Re: Apple ][ plus 13 sector! Problems...

[josephoswaldgg@hotmail.com (Joseph Oswald)]

pausch@saaf.se (Paul Schlyter) wrote in message news:<cditc0$2mo0$1@merope.saaf.se>...

> No, the controller doesn't detect anything by the MSB being 1.  The
> controller merely transfers whatever bits it reads off the disk into
> its 8-bit shift register.  Then it's the responsibility of the software
> (i.e. RWTS) to read a valid nibble by reading the shift register at the
> appropriate time.

Actually, the sequencer code *does* keep track of whether the high bit
of the data shift register is one; this is probably the trickiest part
of understanding the Woz state machine. In write mode, it has to keep
track, because that determines whether the "high numbered" states with
the write pulse = 1 or the "low numbered" state with the write pulse =
0 get executed. _Beneath Apple DOS/ProDOS_ totally skimp on their
description of write mode, but if you look at the schematic, you see
the connection. In read mode, the state machine logic holds off on
shifting in the bits of the next byte when the data register is
high-bit 1, by taking separate branches for "10" and "11"

Just for my own records, <a
href="http://216.239.41.104/search?q=cache:eLu67H9GUXUJ:www.cs.umu.se/~christer/Apple/p6.notes.html+christer+P6&hl=en";>Here</a>
is a link to a Google cache of a P6 rom description.