Date   : Tue, 14 Aug 2001 12:19:59 +0100
From   : "Richard Gellman" <r.gellman@...>
Subject: Re: ANNOUNCE : UEF Specification 0.9

>As for non-standard disc formats - the new 'completely accurate' disc drive
>emulation in ElectrEm is now at the level of all type 1 commands (seek,
>restore, step, step in, step out) + read sector, and so should be able to
do
>all protected discs that just mess with the data after the id mark (which
>I'm willing to bet is most) or relies on non-standard sector ordering (e.g.
>interleaved for greater storage capacity) or an increased number of tracks.

I think (but dont quote me on this) used a trick which involved sending the
head too far into the centre, e.g. track 255 or something equally daft, and
then stepped back a preset number of tracks, thus positioning the head over
data which is not aligned correctly with the rest of the standard disc. This
renders commands like *BACKUP useless, but makes a cunning way to load data
off the disc.

>Within a week I reckon I'll have write sector and read/write track running
>which means even those really devious programs which hide data in the gaps
>or things like that should work. I don't think beta 10 of ElectrEm will
>support writing to FDIs, or possibly any beta, but fully accurate writes to
>UEFs will be implemented.

Will that be in UEF.DLL as well? *hint hint* :)

>But on the topic - does anybody have an example of some protected software
>that would run on an Electron so I can do some proper testing? And also, am
>I right in thinking that the DFS ROM's determing FM or MFM disc types by
>simply trying both? At the minute I've not fully implemented FM support so
>the emulator happily loads MFM discs with either mode selected causing 1770
>DFS *CAT to report FM operation, which is why I ask.

FM=Frequency Modulation (Single Density)
MFM=Modified Frequency Modulation (Double Density)

This is set by a bit in the floppy disc control register (&FE24 on a Master
128). Dunno where it is on an Electron.
(Just read this bit is mapped through to the ^DDEN^ line on the FDC, which
goes low when using double density (MFM).)

>And does anybody know how a disc drive reacts with no disc present? The
>WD1770 datasheet doesn't seem to think any alternate action is taken
>depending on the state of the ready line so I assume it goes ahead and sets
>the motor on anyway, but then not only will no data be found but I'd
imaging
>no index hole pulses (you know, assuming the motor ignored the motor line
>and didn't start) either so according to the datasheet the WD1770 will
enter
>an infinite loop which can't be right.

Something enters an infinite loop. I just tried *. (am I the only one who
knows of this *CAT abbreviation?) on my real BBC Master 128, and its still
spinning.
The 1770 only waits for 6 index pulses after motor start if instructed to in
the fdc command. However there is no logic to detect a timeout. So if
there's no disc, it just goes on infinitely, which is why your beeb just
sits there forever with the disc drive light on.
FYI, there were a few programs written for the bbc which worked under
interrupts to detect a timeout of a disc read/seek command, and would reset
the fdc and cause a BRK on timeout.

-- Richard Gellman

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