[sebhc] h17 and h8d disk images

Dwight K. Elvey dwight.elvey at amd.com
Wed Sep 1 18:34:15 CDT 2004

>From: "Dan Lanciani" <ddl-cctech at danlan.com>
>"Dwight K. Elvey" <dwight.elvey at amd.com> wrote:
>|>|It is true that all original
>|>|HDOS distribution disk have the natural order. This may not
>|>|always be the case. Dan mentions in a previous mail that the
>|>|only staggering he knew of was skewing but it was quite common
>|>|to use a 2:1 or 3:1 interleaving on HDOS disk ( you'll note that
>|>|my transfer program even has this option for creating disk ).
>|>Which H17 driver supported creating interleaved formats?
>| I use many of the calls from the H17. I have lifted the format
>|code from the H17 code and patched in the interleaving. Once
>|the disk is formatted, the H17 doesn't care if it is interleaved
>|or not since it only looks at the headers to find sectors.
>|You can see my code in the H89LDR9.ZIP file, source
>|file H89LDR2.ASM. The idea of interleaving was done by many
>|others before me. I just added it to my code for those that
>|want it. As long as I'm reading and writing through H17
>|routines, it makes no difference other than time.
>I know what interleaving is.  I'm asking which real-world H17 driver
>supported the creation of interleaved disks.  At the time I was really
>into this (I had my own heavily modified SY: driver) and I had never
>come across such a thing.

Hi Dan
 As far as I know, no Heathkit driver ever did this. I know it
was done by others because I learned the trick while looking
at other's disks that were formatted this way. I was in the
process of writing my Forth to run on the H89, at the time
( the one that is available from the ftp ).
How they did it I'm not sure. They may have had special SY:
files. I do remember reading an article about doing this on
the H89. So, like I've said, I was not the first one.
 Like I said, HDOS could care less, once it is formatted. It
just needs valid headers. You only need to know about the
disk's interleave while formatting. All the regular software
works fine with it after that. No special drivers are needed.
 I remember writng my own code to interleave, the first time
using the Forth that I'd implemented. I used this for a friend
that needed it for his BASIC so it wasn't so slow at loading.
He asked me to specifically do this for him so he must have
known about it.

>|HDOS it
>|self has an OS level interleaving like CP/M but it is not easily
>|modified at the OS level as CP/M is.
>Not really.  HDOS interleaved the directory sectors, but not with a fixed
>mapping, i.e., it didn't care if they weren't set up that way.  CP/M BIOS-
>based interleaving was typically a direct mapping done "transparently" on
>each track.

 I just meant that CP/M has a nice table that one can replace. I'm
not sure if I've ever seen anything like that in HDOS, although
it might be there.
>| It sounds like we are still in good shape. You'll just
>|have to watch out for any disk created with my transfer
>|code that someone chose to take advantage of the interleaving
>|option. It sounds like your code will bomb out on them.
>"fail gracefully"

 Yes, fails gracefully. :)
>			Dan Lanciani
>			ddl at danlan.*com
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