[sebhc] Re-creating actual floppies from archive

[Barry Watzman]

Interleave matters only when formatting a disk; it does not matter when
reading or writing.  The sector number is indeed recorded on the disk of a
soft-sector disk, but it's in a "header", not the data (so if the format is
9 sectors of 256 bytes per sector, you really have a full 9*256 of data).
[Also, for what it's worth, the software only looks for a match between the
sector # in the header and the requested sector.  You can have gaps, or odd
sector numbers (sector 237 appearing in a 9-sector track) and that's fine as
long as everything is understood by everyone using the diskette].

Note that while early disk controllers could benefit from this, even with a
2MHz 8080, an 8" controller using a 1771 chip (the very first Western
Digital Chip, single density only) could read at full speed with zero
interleave.


-----Original Message-----
From: sebhc-bounces at sebhc.org [mailto:sebhc-bounces at sebhc.org] On Behalf Of
Lee Hart
Sent: Tuesday, March 14, 2006 8:28 PM
To: sebhc at sebhc.org
Subject: Re: [sebhc] Re-creating actual floppies from archive

Joe Smith wrote:
> All those images and no way to turn them back into disks? That is
> basically what I want to know, I don't really see anything that
> lets you do that.

Heath's standard disk distribution format was 5.25", single-sided,
single density (about 90k per disk) for *both* hard-sector (H17) and
soft-sector (H37) controllers. Obviously, the soft-sector controller
could store a lot more by using double- or extended-density and/or
double-sided. But keeping the same single-density format made it easy to
put your master distribution files on a disk -- both have the same
layout; same tracks, same sector size, same number of sectors.

I think that the Heath format puts the sector numbers *in* in sectors;
thus interleave doesn't make any difference in the data read; just the
physical position it has on the disk. The operating system says "read
sector 1." The disk controller keeps reading until it finds sector 1 and
reads it. The sectors might start anywhere, and go by in any order. Then
the operating system says "read sector 2", and the process starts again.
Interleave is just a way to arrange the sector order so sector 2 happens
to just get to the read head as the controller starts that read sector 2
command. Thus the time delay is minimized. If the controller is slow
(for example, a 2 MHz 8080 and the software was written for a 4 MHz
Z80), all that happens is that the disk controller has to wait a full
revolution for sector 2 to come around again.
-- 
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
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