> On Sep 14, 2017, at 10:44 PM, Fred Cisin via cctalk <cctalk@classiccmp.org>
> wrote:
>
> ...
> Alternatively, you could leave the sectors in sequential order, but not put
> each data block of the file into the sector of the same number.
> Thus, you could put the first data block of the file into the first sector,
> but put the second data block of the file into the third sector, and the
> third data block of the file into the fifth sector.
> Again, if the time to read one sector is not long enough to process the
> incoming data, then you could skip two sectors before the next data block.
An example of that is the DOS-11 DECtape layout (which was also adopted by
RSTS). It has the blocks in a linked list, and the next block allocator starts
looking for a free block 4 blocks beyond the current last block. So, give or
take fragmentation, you get 4:1 software interleave. The block numbers are
still physical numbers.
> ...
There's one other oddity I know of that shows up in the (PDP11/VAX) RX50
format, which is the non-standard track numbering. The first track (the one
with logical sector 0) is physical track 1. But instead of skipping physical
track 0 entirely, that track corresponds to logical track 79 (the last 10
sectors). I'm guessing that it started out with a desire to avoid physical
track 0, but then someone decided not to waste 10 sectors. But I never saw a
real explanation, and the optimization reasons that justify interleave and skew
clearly don't apply.
paul
