Re: Disks that boot on 13 and 16 sector ROMs?
- From: pausch@xxxxxxx (Paul Schlyter)
- Date: Wed, 24 Aug 2005 21:15:20 GMT
In article <Pine.LNX.4.62.0508241459520.15533@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Lyrical Nanoha <LyricalNanoha@xxxxxxxxxx> wrote:
> I've heard of disks that will boot natively on 13 and 16 sector ROMs, how
> do they pull this off? I'd like to mess around with this ability for the
> hell of it, now that I've been able to get DOS 3.2 up on my Apple //e
> (with some help lol).
First, all such disk I've ever seen or heard about have been 16-sector
disks -- well almost anyway; on track 0 there is one sector in 13-sector
format. But the rest of the disk is in 16-sector format.
They pull this off by adding an extra Sector 0 on Track 0. This extra
Sector 0 is in 13-sector format, so these disks have two "Sector 0"
on Track 0: one in 16-sector format an the other in 13-sector format.
The 16-sector format "Sector 0" is a normal 16-sector boot sector.
Thus booting with a 16-sector boot PROM yields just a normal 16-sector
boot.
The 13-sector format "Sector 0" is special: it contains the equivalent
of the 16-sector boot PROM, bot relocated to address $300 (where the
13-sector boot PROM loads sector 0).
Booting such a disk with a 13-sector boot PROM works like this:
1. The 13-sector boot PROM loads the special 13-sector Sector 0 at
address $300 and then executes the code there.
2. The code at $300 does the equivalent of the code in a 16-sector
boot PROM (except recalibrating the disk drive - that has already
been done by the 13-sector boot PROM and need not be done a second
time): read in the normal 16-sector Sector 0 at address $800 and
execute the code there.
3. From this point the code at $800 performs a normal 16-sector boot.
So where do you get the space needed for that extra Sector 0 in
13-sector format on Track 0 ? Baiscally there are two ways:
A: Remove some other sector which isn't needed.
B: Squeeze the remaining sectors together so that the number of sync
nibbles between the sectors are decreased. Making these FF nibbles
normal nibbles rather than sync nibbles saves even more space by
removing those two extra "00" bits in every FF nibble. Sync nibbles
need only be present at one place on the track, not before every
sector. The penalty for that is that the disk sometimes has to spin
one additional revolution before the nibbles are synced, and this
penalty applies only to track 0. Yes, some special code needs to be
written to write such a track 0 to the disk.
--
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Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/
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