Date   : Sun, 09 Sep 2001 07:15:45 -0700 (PDT)
From   : Thomas Harte <t.harte@...>
Subject: Re: XFER

Firstly an interesting announcement : I've done some homework on the PC FDC
and have determined that it should be possible to do DFS disc access even on
new PCs, since if you can fit a valid MFM (double density) track header onto
the disc you can trick them into giving you a raw bit copy of the disc
surface - then in software decode that from FM (single density) form.

I'm also reliably informed that you are usually okay storing
higher-than-intended density information on a disc for a short period, so
for read purposes it should be possible to switch to ISO format (shorter gap
1) whack the data density up to 1mbit and squeeze an MFM header into (less
than) the DFS FM gap 1, then read the track from that with the next
operation and before the header has had time to decompose.

As for writing, the conventional wisdom is that the FM format is engineered
to never have two encoded 0's next to each other (the id, index and data
address marks use an altered clock but apparently maintain this property),
so it seems to me that if you throw a MFM header onto the start of the disc
that lets you write to the entire track, then write the FM encoded data in
MFM you should get a correct single density track, though again with strange
data within what will be gap 1. That shouldn't matter provided you don't
inadvertently generate an FM id/index/data mark within the MFM header. As I
say, I haven't explicitly examined the specifics.

>  I can concur with the statement on 720k disks not working on a 1.44M
drive.
>  I have numerous times attempted to format a 720K disk as 720K on my PC,
and
>  received nothing but "Invalid media or track 0 bad", from a disk I know
>  works. These 720K disks also show up as unformatted when you try to read
>  them.

If you're using experience from the PC, are you sure that is the drive, and
not another 'optimisation' to the PC hardware which causes the port based
data rate selection to be mostly ignored?

>  Except it seems on PCs, where the format program looks at
>  the existing format, determines its 720K and decides thats the limit. PCs
>  are odd.

I thought the idea of marking discs as double density rather than high was
to show that their media would probably cause tightly packed flux changes to
leak into each other over time? And, out of interest, do perpendicular
formats/drives work on standard media?

-Thomas





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