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rotation by an index hole which marks the start of all of the tracks on the disk. This index hole passes over an optical sensor, once every revolution of the disk, thus indicating to the disk drive that the head is about to find the beginning of the currently selected track.

Two standards have evolved for the number of tracks on a 5.25″ disk. These are 48 tracks per inch (tpi) and 96 tpi although drives are now available which will achieve 192 tpi. The useful recording area of a 5.25″ disk is just under one inch band and so 40 or 80 track drives are usual although we all know of the Gemini and Superbrain 35 track 48 tpi formats, when the earliest drives had only that number of tracks. It is fairly obvious that in order to increase the number of tracks on a disk, the read/​write head must write narrower stripe to the disk.

Each track on the disk is divided up into number of sectors, These sectors are generally one of 128, 256, 512 or 1024 bytes long depending upon the disk format chosen by the manufacturer. The Gemini SDDS format uses 128 byte sectors while the DDDS, QDSS and QDDS format use 512 byte sectors. On most disk systems, the sectors are written to the disk by the format program and disks formatted in this way are known as soft sectored disks. Some systems (Apple for instance) use hard sectored disks where the sectors are physically marked on the disk by small holes similar to the index hole.

Consider, for a moment, the Gemini QDDS disk format. Disks in this format have two sides, 80 tracks per side with ten 512 byte sectors per track. The sides are numbered 0 and 1, the tracks are numbered 0 to 79 and the sectors are numbered 0 to 9. On some systems, the sector numbering starts at sector 1 to so that the sectors would be numbered from 1 to 10. Some computer manufacturers start with other numbers but 0 and 1 are the most common values. The side, track and sector information is physically written to the disk during formatting so that the disk controller chip can identify the current side, track and sector by reading the disk. How this information is written to the disk will be looked at shortly but suffice to say that because the sector numbers are held on the disk as a prefix to the data held in those sectors, the disk controller can find a sector and read its data irrespective of the order of the sectors on the track. If the sectors on a track are not held in numerical order (ie: 0, 1, 2,…, 8, 9) then they are said to be skewed,

Sector skew and its companion sector translation are used to improve access times when reading from or writing to a disk. Imagine that a disk has its sectors numbered sequentially from say 0 to 9 and that we wish to read sectors 0 and 1 in that order. Having found and read sector 0, there will probably be a delay while the processor is deciding that the next sector it wants is sector 1. Meanwhile the disk will still be turning and by the time that a request is sent to the disk controller to read sector 1, that sector will have probably passed under the disk head and the controller will have to wait until sector 1 comes around again. To overcome this problem, some manufacturers including Gemini allow the disk sectors to be physically skewed during formatting. If we look at a track on the disk, the sector numbers may look like this:

0 7 4 1 8 5 2 9 6 3

In the example given, if we now read sector 0, two sectors will be allowed to pass under the disk head before sector 1 comes round and the processor should now have plenty of time to make its mind up. Obviously, the amount of skew employed depends very much on the speed of the processor and too much skew is as bad as too little when it comes to slowing down disk access.

Sector translation uses a similar principle to sector skew except that it is a software measure to achieve the same result. With sector translation, a table of sector numbers is held in the computer memory. This table may look Like that given below:

0 3 6 9 2 5 8 1 4 7

What happens now is that although the physical sectors will be in sequential order, the data is read from or written to the sector pointed to by the