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).
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.
Question types you will see
Each type: how to recognise it, the method step by step, and one question to try.
CPU part ↔ function
'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.
Which part of the CPU performs comparison of two values?
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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.
Machine cycle steps and order
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.
In the machine cycle, the step in which the CPU finds out what an instruction means is —
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Decode — the Control Unit interprets the fetched instruction before the ALU executes it. Order: Fetch → Decode → Execute → Store.
Buses and stored-program architecture
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).
The stored-program concept, the basis of modern computers, was proposed by —
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John von Neumann — instructions and data are stored together in the same memory, and the CPU fetches and executes them one after another.
IPO cycle and block-diagram flow
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.
Typing a document, printing it, and saving it on the hard disk — saving belongs to which basic computer function?
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Storage — the four basic functions are Input, Processing, Output and Storage. Saving for future use is storage.
Shortcuts that save time
"Fat Dogs Eat Snacks" = Fetch, Decode, Execute, Store - the machine cycle order. Every instruction, every time.
Correct machine-cycle order?
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Fetch -> Decode -> Execute -> Store.
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.
Which CPU part performs comparisons?
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ALU - comparisons are logic operations.
Mistakes to avoid
Where most students lose marks on this subtopic.
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.
Quick revision
Read this the night before the exam.
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.
Practice: 16 questions
Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.
Topic test · 10 questions
Suggested time 4 min · wrong answers go to your mistake notebook automatically.