I've worked round this in a simple but brute force manner and it now looks as
if reading *and* writing work, more or less. There does still seem to be the
odd bad sector when writing 1440kB disks.
in turn allows the sequencer to lose less time --- this gets the effective
clock rate down to about 1.01us. However we still seem to lose the last sector
on 18-sector disks and there are some disk reads so something is still wrong.
little more reliable at reading long intervals off disk, but does seem to be
doing it. I'm going to definitely need some better sampling logic here and
probably a FIFO. But it'll do for now.
