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From: sansom@trwrb.UUCP (Richard Sansom)
Newsgroups: comp.sys.atari.st
Subject: Re: Twister, Format+ (was: Re: Bug report: 'The Russian Doll')
Message-ID: <1888@trwrb.UUCP>
Date: Wed, 13-May-87 12:04:23 EDT
Article-I.D.: trwrb.1888
Posted: Wed May 13 12:04:23 1987
Date-Received: Sat, 16-May-87 08:18:23 EDT
References: <8705081618.AA15013@ucbvax.Berkeley.EDU> <13104@watmath.UUCP> <967@batcomputer.tn.cornell.edu>
Reply-To: sansom@trwrb.UUCP (Richard Sansom)
Distribution: world
Organization: TRW EDS, Redondo Beach, CA
Lines: 69

In article <967@batcomputer.tn.cornell.edu> braner@tcgould.tn.cornell.edu.UUCP (braner) writes:

>Format+ (the "dead sector" speed-up scheme - I use only 80 tracks, though)
>seems to read a bit faster than Twister.

This has to do with the way the tracks are laid out.  In Format+, all
tracks are written with the following format:

    |BOT|1|2|3|4|5|6|7|8|9|[10]|bad|

Where "BOT" is the beginning of track, and "bad" is(are) the bad sector(s).

Format+ gets its speed from the fact that attempting to read the "bad"
sectors at the end of each track results in an FDC error which returns to
TOS in time to seek to the next track, and continue reading _without having
to wait for sector 1_:

    |BOT|1|2|3|...|9|[10]|bad|(return to TOS)
                                             (seek to next track)
                                                             |BOT|1|2|3|...

Since sector 1 is the next sector to be read, it takes 1 revolution to read
a single track.

On TOS formatted disks, there is no bad sector area at the end of each
track, so the FDC does not return to TOS in time to seek to the next track
_and_ pick up the first sector without having to do another full revolution
of the disk - i.e., it takes 2 revolutions to read each track.

Twister lays out its track with a staggered (twisted) scheme as follows:

    |BOT|1|2|3|4|5|6|7|8|9|10|
    |BOT|9|10|1|2|3|4|5|6|7|8|
    |BOT|7|8|9|10|1|2|3|4|5|6|
    |BOT|5|6|7|8|9|10|1|2|3|4|
    |BOT|3|4|5|6|7|8|9|10|1|2|
    |BOT|1|2|3|...

When reading a "twisted" disk, the read sequence looks something like this:

    |BOT|1|2|3|...|10|(return to TOS)
                                     (seek to next track)
                                                      |BOT|9|10|1|...

The FDC has to wait for sector 1 to show up before it can continue reading.
Since two sectors must be skipped, it takes roughly 1.2 revolutions of the
disk to read each track.

Therefore, the (approximate) relative speeds of the formatting schemes for
reading a disk are:

    formatter   revs/track   efficiency
    ---------   ----------   ----------
    Format+        1.0          100%
    Twister        1.2           80%
    TOS            2.0           50%

As far as writing speeds are concerned, you must turn off write
verification in order to fully take advantage of these fast formats (or
even with TOS formatted disks).  With write verify OFF, the relative write
speeds of the formatting schemes are the same as above.  With write verify
ON, performance will be degraded by (roughly) a factor of 2.

-Rich
-- 
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  /// Richard E. Sansom                   TRW Electronics & Defense Sector \\\
  \\\ {decvax,ucbvax,ihnp4}!trwrb!sansom  Redondo Beach, CA                ///
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