Sector Interleave

[Paul Koning]

> On Nov 30, 2015, at 3:45 PM, Fred Cisin <cisin at 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