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(fdutils.info)Interleave

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Using interleave
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   After having read a sector, the floppy controller needs to "rest"
for a short time.  This time is used to compute checksums, to reset
internal circuitry, etc.  During this time, the floppy disk continues
to rotate, and the "rest" time thusly translates to a certain minimal
gap size.  If a smaller gap is used, the next sector header flies by
the read-write head before the floppy controller is ready again to pick
up the data.  Thus, it has to wait until the next disk rotation until
that sector comes back again.  This leads to an unacceptably low
throughput, as now the system can only read one sector per rotation
instead of all sectors in one rotation.  If we want to use smaller
gaps, we have thus to use _sector interleaving_.  This technique
consists in arranging the sectors in a way such that the next logical
sector does not immediately follow the current sector, but instead
another sector is inserted between two successive sectors.  Instead of
having the following order:
     1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21

   we would use the following order:

     1,12,2,13,3,14,4,15,5,16,6,17,7,18,8,19,9,20,10,21,11,

   This new order allows the floppy controller to rest during the whole
time that sector 12 flies by between reading sector 1 and 2.  This
technique still cuts throughput in half, because two rotations are
needed (one for reading sectors 1 to 11, and the second to read sectors
12 to 21).  However, this is far better than the 21 rotations which
would be needed without interleave.

   This technique allows us to use a gap size of just 1, and thus fit 21
sectors on one track.

   *Usage:* Once formatted, interleaved disks can be used in a similar
way to disks which have simply more tracks.  They can be accessed using
`vgacopy' in Dos, you can boot from them using Lilo, and you may
install any filesystem on them.

   *Interesting Formats:*

     density     tot. cap.    throughput    media description
     5 1/4 HD    1440KB        27KB/s       hd sect=18
     3 1/2 HD    1680KB        26KB/s       hd sect=21
     3 1/2 ED    3360KB        52KB/s       ed sect=42

   You don't need to tell `superformat' to use interleaving, it figures
out by itself when interleaving is needed.  You don't need to tell
`setfdprm' either that a disk is interleaved, as this information is
not needed to read the disk