Computer Fundamentals
🔒 Log in to trackBasic organisation: CPU, memory, I/O
🔒 Log in to trackA computer follows the von Neumann (stored-program) architecture: instructions and data live in the same memory, and the machine repeatedly fetches and executes them.
Block diagram: Input devices -> CPU <-> Memory -> Output devices, with the system bus (data bus, address bus, control bus) carrying signals on the motherboard.
CPU (Central Processing Unit) - the 'brain' - has three parts:
| Part | Job |
|---|---|
| ALU (Arithmetic Logic Unit) | Arithmetic (+, -, x, /) and logic (compare, AND/OR) operations |
| CU (Control Unit) | Directs traffic: fetches, decodes, coordinates all units; does NOT process data itself |
| Registers | Fastest, tiny storage inside CPU (accumulator, program counter, instruction register) |
Machine cycle for one instruction: Fetch -> Decode -> Execute -> Store. CPU speed is measured in GHz (billions of clock cycles per second). Other board parts: chipset, expansion slots, CMOS battery, ports. BIOS/UEFI firmware wakes the hardware at power-on (see software-os topic).
Detailed notes
The basic block diagram
Every computer, from a desktop PC to a smartphone, follows the same plan (the von Neumann or stored-program design):
Input devices → CPU (processes) ↔ Memory ↔ Storage → Output devices, all connected by the motherboard and its buses.
- Input brings data in (keyboard, mouse, scanner).
- Processing happens inside the CPU.
- Storage keeps data permanently (hard disk, SSD).
- Output presents results (monitor, printer). This Input → Process → Output (+ Storage) flow is called the IPO cycle.
Inside the CPU — the three parts
| Part | Job | Everyday picture |
|---|---|---|
| ALU (Arithmetic Logic Unit) | Does all arithmetic (+, −, ×, ÷) and logic (comparisons, AND/OR) | The calculator inside the brain |
| CU (Control Unit) | Directs everything: fetches instructions, decodes them, sends signals; does no calculation itself | The traffic constable |
| Registers | Tiny, extremely fast working slots inside the CPU | The clerk's fingertips |
Registers you should know by name: PC (Program Counter — address of the next instruction), IR (Instruction Register — holds the instruction being executed), MAR/MDR (Memory Address / Data Registers — talk to RAM), and the Accumulator (holds ALU results).
The machine cycle
The CPU repeats four steps millions of times per second:
- Fetch — CU brings the next instruction from memory (its address comes from the PC).
- Decode — CU interprets what the instruction wants.
- Execute — ALU (or another unit) does the work.
- Store — the result is written back to memory or a register. Memory hook: Fat Dogs Eat Snacks.
Buses — the roads between blocks
- Data bus — carries the actual data (both directions).
- Address bus — carries where the data lives (CPU → memory, one direction).
- Control bus — carries command signals (read, write, interrupt). More data-bus lines mean more bits moved at once — that is what "32-bit/64-bit processor" measures.
Why it is called "stored program"
John von Neumann's idea: keep instructions and data together in the same memory, written as numbers. The CPU then simply fetches, decodes and executes them one after another — changing the program needs no rewiring. This single idea separates modern computers from fixed machines like calculators-in-hardware.
Quick revision
- Block flow: Input → Process (CPU) → Output, with Memory and Storage alongside.
- CPU = ALU (calculates) + CU (controls) + registers (fastest storage).
- Machine cycle: Fetch → Decode → Execute → Store.
- PC = next instruction's address; IR = current instruction; Accumulator = ALU result holder.
- Data bus carries data; address bus carries location; control bus carries commands.
- Stored-program (von Neumann) = instructions and data in one memory.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: CPU part ↔ functionvery common5 practice Q
'Which unit performs arithmetic?', 'which unit controls and coordinates?', 'where does the comparison happen?' — a CPU part with units as options.
- ALU = all arithmetic and logic (add, subtract, compare AND/OR). CU = direction only, no maths. Registers = fastest storage slots inside the CPU.
- Named registers: PC holds the next instruction's address, IR holds the current instruction, the Accumulator holds ALU results.
- In 'not a CPU part' questions, memory (RAM) and hard disk are outside the CPU.
Example: Which part of the CPU performs comparison of two values?
The Arithmetic Logic Unit (ALU) — logic operations such as compare, AND, OR belong to the ALU. The Control Unit only directs traffic; it computes nothing.
Type 2: Machine cycle steps and ordervery common4 practice Q
Give the correct order of fetch/decode/execute/store, or ask what happens in one of the four steps.
- Always Fetch → Decode → Execute → Store (Fat Dogs Eat Snacks).
- Meanings: fetch = bring from memory, decode = interpret, execute = do the work (ALU), store = write the result back.
- Reversed pairs (Decode before Fetch, Store before Execute) are the planted traps.
Example: In the machine cycle, the step in which the CPU finds out what an instruction means is —
Decode — the Control Unit interprets the fetched instruction before the ALU executes it. Order: Fetch → Decode → Execute → Store.
Type 3: Buses and stored-program architecturecommon3 practice Q
Which bus carries addresses/data/commands; what the stored-program (von Neumann) concept means; what the motherboard holds.
- Data bus = the data itself (two-way). Address bus = the location (CPU → memory). Control bus = command signals.
- Stored program = instructions and data kept together in the same memory, so programs can be changed without rewiring.
- 'Bits moved at once' describes the data bus width (32-bit/64-bit processors).
Example: The stored-program concept, the basis of modern computers, was proposed by —
John von Neumann — instructions and data are stored together in the same memory, and the CPU fetches and executes them one after another.
Type 4: IPO cycle and block-diagram flowcommon2 practice Q
What IPO stands for, which function saving a file is, or which arrow is correct in the Input–Process–Output block diagram.
- IPO = Input → Process → Output; Storage is the fourth basic function.
- Typing = input; calculating = processing; showing a result = output; saving for later = storage.
- In diagram statements, data always flows input → CPU → output, with the CPU connected to memory.
Example: Typing a document, printing it, and saving it on the hard disk — saving belongs to which basic computer function?
Storage — the four basic functions are Input, Processing, Output and Storage. Saving for future use is storage.
Shortcut tricks
⚡ FDES chant
"Fat Dogs Eat Snacks" = Fetch, Decode, Execute, Store - the machine cycle order. Every instruction, every time.
Example: Correct machine-cycle order?
Fetch -> Decode -> Execute -> Store.
⚡ ALU vs CU
ALU = Actual Labour Unit (does the sums); CU = traffic Constable, Untouched by maths (only directs). If a question says 'performs calculations' -> ALU; 'coordinates/controls' -> CU.
Example: Which CPU part performs comparisons?
ALU - comparisons are logic operations.
Where students lose marks
Saying the CU performs arithmetic - it only controls; arithmetic belongs to the ALU.
Reversing Execute and Decode in the machine cycle.
Thinking registers are slower than RAM - registers are the fastest storage in the whole machine.
Practice sets — 16 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 4 min · wrong answers go to your mistake notebook automatically.