Secondary Storage in Detail
Primary storage (RAM, ROM, cache, registers) is directly reached by the CPU, is very fast and small. Secondary storage (also auxiliary or external memory) is non-volatile, slower, and holds large amounts of data cheaply, making up for the limited size and volatility of primary memory.
Classification
| Access type | Group | Devices |
|---|---|---|
| Sequential | Magnetic tape | ½-inch tape reel, ½-inch tape cartridge, ¼-inch streamer tape, 4-mm DAT |
| Direct | Magnetic disks | Zip disk, disk pack, Winchester disk (HDD) |
| Direct | Optical disks | CD-ROM, WORM (CD-R), CD-RW, DVD, Blu-ray, HD DVD |
| Direct | Memory storage devices | Flash (pen) drive, memory card (SD, microSD), RAM disk |
Sequential versus direct access
| Feature | Sequential | Direct (random) |
|---|---|---|
| Method | Must pass through the earlier data to reach the record | Jump straight to any location |
| Access time | Varies with the position of the data | About the same everywhere |
| Best for | Batch jobs, payroll, backup, archiving | Online booking, banking, databases |
| Example | Magnetic tape | Magnetic disk, SSD, optical disk |
Magnetic tape
A plastic ribbon, usually ½ or ¼ inch wide and 50 to 2,400 feet long, coated with iron oxide or chromium dioxide and wound on reels or kept in a cartridge. Bits are stored as tiny magnetised and non-magnetised spots.
- The tape runs from the supply reel to the take-up reel.
- The write head magnetises spots in the pattern of the data.
- The read head senses the pattern and turns it into electrical signals.
- The signals are converted into binary and sent to the computer.
It is sequential, slow, high-capacity and very cheap per bit, so it is used for backup and archives.
Magnetic disk
A thin circular platter coated with magnetic material on both sides. The surface is divided into tracks (invisible concentric circles), and each track into sectors (wedge-shaped parts, usually 512 bytes each).
| Parameter | Meaning |
|---|---|
| Seek time | Time to move the head over the correct track |
| Latency (rotational delay) | Time for the correct sector to spin under the head |
| Transfer rate | Speed of reading or writing once the head is in place |
Total access time = seek time + latency + transfer time
| Type | Year | Portability | Capacity | Status | Key feature |
|---|---|---|---|---|---|
| Winchester disk | 1973 (IBM 3340) | Fixed inside the PC | Hundreds of GB to TB | Still used | Sealed, airtight; rigid platters; the standard HDD |
| Bernoulli disk | 1982 (Iomega) | Removable cartridge | Up to about 230 MB | Obsolete | Flexible disk lifted by air pressure (Bernoulli principle); it flops away from the head if dropped, so it is crash-proof |
| Zip disk | 1994 (Iomega) | Pocket-sized cartridge | 100 MB to 750 MB | Obsolete | Replacement for the 1.44 MB floppy; died when pen drives arrived |
Optical disk
A disk with a thin reflective layer. Data is a spiral track of tiny pits (depressions) and lands (flat areas). A laser is shone on the spinning disk: a land reflects the light back, a pit scatters it, and a sensor reads these two states as 1 and 0. There is no physical contact with the surface.
| Type | Introduced | Capacity | Laser | Sub-types |
|---|---|---|---|---|
| CD | Early 1980s | 700 MB | Infrared (780 nm) | CD-ROM (read only), CD-R (record once, WORM), CD-RW (rewritable) |
| DVD | Mid 1990s | 4.7 GB (single layer), 8.5 GB (dual) | Red (650 nm) | DVD-ROM, DVD-R, DVD-RW, DVD+R, DVD+RW |
| Blu-ray (BD) | Early 2000s | 25 GB per layer (50 GB dual) | Blue-violet (405 nm) | BD-ROM, BD-R, BD-RE |
| HD DVD | Mid 2000s | 15 GB (30 GB dual) | Blue (405 nm) | HD DVD-ROM, -R, -RW; format discontinued |
Optical disks are portable, light and removable, but hold less than HDDs and SSDs and are sensitive to scratches and dust. Magnetic disks, by contrast, are sensitive to magnetic fields and shock.
Flash memory
- Electronic, non-volatile and based on EEPROM technology: it can be erased and rewritten electrically.
- Invented by Fujio Masuoka at Toshiba (1980). The name comes from the way erasing a block reminded him of a camera flash.
- Direct access, no moving parts, silent, shock-resistant, low power and very fast to read.
- Used in pen drives, SSDs, SD and microSD cards, smartphones and tablets.
Storage devices compared
| Feature | Magnetic tape | Magnetic disk (HDD) | Optical disk | Flash (SSD, pen drive) |
|---|---|---|---|---|
| Access | Sequential | Direct | Direct | Direct |
| Speed | Slow | Moderate | Slow to moderate | Very fast |
| Moving parts | Yes (reels) | Yes (platters, head) | Yes (spinning disk) | No |
| Capacity | Very high | Very high (TBs) | Low to medium | Medium to high |
| Cost per GB | Very low | Low | Low | Moderate to high |
| Best for | Backup, archives | Main storage | Media distribution | Portable, fast storage |
Keep practising
Notes are only half the work. Test yourself with questions and discuss doubts with other aspirants.
Prepared for exam revision from standard computer textbooks, Microsoft documentation and official sources. Features differ between Excel and Word versions (2016, 2019, 2021, 365) so check which version your exam follows. Found a mistake? Tell us.