Primary Storage
Primary strorage refers to the memory inside the computer. There are basically two types of memory.
The first is that which has instructions and data stored in it which is required to be kept even when the computer is turned off. This is called READ ONLY MEMORY and the contents cannot be changed without replacing the memory chip or reprogramming the instructions and data using specialised equipment.
The second is that which is used to strore the data and instructions required once the computer is started. These memory chips loose all of the stored data and instructions once the power is turned off. The contents of the chips can also be changed under program control at anytime that the changes are required.
Firmware
Firmware is software that is embedded onto a microchip. It usually takes the form of Read Only Memory (ROM) in that it is formed as an actual computer circuit. However, it is now more common to store firmware now onto a Erasable Programmable Read Only Memory chip (EPROM) as the data can be erased by the use of ultraviolet light and rewritten by a special device called an EPROM writer.
The most common use for firmware is in situaltions like the computer's Basic Input/Output System BIOS which must be maintained even when the computer is turned off, computer software such as Nintendo games that are loaded by pluggin in a special module containing the EPROM and embedded devices such as controllers for elevators, watering systems, traffic lights etc.
Secondary Storage
Tape Storage
Tape storage is usually used for backup purposes. The reasons for thes are as follows.
a) Tape storage is sequential - that is the data is accessed in a linear fashon. To find a particular piece of data, the tape is wound to the start and then searched from the beginning to the position on the tape where the data is to be found.
b) They are reletively slow, primaraly because of the manner in which the data is accessed.
c) They are relatively inexpensive. A large amount of data can be stored on a tape that costs very little compared to other devices. A 10gb DAT tape will cost about the same price as a 100 mb Zip disk or 40 1mb floppy disks.
Audio Tapes
These are rarely used now and were used on older systems. The data was stored as sounds and needed to be converted to and from audio to allow for storage and retrieval. They did not hold much data and were not very fast or reliable.
Digital Tapes
These come in a variety of shapes and sizes. The typical computer in movies often had large tape spools wizzing around. Today, some computers still use these large tape spools but a typical system would just have a casette which would be placed into the system similar to a VCR.
DAT tapes
These are Digital Audio Tapes and are reletively very small and hold a large amount of data - 10 gigabytes as a typical amount on a 120m tape.
Disk Storage
The most common storage meduim is Disk storage. This is because the data can be retrieved using a Random Access method. That means the data can be reletively immediately accessed as the mechanism used can directly go to the part of the disk that holds the data and retrieve just that data.
The advantages of disk storage is that access is very fast but the disadventage is that disk storage is relatively expensive compared to tape storage. (see prices listed above).
Hard Disk Drive (HDD)
There are basically two classes of HDD - those used in large commercial systems and those used in personal computers. The commercial systems store hundreds of Gigabytes of data and the systems look much like a typical washing machine!
For personal computers, the following is relevant. HDD's come in many sizes. Early HDD's held as little as 5mb of data although a typica drive for 1999 holds 4gigabytes of data. As weel, they come in a variety of forms such as 2 inch to 5.25 inch with a typical drive being 3.5inch. Modern drives also rotate at speeds of up to 20,000 revolutions per minute.
The way these HDDs attach to the system is also important. IDE drives are very common but are considered the slow cousins of the SCSI drive. SCSI drives are used in servers where very high speed and very fast data access is required. IDE drives are used in Small Office Home Office (SOHO) systems where speed is not as critical. However, some new IDE drives out-perform some new SCSI drives!
CD-ROM drive
Data stored on a CD-ROM is read only. Typically the data is stored using a CD-ROM burner and then the CD-ROM is used for distribution of data and information.
Floppy Drives
The first floppy drives were 8 inches in diameter and stored 180 kbytes of data on one side. The sizes have got smaller (5.25 inches and 3.5 inches) and the capacity has increased to a typical 1.44 mb. Latest techology (1999) allows much higher data volumes to be achieved but very little of this has become commercially available.
ZIP and Jaz drives
These are a variation on the floppy drive requiring a special drive. They hold up to 100 mbytes of data.
Author: Mike Leishman