Scor­pio News

  

January–March 1987, Volume 1, Issue 1











Page 26 of 63











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

Two standards have evolved for the number of tracks on 4 5.25" disk These are 43 tracks per inch (tpi) and 96 tpi although drives are now available which will achieve 192 tpi The useful recording aren of a $.25” disk 4s just under one inch band and #0 40 or 80 track drives are usual although we all know of the Gemini and Superbrain 35 track 48 tpi formate, when the earliest drives had only that number of tracks Ie is fairly obvious that in order to increase the number Of tracks on a dick, che read/write heed must write oarrower stripe to the disk.

number of sectors, Thes

Each track on the disk {2 divided up into generally one of 128, 25

fhe DDDS, QDsS and QDDS format Sectors are written to the disk by the format progr: this way ace known ae soft sectored disks. Som (apple for instance) use hurd Sectored diske where the physically marked on the disk by email holes similar to the index hol

Consider, for a moment, the Gemini QDDS disk format. Diake in this format have two sides, 60 tracks per side with ten 512 byte sectors per track. The sides are pumbered 0 and 1, the tracks are numbered 0 to 79 and the sectors are numbered 0 to”. On some systems, the sector numbering atarte at tector 1 to thet the Bectors would be numbered from 1 to 10. Some computer manufacturers start vith Other numbers but G and 1 are the most common velvet track and tector information is physically written to the diek during formatting so that the disk controller chip can identify the current side, track and vector by Atk will be looked at

rreheld on the disk eeoller can find « ctor on the track. Zee Be

Sector skew and its companion sector translation are used to improve access times when reading from Or writing toe 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 & delay while the processor is deciding that the next sector it wante 18 #ector 1 Meanuhile che disk will still be turping and by the time that « request is sent tothe 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 iisks sectors to be physically skewed during formatting. If ve look at 4a track on the disk, the sector tumbers’ aay Look Like thii

o7s1es2963

two sectors vill be allowed to found and the processor should

sple given, if we now read sector the disk head before sector 1 cor Row have plenty of time to make ite mind up. Obviously, the amount of skev fonployed depends very much on the speed of the processor and too much skew is as bad ax too Little when it comes to slowing down disk acce

Sector translation uses # similar principle to sector skew except that it i 8 Software measure to achieve the sane result. With sector translation, a table of sector numbers is held in the computer memory. This table may look Like that given below:

oseszseia7

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


This is an OCR’d version of the scanned page and likely contains recognition errors.











Page 26 of 63