Re: Apple2 disk controller questions

["Fabrice Frances" <frances@ensica.NOSPAM.fr>]

Damn I was without news access during five days... 8-(
Thanks a lot for this detailed answer, David, it's great !

Now I understand my error !  I was counting 250x1024 bps
as the transfer rate instead of 250000bps...
Poor me, I have assumed this 256000 bps for several years,
I should have asked myself where the clock for
such a rate is... (the 'Mega' on those 8Mhz crystals is
surely not the same as on those hard drives ;-)
Ok, so this means 6250 bytes per track for a MFM...
(your 7250 theoretical limit is a typo, isn't it ?)

In fact, 50000 bits can be used on a track, whatever the
encoding format is, am I right ? The encoding scheme
then decides to reserve one bit out of two for clock purpose
(FM), or less costly clock schemes (GCR and MFM).
So, I'm still wondering why those .nib images hold 6656 bytes...
I can see two reasons, could you help ?
- the rate is 255kbps instead of 250kbps, as David Wilson states
in his table... can anyone look into an Apple2 controller and read
the crystal frequency in order to be sure ? this would mean
6375 bytes per track...
- more practically, modern computers use 512-bytes sectors,
so the emulators have chosen to use a size of 13x512=6656
bytes per track in order to accelerate virtual seek operations...

Anyway, David, thanks again for your explanations, I guess
I have something to thank you... remember our discussion
about Applesoft Basic and the hidden Microsoft signature ?
I now have the original Basic65 from which it is derived...
there is only a few bytes missing here and there, because
it was patched to run on the Microtan65 (those patches are
visible like a big nose in a face, so most of them can be
restored to the original code)...
Well, I will give it to you Apple2 people if you all enter the
1K 6502 programming contest ;-)
(some of you already have manifest their intention to
participate, but I guess there are more people here
who could join that nice occasion to live their youth again 8-)