Computer Fundamentals
🔒 Log in to trackBasic organisation: CPU, memory, I/O
🔒 Log in to trackEvery computer follows the von Neumann (stored-program) plan. Data comes in through input, the CPU processes it, memory and storage hold it, and output shows the result. The CPU has an ALU, a control unit and registers. It repeats the machine cycle: fetch, decode, execute, store.
The basic block diagram
Every computer, from a desktop to a smartphone, follows one plan.
Input devices, then the CPU, with Memory and Storage alongside, then Output devices. The motherboard and its buses connect them all.
- Input brings data in (keyboard, mouse, scanner).
- Processing happens inside the CPU.
- Storage keeps data permanently (hard disk, SSD).
- Output shows results (monitor, printer).
This flow is the IPO cycle: Input, Process, Output. Storage is the fourth basic function.
Inside the CPU
| Part | Job | Everyday picture |
|---|---|---|
| ALU (Arithmetic Logic Unit) | All arithmetic (+, -, x, /) and logic (compare, AND, OR) | The calculator inside the brain |
| CU (Control Unit) | Fetches and decodes instructions, sends signals. Does no calculation | The traffic constable |
| Registers | Tiny, very fast slots inside the CPU | The clerk's fingertips |
Four registers to know:
- PC (Program Counter) holds the address of the next instruction.
- IR (Instruction Register) holds the instruction being executed.
- MAR and MDR (Memory Address and Data Registers) talk to RAM.
- Accumulator holds ALU results.
Watch: Registers are the fastest storage in the whole machine. They are faster than RAM.
The machine cycle
The CPU repeats four steps millions of times each second.
- Fetch: the CU brings the next instruction from memory. Its address comes from the PC.
- Decode: the CU works out what the instruction wants.
- Execute: the ALU or another unit does the work.
- Store: the result is written to memory or a register.
Say "Fat Dogs Eat Snacks". CPU speed is measured in GHz, the billions of clock cycles per second.
Buses
A bus is a set of wires between blocks.
| Bus | What it carries | Direction |
|---|---|---|
| Data bus | The data itself | Both ways |
| Address bus | Where the data lives | CPU to memory |
| Control bus | Command signals (read, write, interrupt) | Control signals |
More data-bus lines move more bits at once. That is what "32-bit" or "64-bit" processor measures.
The stored-program idea
John von Neumann's idea: keep instructions and data together in the same memory. The CPU fetches, decodes and executes them one after another.
To change the program, you change memory. You do not rewire the machine. This separates modern computers from fixed machines.
Rule: Stored program means instructions and data live in one memory.
Question types you will see
Each type: how to recognise it, the method step by step, and one question to try.
CPU part and its function
The question asks which unit does arithmetic, which controls, or where a comparison happens. The options are CPU parts.
ALU: all arithmetic and logic.
CU: direction only, no maths.
Registers: fastest storage inside the CPU.
Cross out RAM and hard disk. They are outside the CPU.
Each CPU part has one job.
Which part of the CPU performs comparison of two values?
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Comparison is a logic operation.
Logic operations belong to the ALU.
Arithmetic Logic Unit (ALU)
Machine cycle steps and order
The question asks for the correct order of fetch, decode, execute and store. Or it asks what happens in one step.
Recall the chant: Fat Dogs Eat Snacks.
Fetch: bring from memory. Decode: understand.
Execute: do the work. Store: write the result.
Reject options that put Decode before Fetch.
The four steps always run in this fixed order.
In the machine cycle, in which step does the CPU find out what an instruction means?
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Understanding the instruction is decoding.
Order: Fetch, Decode, Execute, Store.
Decode
Buses and stored-program idea
The question asks which bus carries addresses, data or commands. Or it asks who proposed the stored-program idea.
Data bus: the data, two ways.
Address bus: the location, CPU to memory.
Control bus: command signals.
Stored program: John von Neumann.
Each bus has one duty, and the stored-program idea has one owner.
Who proposed the stored-program concept, the basis of modern computers?
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The idea keeps instructions and data in the same memory.
It is credited to John von Neumann.
John von Neumann
IPO cycle and block diagram
The question asks what IPO stands for, or which basic function an action such as saving belongs to.
IPO is Input, Process, Output.
Typing: input. Calculating: processing.
Showing a result: output. Saving for later: storage.
Every action maps to one of four basic functions.
Saving a document on the hard disk belongs to which basic function?
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Saving keeps data for later use.
That is storage, the fourth basic function.
Storage
Registers by name
The question describes what a register holds: the next instruction address, the current instruction or the ALU result.
Next instruction address: PC.
Instruction being executed: IR.
ALU results: Accumulator.
Talking to RAM: MAR and MDR.
Each named register holds one kind of value.
Which register holds the address of the next instruction to be fetched?
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The fetch step gets its address from one register.
That register is the Program Counter.
Program Counter (PC)
Shortcuts that save time
"Fat Dogs Eat Snacks" gives Fetch, Decode, Execute, Store. This is the order for every instruction.
What is the correct machine-cycle order?
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Say the chant: Fat Dogs Eat Snacks.
Fetch, Decode, Execute, Store.
Fetch, Decode, Execute, Store
If the question says performs calculations or comparisons, answer ALU. If it says controls or coordinates, answer CU.
Which CPU part performs comparisons?
Show solutionHide solution
Comparing is a logic operation.
Logic belongs to the ALU.
ALU
Mistakes to avoid
Where most students lose marks on this subtopic.
Saying the CU does arithmetic.
The CU only controls. Arithmetic belongs to the ALU.
Swapping Execute and Decode.
Decode comes first, then Execute. Fetch is always first.
Thinking registers are slower than RAM.
Registers are the fastest storage in the machine.
Placing RAM or the hard disk inside the CPU.
The CPU has ALU, CU and registers. Memory and disk are outside.
Forgetting storage in the basic functions.
The four functions are Input, Processing, Output and Storage.
Quick revision
Read this the night before the exam.
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 holds the next instruction's address. IR holds the current instruction. Accumulator holds ALU results.
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.