False. The Control Unit coordinates all activities. The ALU only performs arithmetic and logical operations.

True. Registers are located inside the CPU and are accessed in a single clock cycle, making them the fastest memory type.

False. RAM is volatile memory — its contents are lost when the power is switched off. ROM is non-volatile.

True. The von Neumann architecture is based on the stored program concept, where both data and program instructions are stored in the same memory.

False. The Data Bus carries data between the CPU and memory. The Address Bus carries memory addresses specifying the location to read from or write to.

False. Cache memory is faster than RAM. It sits between the CPU and RAM specifically to provide faster access to frequently used data.

True. The Instruction Register (IR) holds the instruction that has just been fetched from memory and is currently being decoded and executed.

The von Neumann architecture is a computer design model proposed by John von Neumann. Its key feature is the stored program concept — both data and program instructions are stored in the same memory. The CPU fetches instructions sequentially, decodes them, and executes them in a continuous cycle (fetch-decode-execute-store).

A bus is a set of parallel wires (communication pathway) that transfers data between computer components. The three types are: 1. Data Bus — carries actual data between CPU, memory, and I/O devices 2. Address Bus — carries memory addresses to specify the read/write location 3. Control Bus — carries control signals (read, write, interrupt) from the CPU

Registers are small, high-speed storage locations inside the CPU used to hold data and instructions temporarily during processing. Two important registers are: 1. Program Counter (PC) — holds the address of the next instruction to be fetched 2. Accumulator — stores intermediate results of ALU operations

Block Diagram of a Computer System:

• Accepts data and instructions from the user
• Converts human-readable data into machine-readable form
• Examples: keyboard, mouse, scanner, microphone, joystick

2. Central Processing Unit (CPU): The CPU is the brain of the computer. It has three sub-units:

• Arithmetic and Logic Unit (ALU):
• Performs all arithmetic operations (addition, subtraction, multiplication, division)
• Performs logical operations (AND, OR, NOT, comparisons like greater than, equal to)
• Receives data from registers and returns results to the accumulator

• Control Unit (CU):
• Fetches instructions from memory
• Decodes (interprets) the instructions
• Sends control signals to ALU, memory, and I/O devices
• Coordinates the entire fetch-decode-execute cycle
• Does NOT perform calculations itself — only directs operations

• Registers:
• Small, ultra-fast storage locations inside the CPU
• Hold data temporarily during processing
• Key registers: Program Counter, Instruction Register, Accumulator, MAR, MDR

• Primary Memory: Directly accessed by CPU
• RAM (Random Access Memory) — volatile, read-write, stores running programs
• ROM (Read Only Memory) — non-volatile, stores boot-up instructions
• Cache — fast memory between CPU and RAM
• Secondary Memory: Large, non-volatile storage
• HDD, SSD, pen drives, optical discs

• Receives processed results from the CPU
• Converts machine-readable data into human-readable form
• Examples: monitor, printer, speaker, projector

The von Neumann architecture, proposed by mathematician John von Neumann in 1945, is the foundation of most modern computers. Its key principles are:

Core Principles:

1. Stored Program Concept: Both data and program instructions are stored in the same memory. This was revolutionary — earlier computers had to be rewired for each new task.

2. Sequential Execution: Instructions are executed one after another in order, unless a branch instruction changes the flow.

3. Single Memory for Data and Instructions: There is one unified memory that holds both data and programs.

4. Centralised Processing: A single CPU processes all instructions using the fetch-decode-execute cycle.

The Machine Cycle (Instruction Cycle):

• Fetch: The CPU fetches the next instruction from memory, using the address in the Program Counter
• Decode: The Control Unit decodes the instruction to determine what operation to perform
• Execute: The ALU performs the operation (arithmetic or logical)
• Store: The result is stored in memory or a register

Components of von Neumann Architecture:

Significance:

• Made computers programmable without rewiring — just load a new program
• Unified memory design simplified computer architecture
• Enabled the development of general-purpose computers
• The fetch-decode-execute cycle is still the basis of modern CPUs
• Introduced the concept of stored programs, enabling software as we know it

Limitation (von Neumann Bottleneck): The single bus between CPU and memory creates a bottleneck — the CPU cannot fetch data and instructions simultaneously. This is addressed in modern architectures through caches, pipelining, and Harvard architecture (separate data/instruction caches).

• Located inside the CPU
• Fastest memory, accessed in a single clock cycle
• Very small capacity (a few bytes to a few hundred bytes)
• Hold data currently being processed
• Types: Program Counter, Instruction Register, Accumulator, MAR, MDR
• Most expensive memory per unit

• Located between CPU and RAM
• Very fast (faster than RAM, slower than registers)
• Small capacity (KB to a few MB)
• Stores frequently accessed data and instructions
• Levels: L1 (fastest, smallest, inside CPU), L2 (larger, slightly slower), L3 (largest, shared among cores)
• Reduces the number of times the CPU must access slower RAM

• Primary main memory, directly accessed by CPU
• Volatile — data lost when power is off
• Read-write — data can be read from and written to
• Medium capacity (4 GB to 64 GB in modern systems)
• Stores currently running programs and their data
• Two types:
• SRAM (Static RAM): Faster, more expensive, used for cache
• DRAM (Dynamic RAM): Slower, cheaper, needs periodic refresh, used for main memory

• Non-volatile — retains data even without power
• Read-only (in normal operation) — data is written during manufacturing
• Stores essential boot-up instructions (BIOS/firmware)
• Types:
• PROM: Programmable once by the user
• EPROM: Erasable using UV light, reprogrammable
• EEPROM: Electrically erasable, can be rewritten

• Non-volatile, permanent storage
• Large capacity (hundreds of GB to TB)
• Slowest access time (compared to primary memory)
• Not directly accessed by CPU — data must be loaded into RAM first
• Types:
• HDD (Hard Disk Drive): Magnetic storage, large capacity, slower
• SSD (Solid State Drive): Flash storage, faster than HDD, no moving parts
• Optical Discs: CD, DVD, Blu-ray
• Pen Drives / Flash Drives: Portable, USB-based

Memory Hierarchy Summary (Fastest to Slowest):