[sebhc] h17 and h8d disk images

Dwight K. Elvey dwight.elvey at amd.com
Wed Sep 1 14:36:54 CDT 2004

>From: "Dave Dunfield" <dave04a at dunfield.com>
>1) I had not-to-long ago just done the Altair simulator which has the
>   NorthStar hard sectored single-density disk system in it - this system
>   DOES NOT use headers, and sector numbering HAS TO BE the physical

Hi Dave
 Actually, single-density and double-density both use headers,
you just don't, usually, have as direct access to them as Dan's
methods would have. Both can also have different physical and
logical interleaving. I used to play with the physical sector
interleaving on an original IBM PC to speed up transfers.

>2) The H8 simulator caches the disk a track at a time, so interleave
>   factor makes absolutely no difference, except for the fact that I

 This makes perfect sense Dave. I'm not criticizing, only making the
point to Dan that both of the formats we are using now do not account
for interleaving. We both have good reasons for ignoring it. You
have no need for it in the simulator and I gather the data through
H17 calls so don't see it. I'm only aware of it when I format the
disk. Even then, once format is done, the software ignores it.
It is only a timing issue if such a disk is used for real systems.

>I actually has been assuming that the .H17 format always placed the
>sectors in sequential order in the file (I guess I assumed that when
>transferring, he just starts with the first logical sector and keeps
>reading the next one till the end of the disk - but now I realise that
>he may be reading and transferring entire tracks, including the header
>information, so at least he COULD be preserving the interleave).

 All of the .H8D files that I and others have creates have all been
in logical sector order. It looks like so far, the ones that
Dan has created all have logical order, even though created by
physical order. As long as the images for H8D are kept in logical
order, we don't have any issues. The potential problem is that
Dan's method drops the sector headers and could potentially have
scrambled sectors.
 I think we should always assume that the order for the H8D files
are in logical sector order. Anything else would not make sense since
there is no additional information to determine the physical
order of the original. Your format has the potential by using
the comments at the beginning of each sector.
 I see this a more of a problem for Dan since he has assumed that
all disk are created with matching physical and logical order.
 Recreating the exact interleaving is not essential for image
transfer. It is more of an issue of convenience of use once one
creates the new disk. I was even reluctant to add the feature
to my transfer program except I remember the enormous increase
in the loading of BASIC programs when used.

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