What is the main purpose of cache memory?

A. Reducing access time for useful data and instructions
B. Permanently saving every file
C. Cooling the CPU
D. Increasing screen size

Question 10. In the basic Von Neumann model, instructions and data:

A. Are always held in separate memories.
B. Cannot be held in memory.
C. Use shared memory.
D. Are executed directly from a hard disk only.

Question 11. What is the main function of a heat sink?

A. Decoding instructions
B. Helping remove heat from the processor
C. Holding memory addresses
D. Performing arithmetic comparisons

Question 12. When comparing two different CPUs, which statement is correct?

A. The CPU with more GHz must be faster at every task.
B. Core count can never affect performance.
C. Cache is only permanent file storage.
D. Performance depends on design and workload as well as clock speed.