Theory
One diagram to rule them all
Chirag opens a dusty cabinet with a screwdriver and says: "Motherboard, processor, RAM, hard disk. Now forget the parts. Everything you will ever see in here, and in a ₹50 crore supercomputer, and in your phone, fits one diagram sketched in the 1940s."
He is right. Learn this one map and every machine becomes familiar territory. Exams agree: "draw and explain the block diagram" is a guaranteed question.
Theory
A restaurant kitchen
Orders come in from waiters (input). The head chef reads each order and directs the kitchen: "you chop, you fry, now plate it" (control unit). The cooking hands actually chop and fry (ALU). The fridge and store-room hold ingredients and finished dishes (memory). Serving counters send food out (output). No unit cooks alone; the head chef coordinates everything.
Theory
The diagram itself
Draw it exactly like this in exams, arrows included:
⎡ CPU ⎤
INPUT → ⎢ CU | ALU ⎥ → OUTPUT
⎣ (registers) ⎦
↕
MEMORY
Data enters, the CPU processes it, results leave. Memory sits in constant two-way exchange with the CPU: instructions and data go up, results come back down.
Theory
What each unit does
- Input unit: brings data in, converted to binary (keyboard, mouse, scanner).
- CU (Control Unit): the director. Fetches instructions, tells every unit what to do and when. It does no calculation itself.
- ALU (Arithmetic Logic Unit): the worker. All arithmetic (+, -, ×, ÷) AND all logical comparisons (is 78 > 65?).
- Registers: a handful of tiny, fastest storage slots inside the CPU holding what is being worked on right now.
- Memory unit: holds data and instructions; Output unit turns binary results back into human form.
Quiz
When the computer checks whether marks 78 is greater than the pass mark 35, which unit does that comparison?
- ALU
- Control Unit
- Memory unit
- Input unit
Show the answer
ALU
Comparisons are logical operations, the L in ALU. Most students hand anything that sounds like "deciding" to the Control Unit, but the CU only directs traffic; it never calculates or compares. The ALU does both arithmetic and logic. That distinction is a repeat exam question.
Think first
Trace one calculation
You type 5 + 3 on a calculator app and press =. Walk the journey through the block diagram in your head: which units touch it, in what order?
Show the answer
Input (keys) sends 5, +, 3 in binary → memory holds them → CU fetches the instruction and orders the ALU to add → ALU computes 8 into a register → result goes to memory → output (screen) shows 8.
Six stops, a fraction of a second, and the same trace works for every operation a computer ever performs.
Watch out
Three places marks leak
One: giving the CU calculation duties ("CU processes data"). It directs; the ALU processes. Two: drawing the diagram without arrows, the arrows ARE the data flow and carry marks. Three: writing "CPU" as a unit beside CU and ALU. The CPU is the box that contains CU + ALU + registers, not a fourth sibling.
Formula
Exam recipe: the 5-mark version
Draw the diagram with all arrows and labels. Then one crisp line per unit: input (data in, binary), CU (directs, no calculation), ALU (arithmetic + logical operations), registers (fastest, inside CPU), memory (holds data + instructions), output (results in human form). Diagram + six lines = full marks, every time this is asked.
Theory
This map runs the whole subject
Unit 2 of this subject zooms into the middle of this diagram (processor, memory types, buses connecting them). Unit 3 zooms into the edges (input and output devices). Even C programming next semester is this diagram in code: scanf feeds input, expressions run in the ALU, printf is output. Pin every future topic to this map.
Summary
Key takeaways
- Five functional units: input, CU, ALU (+ registers inside the CPU), memory, output.
- Flow: input → CPU ↔ memory → output; arrows carry marks in the diagram.
- CU directs everything but calculates nothing.
- ALU does all arithmetic AND all logical comparisons.
- Registers are the CPU's tiny fastest workspace.
- Memory hook: the kitchen, head chef directs, hands cook, fridge stores.