Date   : Sat, 29 Oct 2005 13:15:13 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: Formatting a disk beyond 80 tracks

"David Hunt" <dm.hunt@...> wrote:
> > http://www.mdfs.net/Software/BBC/FormDFS.txt - and got some interesting
 
> In a similar vein, I used to have a program that staggered the sectors of a
> disc so a track could be read without waiting for a revolution of the disc
> to pass first. You could hear the drive head clicking across at almost twice
> the speed. This made loading programs so much faster. Mind you with the
 
The format program mentioned above staggers each track by 3
sectors. I arrived at this by experimentation. It seemed to result
in the fasted disk access under various situations.
 
In case people don't know what track staggering is:
 
Often, when a disk is formatted, each track is laid out with the
sectors numbered sequentially from the index pulse, starting at
the same starting number. With DFS this is:
 
                  sectors
  track 0:  0  1  2  3  4  5  6  7  8  9
  track 1:  0  1  2  3  4  5  6  7  8  9
  track 2:  0  1  2  3  4  5  6  7  8  9
 
However, a disk drive takes some time to step from one track to
another. Imagine a file starting at track 0/sector 8 and occupying
four sectors. The drive reads sector 00:8 and 00:9, then steps the
head to track 1. However, by the time the disk has stepped the
head the disk has rotated slightly. The disk head will now be
somewhere near sector 2, so has to wait for eight more sectors to
spin past before it can read sector 1:00 and 1:01.
 
Staggering the format tries to arrange the sectors so that by the
time the drive head has stepped to the next track sector 0 is
ready to be read. Using a stagger of 3 gives this:
 
                  sectors
  track 0:  0  1  2  3  4  5  6  7  8  9
  track 1:  7  8  9  0  1  2  3  4  5  6
  track 2:  4  5  6  7  8  9  0  1  2  3
 
Now, after reading sector 00:9 the drive will step to track 1.
During this time the disk will rotate slightly, and instead of
sector 2 being under the head, sector 9 is. So, the drive only has
to wait for half a sector to go past before sector 0 is ready
instead of eight or so in the previous example.
 
As David said, there is a very noticable difference between a disk
waiting 85% of a rotation between each track and 5% of a rotation.
 
Zzzzzzz instead of chug chug chug chug.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
There are three food groups: brown, green and ice cream.