> On Nov 30, 2015, at 3:45 PM, Fred Cisin <ci...@xenosoft.com> wrote:
>
> Oversimplified remedial tutorial:
> Ideally, the system reads a sector, does what it has to do with the content,
> and goes back for the next one, and can read every sector of the track in a
> single revolution.
> ...
> It is USUALLY the same on every track, but there are rare exceptions. And
> different disk formats from the same manufacturer may be different.
Your writeup was aimed at floppy disks, but interleave may also appear on hard
drives. I don't remember it in reasonably modern systems, but it shows up on
CDC 6000 systems. There the same drive model may be either interleaved ("2:1
interleave") or not ("1:1 interleave" [sic]) depending on the CPU. The
original 6000 series CPUs (or more precisely, their PPUs and I/O channels) are
too slow for non-interleaved transfer with the stock CDC drivers, so
interleaving is used. The 170 series have PPUs and channels that go twice as
fast, so they can handle non-interleaved transfers without losing revolutions.
And clever programming such as used in PLATO enables non-interleaved access
even on the 6000 series.
Use of interleaving when not needed comes with a 2x performance penalty, which
is why PLATO did a bunch of magic to avoid using it.
paul
