🚀 go-pugleaf

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

This does not completely explain what I have observed from reading
the TOS driver code and from experiments.  The driver issues a FDC
"seek-with-verify" command before every I/O operation, even if the
head is already on the correct track.  The FDC steps the head to
the desired track, waits for the next sector header to arrive, and
then reads the track number written there to check that the head
is indeed on the correct track.  This would seem to guarantee that
at least one valid sector will be missed between reads and would
explain the fact that TOS takes two revolutions per track read,
even if there were no "bug" in the read-multiple-sectors FDC command.

Robert Fischer and I have patched the driver code to bypass the
seek when the head is already on the same track.  This eliminates
the "dead" revolution in the case of a read ending at sector 9
followed by another read beginning at sector 1 of the SAME track.
When seeking to another track, 2 sectors are missed due to the seek,
but it is possible with that patch to get roughly 80% efficiency
with standard format disks by "twisting" the order in which sectors
are read, e.g.

    |1|2|3|...|9|
                 (seek)
                      |2|3|...|9|
                                |1|
                                   (seek)
                                        |4|5|...|9|
                                                  |1|2|3|
                                                        (seek)
                                                              |6|7|8|9|
                                                                       ...

Note that seeking to the same track also occurs when TOS writes
with the verify flag on, for it writes the data, seeks to the same
track, and then does the verify operation, causing a dead revolution
between the write and the verify.

What I do not understand is why FORMAT+ works.  According to my
understanding, the bad sector would terminate the previous read
immediately, but the following seek-with-verify would wait for sector
1 of the new track, causing it to be missed by the following read.
Is it perhaps the case that the FDC remembers enough of the sector
header so that a read immediately following a seek-with-verify can
catch the same sector?  Or does the read-multiple-sectors command
in fact terminate properly, and FORMAT+ works because the bad sector
causes the seek-with-verify to terminate early?  Or is something else
going on?

--Mike Fischer
        Arpanet:  fischer@yale.arpa
        UUCP:     ...!imagen!yale!fischer
                  ...!ucbvax!decvax!yale!fischer
        Bitnet:   fischer@yalecs
-------