Re: GCR specification - where?

[russotto@wanda.vf.pond.com (Matthew T. Russotto)]

In article <5dtns1$ajp@staff.cs.su.oz.au>,
Fred the Wonder Worm <ftww@cs.su.oz.au> wrote:
}
}I've recently unearthed my Apple ][+ and was somewhat surprised to find
}that everything was in working order (I did clear the spiders out of the
}case first, though).  Anyway, I want to find out precisely how the data
}is laid out on the disk; much searching through FAQs and various other
}places has revealed little more than that GCR (group code recording) is
}used.

Best bet is to track down a copy of Beneath Apple DOS by Worth and
Lechner (sp?).  Although no one seems to have a description of the
T0S0 nibblization for 13-sector disks. 

}So now what I want is a description of how data is organized on the disk
}under GCR, and how this is translated into bytes to move into memory.
}Obviously I could look at RWTS, but it's likely I would misunderstand
}something, and in any case the timing considerations would not be spelt
}out. 

Sure they are.  Just count cycles :-).  Though somewhere in the dusty
attic of my head I remember that it takes 32 cycles to write a disk
nibble and 40 seconds to write a self-sync nibble.  So first you write
a few self-syncs, then every 32 cycles you need to load the data latch
with a new value until you're done writing nibbles.  Then I think you
write some more self-syncs, but I'm not sure.  In any case, it's all
in the book. 

}PS: While I'm here, I'd also like descriptions of what happens with the
}    stepper motors in the drive.  I have a document by Neil Parker which
}    mentions the four phases, but doesn't say what happens if more than
}    one phase is on at once, or how long programs should wait before
}    assuming the head is where they want it to be.

Same book.  If you turn more than one phase on at once, you can get
the motor halfway between the magnets, which is how quarter-tracking is
done.
-- 
Matthew T. Russotto                                russotto@pond.com
"Extremism in defense of liberty is no vice, and moderation in pursuit
of justice is no virtue."