In the beginning was the disk and the disk was where it was at. It was the
source from which all knowledge flowed into the brain (CPU). And that knowledge
needed storing somewhere. Well, OK before disk there was tape and before that
punched cards. But having got your information into the computer some way is
required of keeping it. When you switch off anything stored in RAM will be lost.
It must be recorded to be kept. Just as you have a wardrobe for keeping your
clothes or a chest of drawers so a business has a set of filing cabinets in
an office. Suppose this office has just one room where it keeps all its information.
In this room it can decide how many filing cabinets it can have: say 9, 10 or
16. The boss can also say how they are arranged: all side by side along one
wall or around the walls, stacked on top of each other etc. For each drawer
the filing clerk can decide how many files are kept. For standardisation we
shall say that he or she keeps a set number in each drawer - perhaps 40 - with
an index or catalogue of all the files kept. Just as a library has a catalogue
of all its books.
Now, in days of old computer data used to be held on long spools of magnetic tape. Without going into technicalities we shall just say that just as a magnet passed over a pile of iron filings can rearrange those filings (remember your elementary school physics?) so a tape head can rearrange the particles stuck to a piece of tape such that a group that passes in a given moment of time represents an item of data be it audio, video, text or whatever. Now suppose that tape were flattened out to form a disk (rather like a gramaphone record). We can now rotate the disk so that it passes underneath the head instead of across it and we can move the head across the disk. The principles of electromagnetism can be used to put data onto the disk. And we have speeded up access to our information. Not only that, but we are now able to access any part of our information store at any time ie random access. A tape only gives us serial access since only one item of information can be "seen" at once before we can get at the next - a continuous stream of data. It's a bit like standing on the platform of a station watching the carriages go by of one train before you can see the one behind travelling at a different speed instead of standing on the bridge where you can direct your view where you like.
So, a computer disk is a thin flat bit of plastic coated in a magnetic material. This is why it is called a floppy disk: it is so thin it flops about. In order to protect it it is placed in a plastic jacket. Well, it doesn't unzip but you can lock the zip to stop people getting inside it. Or to put it another way, you can stop anyone adding information (writing) to it by covering up a notch or hole. This is known as the write protect notch so to stop anyone altering the disk (perhaps yourself accidentally) you write protect it. Confusion can arise in that there are several sizes of disk (or diskette - spelling a computer disk with a 'c' is incorrect): the most common are 5¼ inches and 3½ inches. Other sizes need not concern us. The former was the first and had a flimsy jacket which could be bent etc causing damage so when the latter was brought out a hard jacket was added instead making it more robust.
Now just as our office above can organise its filing cabinets so a computer can organise its disks. In fact, it needs to do this so that it can find your information. Most people know that a gramaphone record has one groove containing the audio information which at a glance appears to be many grooves. It is, in fact, only one groove so that the needle can travel from the edge to the centre. But if the needle did not need to touch the surface but could instead "sense" the information then many grooves could exist. A computer splits a disk up into TRACKS in just this kind of way - a process called formatting. Remember that the head which passes over the disk does not actually touch the spinning surface but passes above a window in the jacket and creates a magnetic field in order to read what is on the disk. A disk drive is the piece of hardware that reads information from and writes information to the disk. Because of the state of the technology early disk drives were only 40 track but more modern drives were 80 track because, basically, the area that produced the magnetic field was able to be made thinner. A technical fix enabled these drives able to read the older style disks. More information could be stored as it was packed tighter and more condensed.
Each track is split into a number of SECTORS of a fixed number of BYTES (usually 256 or 512). Thus each part of the disk has its own unique address (filing cabinet 10 drawer 3, for example, track 10 sector 3) so that if we want something all we have to do is find its address and then start reading from that point (just as you might start reading a book from a certain page). When a disk is formatted (or prepared ready for use) these sectors are created. The size of each sector depends on the disk system in use. Every sector on a particular disk is the same size (although some software companies protect their disks from being copied by varying the size using special software).
So how do we know what's on our disk? A computer stores information in files just as our filing cabinet has files. The file could be a program, data, a picture or whatever. It is a collection of related information. Files can be of differing sizes. We need some kind of index or table of contents in other words a catalogue or directory so that we know what files we have. This starts at the very beginning of the disk and is sited at track 0. Since computers start numbering at zero (mathematically this is the very first number) it starts at track 0 sector 0. It contains the names of all the files on your disk, how big they are and where to find them just as a telephone directory contains the names addresses and phone numbers of people or you might have a catalogue of what's in those filing cabinets. Note some systems store this slightly further on than track 0 sector 0. There are also systems which make use of multiple, linked catalogues.
Floppies (cont) Hard disks Raid Care and Attention CD ROMS CD Recording Viruses Backups
Contents Introduction Communications Input Numbers Systems Operating systems Disks History Glossary Links Contact