Re: please help--tracks, sectors, VTOC, and disk geometry

[Ed Eastman <noone@nowhere.net>]

jutland1916@hotmail.com wrote:
> When using a sector editor <snip>  are the sectors it displays the physical or the logical sectors?

Logical.

Physical sector location is relatively arbitrary on the media so it 
doesn't really matter where on the track it is located.  The sectors on 
a real disk are stuck physically in a specific location on the track 
that was decided by the format program.  From then on unless you 
reformat, that sector is physically on the same section of the diskette. 
 Beneath Apple Dos does a good job explaining this, it does not readily 
explain interleave properly.

The physical layout of the sectors (skew) simply allow the relatively 
slow apple II to read sectors as they pass under the head at an optimal 
rate.  Since the apple II's read sector (or block) routine is not fast 
enough to read and decode adjacent physical sectors, sectors are laid 
out by the format program non sequentially along the track for optimal read

Intuitively one would normally lay out sector numbers sequentially along 
the track this way:
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

But the Apple II needs more time between sector reads/writes so sectors 
are generally skewed so Dos3.3 or ProDOS can read sequentially as fast 
as the Apple II can decode the data.
0 6 11 1 7 12 2 8 13 3 9 14 4 10 15 5

In the first numbering scheme, assuming rotational speed of 300ms per 
revolution and wanting to read the sectors sequentially from 0 to 15. 
The Apple II could read one sector, decode it and then have to wait one 
full revolution to read the next sector as the next sequential sector 
had already gone by.  That's 4800ms for one full track read.  In the 
second number scheme, we can read 6 sectors per rotation or one track in 
900ms. (3 passes)

So the sector editor doesn't really care where a sector is on the 
physical media.

Thankx,
Ed