CPU, Memory and Storage: How a Computer Processes Data

Learning goal

Recognise the different jobs of the processor, working memory and long-term storage. Practise following a simple task through a computer and reading capacity units carefully.

The CPU does the processing

The central processing unit (CPU) executes program instructions. Its arithmetic logic unit (ALU) performs calculations and logical comparisons. The control unit (CU) interprets instructions and coordinates their execution. Registers hold small amounts of data, instructions or intermediate results inside the CPU. Cache holds copies of frequently accessed information to reduce the time spent accessing main memory. Read-only memory (ROM) is non-volatile: it retains its contents without power. It is commonly associated with instructions that change infrequently. NCERT: Computer System

Working memory and storage

Ordinary main RAM is volatile. Information held only there is lost when power is removed. Secondary storage keeps saved information without continuous power. An HDD stores data magnetically and uses moving mechanical parts. An SSD usually stores data in NAND flash memory and has no moving mechanical parts. Flash storage is therefore different from volatile RAM, even though both use electronic components.

An SSD can reduce delays caused by mechanical disk access. That does not mean every task becomes equally faster, or that an SSD cannot fail. For examination purposes, first identify the device's role and technology; avoid treating a speed claim as a definition. Intel: SSD technology

Bits, bytes and capacity

A byte contains eight bits. Standard decimal prefixes and binary prefixes have different meanings: 1 kB is 1,000 bytes, whereas 1 KiB is 1,024 bytes. Similarly, 1 MB is 1,000,000 bytes and 1 MiB is 1,048,576 bytes. Some learning materials use KB for 1,024 bytes. In a numerical question, follow its explicitly stated convention; do not silently switch between conventions. NIST: Binary prefixes

Worked everyday example

Imagine calculating the cost of six notebooks at ₹25 each. The keyboard provides input, the CPU carries out the multiplication, and the screen shows ₹150. Saving the result creates a stored copy; merely seeing it on screen does not prove it has been saved.

Now suppose a small practice file occupies exactly 4 KiB, ignoring file-system overhead. Its size is 4 × 1,024 = 4,096 bytes, or 4,096 × 8 = 32,768 bits. Write the unit after each calculation to prevent confusion.

Exam traps

• ALU and CU have different functions: calculation versus control.
• Non-volatile means retaining data without power, not being immune to failure.
• A larger number is not automatically larger capacity: compare the units first.

Self-checks

1. Which CPU part performs the arithmetic in 25 × 6? The ALU. The CU coordinates instruction execution; it is not the arithmetic unit.
2. A saved document is on an SSD. Must its contents disappear when the computer shuts down normally? No. SSD flash is non-volatile; ordinary RAM is volatile.
3. Is 2 KiB equal to 2,000 bytes? No. It is 2 × 1,024 = 2,048 bytes. Two decimal kilobytes equal 2,000 bytes.

These are original practice checks, not official previous-year questions.