Re: Disks that boot on 13 and 16 sector ROMs?

[pausch@saaf.se (Paul Schlyter)]

In article <1124923204.893573.95280@g14g2000cwa.googlegroups.com>,
josephoswaldgg@hotmail.com <josephoswald@gmail.com> wrote:

> Paul Schlyter wrote:
> 
> ...
>>
>> 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.
> 
> Maybe I'm missing something simple, but how can a 13-sector controller
> read the 16-sector disk contents?

Actually it can to some extent.  If you have a 16-sector disk controller
card and a 13-sector state machine ROM, try to but it on your disk
controller card (but keep the 16-sector boot ROM).  Yes, there'll be
an awful lot of read errors and retries, but I think I've managed to
at least boot DOS 3.3 in that way.  It's not enjoyable though....
And since the checksum is only one byte, one out of approximately 256
read errors will appear to succeed....


> Seems like it would have to be the other way around: 16-sector
> controller reads one sector (or so) in 16-sector format, enough to load
> a 13-sector RWTS routine for the completion of the boot.

You're absolutely right about that!  It is the other way around, and
I got it backwards when I described it.  The only thing I can blame
is my own memory: after all it is a few decades since I last dabbled
in that.

So these "double boot" disks are actually 13-sector disks, with a
special extra Sector 0 in 16-sector format on track 0.

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