Storage system

Computer storage holds the data and programs that the processor uses. Storage is arranged in two levels: fast, directly accessible primary storage (main memory) and large, permanent secondary storage. This note compares them, explains how data is stored and retrieved, and describes each common memory and storage device asked in NEC MCQs.

Key concepts

Primary vs secondary storage

Feature	Primary	Secondary
Access by CPU	Direct	Through I/O, loaded into RAM first
Speed	Very fast	Slower
Capacity	Small (GB)	Large (hundreds of GB–TB)
Volatility	RAM volatile	Non-volatile
Cost per bit	High	Low
Examples	RAM, ROM, cache	HDD, SSD, CD/DVD, tape, flash drive

Units – 1 byte = 8 bits; nibble = 4 bits; 1 KB = 1024 bytes; MB, GB, TB, PB each × 1024.

Data storage and retrieval methods

• Sequential access – data read in order from the beginning (magnetic tape).
• Direct (random) access – any location reached directly (disks; RAM is random access).
• Indexed sequential – records stored sequentially with an index for direct lookup. Access time = seek time + rotational latency + transfer time (for disks).

RAM (Random Access Memory) – read/write, volatile working memory.

• SRAM – flip-flops, no refresh, faster, costlier; used for cache.
• DRAM – capacitor-based, needs periodic refresh, denser and cheaper; used for main memory (SDRAM, DDR4, DDR5).

ROM (Read Only Memory) – non-volatile, holds firmware (BIOS/UEFI boot program).

Type	Programming / erasing
Mask ROM	Written at manufacture; cannot change
PROM	Programmed once by user (fuses blown) – one-time programmable
EPROM	Erased by ultraviolet light through a quartz window, then reprogrammed
EEPROM	Erased and reprogrammed electrically, byte by byte, in circuit
Flash	A type of EEPROM erased in blocks; fast; used in USB drives, SSDs

Magnetic tapes – plastic tape coated with magnetic oxide; sequential access; very cheap per GB; used for backup and archival (LTO cartridges). Data recorded in tracks (frames); density in bpi; records separated by inter-record gaps.

Cartridge tape – tape enclosed in a cartridge for easy handling (QIC, DAT, LTO); used for server backups.

Magnetic disks – circular platters coated with magnetic material; data in concentric tracks, divided into sectors; the same track on all platters forms a cylinder; read/write heads on an access arm. Direct access. Floppy disks (3.5 inch, 1.44 MB) were removable magnetic disks.

Hard disks – sealed, rigid platters spinning at 5400–15 000 rpm; capacities up to many TB; interfaces IDE/PATA, SATA, SCSI, SAS. SSD (solid-state drive) uses flash memory: no moving parts, faster, more shock-resistant.

Optical disks – data written/read with a laser as pits and lands on a spiral track.

Disk	Typical capacity	Laser
CD	650–700 MB	Infrared 780 nm
DVD	4.7 GB (single layer), 8.5 GB (dual)	Red 650 nm
Blu-ray	25 GB (single layer), 50 GB (dual)	Blue-violet 405 nm

Compact disks – CD-ROM (read only, pressed at factory), CD-R (write once – WORM), CD-RW (rewritable, phase-change layer).

Zip drive – removable magnetic disk system by Iomega (1994), 100 MB, later 250/750 MB; replaced by flash drives and CD-RW.

Flash drives – USB pen drives using NAND flash; portable, plug-and-play, non-volatile, limited write cycles. Memory cards (SD, microSD) work the same way.

High-yield points

• EPROM is erased by UV light; EEPROM electrically; PROM is one-time programmable.
• SRAM (flip-flops, no refresh) for cache; DRAM (capacitors, refresh) for main memory.
• Tape = sequential access; disk = direct access.
• Disk access time = seek time + rotational latency + transfer time.
• CD ≈ 700 MB (780 nm), DVD 4.7 GB (650 nm), Blu-ray 25 GB (405 nm).

Formulas

Disk capacity $=$ surfaces $\times$ tracks/surface $\times$ sectors/track $\times$ bytes/sector. Average rotational latency $= \dfrac{1}{2}\times\dfrac{60}{\text{rpm}}$ seconds.

Worked example

A disk has 4 surfaces, 1000 tracks per surface, 500 sectors per track and 512 bytes per sector, spinning at 6000 rpm. Capacity and average latency?

• Capacity $= 4 \times 1000 \times 500 \times 512 = 1\,024\,000\,000$ bytes ≈ 1.02 GB (≈ 0.95 GiB).
• One rotation $= 60/6000 = 10$ ms; average latency $= 5$ ms.

Exam tips

• EPROM is erased with UV light; EEPROM electrically – a classic swap.
• RAM is volatile; ROM, flash, disks and tape are non-volatile.
• Magnetic tape is sequential access; disks are direct access.
• SRAM for cache (fast, no refresh); DRAM for main memory (needs refresh).
• CD-R is write-once (WORM); CD-RW can be rewritten.

Exam-style questions

From Calmon's question bank, which has 25 questions on this topic.

1. A ROM chip that is supplied blank and can be programmed only once by the user is called:

• A. DRAM
• B. PROM
• C. EEPROM
• D. Mask ROM

Answer: B. A Programmable ROM is written once by the user with a PROM programmer (by blowing fuses); after that its contents cannot be changed.

2. In a hard disk pack, a cylinder is:

• A. The spindle that rotates the platters
• B. A single track on the top surface
• C. A group of adjacent sectors on one track
• D. The set of tracks at the same position on all the recording surfaces

Answer: D. All tracks at the same radial position on every surface form a cylinder; they can be accessed without moving the read/write heads.

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