1) Overview

A computer is a system of connected parts. Input devices feed data, the CPU processes it using memory, and output devices show results. Everything talks over buses controlled by the Control Unit.

2) Standard block diagram (ASCII)

Blocks to remember

• Input Unit → Keyboard, mouse, scanner, mic, camera.

• Output Unit → Monitor, printer, speakers, projector.

• CPU (Central Processing Unit) → ALU, CU, Registers.

• Memory Unit (Primary) → RAM, ROM, Cache.

• Secondary Storage → SSD/HDD, USB, DVD (via I/O controllers).

• System Bus → Data Bus, Address Bus, Control Bus.

• I/O Controllers/Adapters → Interface between devices and the system bus (e.g., USB, disk, display adapters).

3) What each block does (short & clear)

A) Input Unit

• Converts human/world data into machine-readable form (binary).

• Sends data to memory or directly to CPU via controllers.

B) Output Unit

• Converts machine results into human-readable form (text, images, sound).

• Examples: Monitor (video), Printer (paper), Speakers (audio).

C) Memory Unit (Primary)

• RAM (Read/Write, volatile): Working area for programs and data.

• ROM/Flash (Read-mostly, non-volatile): Boot firmware (BIOS/UEFI).

• Cache (L1/L2/L3): Very fast memory close to CPU; stores recent instructions/data to speed up processing.

D) Secondary Storage (Auxiliary)

• Non-volatile long-term data (SSD/HDD/USB).

• Slower than RAM; data is loaded into RAM before CPU uses it.

E) CPU Internals

• ALU (Arithmetic Logic Unit): Calculations (add, sub, multiply) and comparisons (<, =, >).

• CU (Control Unit): Directs operations—fetches instructions, decodes them, controls buses and timing.

• Registers: Tiny, ultra-fast storage (e.g., accumulator, instruction register, program counter, status/flags).

F) System Bus (the highways)

• Data Bus: Carries actual data (bi-directional).

• Address Bus: Carries memory/device addresses (usually CPU → others).

• Control Bus: Carries control signals (Read/Write, clock, interrupts).

G) I/O Controllers (a.k.a. device controllers/adapters)

• Speak both device language and system bus language.

• Examples: Disk controller, USB controller, GPU/display adapter, Network interface.

4) How work flows (Fetch–Decode–Execute cycle)

1.   Fetch: CU uses Program Counter (PC) to get the next instruction from RAM into the Instruction Register.

2.   Decode: CU interprets the instruction (what operation? which operands?).

3.   Execute: ALU performs the operation; results go to a register or RAM.

4.   Update PC: PC moves to next instruction; repeat.

Data path example (adding two numbers from keyboard):

• Input → stored in RAM → CPU fetches instruction “ADD A, B” → ALU adds → result to register/RAM → output to screen or save to storage.

5) Helpful “zoom-ins”

• Cache levels:
  L1 (smallest, fastest) inside each core → L2 (bigger) → L3 (shared). Closer = faster access.

• Virtual memory (simple):
  When RAM is full, the OS temporarily moves some data to disk/SSD (paging). Slower than true RAM, but prevents crashes.

• Interrupts:
  Devices “raise a hand” to signal the CPU (e.g., key pressed, packet arrived). CPU pauses, handles it, then continues.

• DMA (Direct Memory Access):
  For big transfers (like disk → RAM), a DMA controller moves data without tying up the CPU.

6) Typical exam diagram (labels to remember)

• Input Unit, Output Unit, Memory (RAM/ROM/Cache), CPU (ALU, CU, Registers), System Bus, Secondary Storage, I/O Controllers.

• Arrows showing Input → Memory/CPU → Output, with Storage off the bus via controllers.

7) Common confusions cleared

• Memory vs Storage: RAM is temporary & fast; SSD/HDD is permanent & slower.

• ALU vs CU: ALU calculates; CU controls.

• Cache vs RAM: Both are volatile; cache is smaller & faster, sitting closer to CPU.

• Address vs Data bus: Address bus selects where; data bus carries what.

8) Mini example to write

“In the block diagram, input from the keyboard is accepted by the input unit, stored in RAM, processed by the CPU (ALU under the CU’s control), and the result is sent to the monitor via the output unit. All components communicate using the system buses.”

9) Practice questions (with brief answers)

1.   Name the three main parts inside the CPU and their roles.
Ans: ALU (calculations/logic), CU (control/coordination), Registers (ultra-fast storage).

2.   Differentiate between Data Bus and Address Bus.
Ans: Data Bus carries data; Address Bus carries the memory/device address to read/write.

3.   Why is cache memory used?
Ans: To reduce average memory access time by keeping recent/frequent data close to the CPU.

4.   Explain the Fetch–Decode–Execute cycle in two lines.
Ans: CPU fetches an instruction from RAM, decodes it to understand the operation, executes it using ALU/registers, then fetches the next.

5.   Where does secondary storage fit in the block diagram?
Ans: Behind I/O controllers; data is moved to RAM before the CPU can process it.

10) One-page recap

• Blocks: Input, Output, CPU (ALU+CU+Registers), Memory (RAM/ROM/Cache), Secondary Storage, I/O Controllers, System Bus (Data/Address/Control).

• Flow: Input → RAM → CPU (F–D–E) → Output; long-term data in SSD/HDD.

• Key ideas: Cache speeds access; interrupts notify CPU; DMA offloads big transfers.

• Don’t mix up: RAM ≠ storage; ALU ≠ CU; address ≠ data bus.

• Goal: Turn raw input data into useful information efficiently and reliably.