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

[pausch@saaf.se (Paul Schlyter)]

In article <qhhds3plac.fsf@ruckus.brouhaha.com>,
Eric Smith  <eric-no-spam-for-me@brouhaha.com> wrote:
> The final case occurs if the the start of the read is at the 0 bit.  Because
> the controller detects that it has read a full nibble by the most significant
> bit of the shift register being 1
 
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.
 
It's a tight loop in the RWTS which decides when a valid nibble is in
the shift register (= when the high bit is a 1):
 
nibloop: LDA (addr_of_shift_reg),X BPL nibloop
 
where X contains the disk controller slot number multiplied by 16.
 
Note that there's no timeout or any other exit out of this loop if
the high bit never should become 1 for some reason (e.g. hardware
failure).  There's no time for such "fancy" stuff if one wants to
make sure to not miss any new bit which arrives in the shift register
every 4'th clock cycle.
 
After a new valid nibble has been read, one has some 30 clock cycles
to do some other stuff, which in the RWTS of DOS, ProDOS, CP/M (yep,
the RWTS of the various CP/M implementations of Apple II are all
written in 6502 assembly) and most other OS'es is just storing the
nibble in a buffer.  The RWTS of Apple Pascal though mamanges to
actually de-nibblize the data as well, so when the sector has been
read, it's been de-nibblized as well, and the next sector can be
read+denibblized "on the fly", enabling Apple Pascal to read one
track in just one disk revolution.  In the other Apple II OS'es
de-nibblizing is done separately after the sector read -- and this
causes the RWTS to miss the next sector on the disk, forcing a read
of one entire track to take at least two disk revoutions.
 
The fast RWTS read code of Apple Pascal has to pay a price though:
that code is very large, compared to the much more compact RWTS'es of
the other Apple II OS'es.  The Apple Pascal RWTS uses a well-known
loop optimization trick: if the loop is to always be executed a fixed
number of times, it executes faster if it is replaced with linear
code.  This is called "loop unrolling".
 
 
 
-- 
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