9 min readConcept sheetOften a 5- or 10-mark answer

Computer storage and storage devices

Primary memory holds work in progress; secondary storage keeps data permanently on magnetic, optical or flash media.

Smallest unitBit; 8 bits = 1 byte

Primary memoryRAM, ROM, cache: fast, close to the CPU

Secondary storageMagnetic, optical, flash

AccessDisks: direct. Tape: sequential

Storage units

Unit	Equals	Rough idea
Bit	0 or 1	One switch, on or off
Byte	8 bits	One character
Kilobyte (KB)	1,024 bytes	A short paragraph of text
Megabyte (MB)	1,024 KB	One phone photo is a few MB
Gigabyte (GB)	1,024 MB	A full-length film; RAM sizes
Terabyte (TB)	1,024 GB	A large hard disk

Working with storage units

Problem

A 16 GB memory card stores product photos of 4 MB each. How many photos fit? Also, how many bits are in 3 KB?

Working

16 GB = 16 × 1,024 MB = 16,384 MB
16,384 MB ÷ 4 MB = 4,096 photos
3 KB = 3 × 1,024 bytes = 3,072 bytes
3,072 bytes × 8 bits = 24,576 bits

Answer

4,096 photos; 24,576 bits (using 1 GB = 1,024 MB and 1 KB = 1,024 bytes)

Primary vs secondary storage

Primary (main memory)RAM, ROM and cache. Used directly by the CPU. Fast, costly per GB, smaller. RAM is volatile: it loses data when power goes off.

Secondary storageHard disks, SSDs, optical discs, flash drives and tape. Non-volatile, cheaper per GB, much larger, slower.

Disk makers count 1 GB as 1,000 MB, so a new disk shows slightly less space on the computer.

Secondary storage devices

Type	Examples	Strengths	Weaknesses
Magnetic disk	Hard disk drive (HDD), external hard disk	Large capacity, low cost per GB, direct access	Moving parts; can be damaged if dropped
Magnetic tape	Backup tape cartridges	Very cheap for huge backups and archives	Sequential access only; slow to find one file
Optical disc	CD (about 700 MB), DVD (4.7 GB single layer), Blu-ray (25 GB single layer)	Cheap, portable, read by laser	Small capacity today; scratches easily
Flash memory	USB pen drive, SSD, SD memory card	No moving parts, fast, shock-resistant	SSDs cost more per GB; limited number of writes

Sequential vs direct access

Sequential accessData is read in order from the start, like an old cassette. Used by magnetic tape. Fine for backups and payroll runs.

Direct (random) accessThe device jumps straight to the data needed. Used by disks and flash. Needed for looking up one customer quickly.

In Sri Lankan business

A Sri Lankan bank needs fast disk and flash storage so ATM and app balance checks are instant, but it also keeps backup copies on tape or at a separate site, so customer records survive a fire, flood or system failure.

Exam angle

Short answer: “What is the difference between volatile and non-volatile memory?” Volatile memory (RAM) loses its contents when power goes off. Non-volatile storage (ROM, disks, flash) keeps data without power.

Structured question: “Compare magnetic, optical and flash storage.” Give examples, how each stores data, capacity, speed, cost and durability. A table works well.

Essay: “Discuss the types of computer storage and storage devices used in business.” Explain storage units, primary vs secondary storage, magnetic disks and tapes, optical discs and flash memory, with business uses.

Remember it

Secondary storage = M-O-F: Magnetic (HDD, tape), Optical (CD, DVD, Blu-ray), Flash (USB, SSD, SD card). Tape is sequential; disks are direct.