Discuss the characteristics and functionalities of auxiliary memories such as magnetic disks, magnetic tapes, and optical disks?
Discuss the characteristics and functionalities of auxiliary memories such as magnetic disks, magnetic tapes, and optical disks?
Quick answer
Auxiliary (secondary) memories hold large amounts of data permanently, at a low cost per bit. They are not directly accessed by the CPU. Their data is first brought into main memory. They are slow, but non-volatile.
Magnetic disk
A magnetic disk has circular platters coated with magnetic material. The platters spin, and a read/write head moves over them. Data is stored in tracks and sectors. It is a direct access device. The access time is seek time plus rotational latency plus transfer time. It has a large capacity (hundreds of GB to TB), a reasonably low cost and a fast transfer rate. It is used as the main storage for the operating system, programs and files. It is also the base of virtual memory. Its drawbacks are moving parts, noise, and damage from shocks.
Magnetic tape
A magnetic tape is a long plastic strip coated with magnetic material, held in reels or cartridges. Data is written in tracks along the tape. It is a sequential access device. To reach a record, the tape must be run past all earlier records, so the access is slow. But it has a very low cost per bit and a very high capacity, and it is easy to store. It is used for backup, archives and bulk data. A record can be read only in order, so it is not suitable for random access.
Optical disk
An optical disk stores data as pits and lands on a spiral track. A laser reads them by the difference in reflected light. CD-ROM holds about 700 MB, DVD about 4.7 GB, and Blu-ray about 25 GB. Read-only, write-once and rewritable forms exist. Optical disks are cheap, light and long lasting, and they are good for distributing software, music and movies. They are slower than a magnetic disk and have less capacity.
Comparison
| Point | Magnetic disk | Magnetic tape | Optical disk |
|---|---|---|---|
| Access | direct | sequential | direct |
| Speed | fast | slow | medium |
| Capacity | large | very large | small to medium |
| Cost per bit | low | lowest | low |
| Main use | primary storage | backup, archive | software and media distribution |
The functions of these memories are permanent storage, backup, data exchange between machines, and extending the main memory through virtual memory.
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