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Storage 1… or … Memory? Both hold data and/or instructions but: Memory resides in semiconductor chips ON the motherboard Storage refers to space that is OFF the motherboard: typically some kind of disk drive that supports reading and (usually) writing. 1. These notes treat the terms storage and secondary storage as synonymous.
Storage/Memory Compared • Storage is non-volatile: data is not lost when power is off. But most memory is volatile - apart from a small amount of ROM, data is lost when power is off. • Secondary storage gives much lower retrieval speeds than does memory • Secondary storage has much higher capacity than memory.
Secondary Storage Media NB Apart from magnetic tape, all these media allow random access. The reading device goes directly to the required location, without having to trawl through preceding items.
Hard Disks Pictures from www. howstuffworks. com
How Hard Disks Work • A set of metal disks, mounted above one another, are coated with a metal oxide then highly polished. Data is stored on the disk surfaces as magnetised spots within concentric tracks and wedge shaped sectors.
How Hard Disks Work • The highly polished disk surfaces offer great precision. This, and the fact the disks spin at speeds up to 3, 000 inches per second (170 mph) - the read/write head flying above the disk without touching it enables vast amounts of data to be stored (compared with tape’s much lower storage rates per square centimetre). Foreign Matter Read/Write Head Disk Surface This diagram shows the importance of keeping disks free from dust. The gap between read/write head and disk surface is very small.
Fixed v Removable Drives • The traditional hard drive is an integral part of the computer, housed in a disk bay within the case that also contains the motherboard. • Jaz and Zip drives are removable, enabling large amounts of data to be carried from one machine to another. (It may be, however, that the relatively large storage capacities of write-enabled CD’s and the much larger capacities of DVD - both discussed later - will exert a “squeeze” on the more expensive removable drives, which don’t hold more than 2 GB at present. )
Hard Disk Statistics • Capacity - at time of writing a typical PC hard disk will store 8 to 20 gigabytes • Speed - two ways of measuring this: – data rate: modern disk drives deliver 40 megabytes per second to the cpu – seek time: from cpu requesting a file, to the first byte of data from that file being supplied, is now typically 10 milliseconds or less
Creating More Space All binary files contain repetition. Compression software like Winzip can reduce the space they take up, but there is a performance cost since the files must now be decompressed before they can be used. So before resorting to file compression, be sure to get rid of unwanted files and archive old files.
Optimising a Hard Drive Where possible, files saved to disk are stored in contiguous memory locations. But as the disk gets full, new files get chopped up and stored wherever a few kilobytes of space can be stored. Retrieval times get slower as more locations have to be searched. You can run a defragmentation program once a month. (There is always one in utilities packages like Norton or the tools that come with Windows. ) This will try to shuffle files into contiguous addresses, so optimising storage and speeding up access.
Repairing a Drive Besides the problem of fragmentation, hard disk errors can also cause perform -ance to decline. Disk repair programs are another example of utilities software. They search for and repair disk errors, and should also be run at regular and frequent intervals - once a month, say.
Virus Protection • A virus is a program that disrupts your computer. Symptoms range from irritating messages onscreen, to total wipe-out of your hard disk. • You need to check regularly for viruses (especially if you use the net). Anti virus software is available, to check for known viruses every time you boot up. (Two things: one, that anti-virus software will only check for known viruses, so you need to keep updating it; two, that it checks on boot-up - so don’t leave your computer permanently switched on. Boot up once a day!)
Backing Up • Virus, disk failure, loss, theft and damage to floppies - all can cause data to be lost. Always back up anything that would cause you grief if lost. • Floppies are easy to back up and you should do so at least once a day if they contain important work - such as an assignment ….
Tape Drives • Tape only supports sequential access to data - much slower than random access. As a backup medium this is not important - retrieval won’t often be needed. • Tape drives enable data to be dumped from hard disk to tape. Like other storage media, they may be inside the computer or external to it. Usually they’re external, to allow backing up of more than one computer. • Most tape drives come with their own back up software to automate the process.
Tape Backup Software • Can be set to run automatically, allowing overnight backups while the computer is not being used. • Full backups copy all files. Incremental backups copy only the files that have been changed since the previous backup. • Most backup software compresses data to maximise storage capacity.
Types of Tape Drive • QIC (quarter inch cartridge) is still widely used, storing 4 gigabytes (GB) of data • Travan is faster, and stores up to 20 GB • DAT (digital audio tape) is the fastest and most expensive. Current versions can store up to 70 GB
An Alternative Back-up Strategy It is increasingly common for companies who need to back up data to dispense with in-house methods. Instead, they go outside the company to rent server space from specialists in back up provision.
Floppy Disks • Are read and written to in the same way as hard disks, but at lower speeds. The disk is floppy but, since the old days of 5. 25 disks, comes in a rigid plastic casing • Are useful for transferring and backing up small files. (Their use as an archiving medium is declining due to the much higher capacity of CD and DVD. )
Looking After Floppy Disks • Keep ‘em clean! • Use the write-protect hole to prevent accidental erasure of important data. It’s the same principle as on audio cassette tape.
CD ROM CDs are made from plastic, with tiny bumps spiralling out from the centre. These bumps are then covered with a thin layer of reflective aluminium which is itself covered with acrylic, on which a label is printed. Bumps
Reading a CD’s Bumps The bumps on a CD are tiny: reading them calls for very precise hardware. A CD drive has 3 basic elements: – motor to spin the CD at 200 - 500 rpm – laser and lens unit to read the bumps – tracking system to move the laser unit - at micron resolutions and allow the beam to follow the spiral track There’s no principled difference between music CDs and data CDs. CD players can’t do anything with a data CD because they have no means of processing its information but any PC with CD Drive, speakers and sound card can play music CDs as well as read data CDs.
CD Drive Components
CD Speed and Capacity • Early on, speed was a critical factor, as some drives read too slowly for quality sound and visual animation. Now the slowest drives read fast enough for this. (You’ll see drives advertised as 4 speed, 8 speed etc. This refers to how many times faster than the original read-speed of 150 Kb per second. On most uses, there is no difference between 24 and 12 speed!) • CD capacity is either 650 MB or 700 MB
CD Writers • The means of writing (and that includes copying) CD’s are now easily available. (Blank disks for CD writers attached to a PC are much cheaper than those for use with hi-fi CD copier systems. ) • CD’s are therefore an excellent archiving and back-up medium for small to medium sized companies.
DVD ROM • Digital Versatile (or Video) Disk. Similar to a CD-ROM but with a capacity of over 4 MB - and this is likely to rise to nearer 20 MB. • DVD ROM disks will hold a two hour film at better resolution than video tape. • DVD ROM drives also read CD’s.