[Coco] cache sector 0? related question

[Gene Heskett]

On Saturday 25 January 2014 12:30:32 Aaron Wolfe did opine:

> Here's another one for the experts:
> 
> Is the area in LSN0 that actually contains data ever bigger than 64
> bytes? NitrOS9 docs show PD.OPT section is from 0x20 to 0x3F... then..
> nothing?  Of course I ask because 64 bytes is much easier to deal with
> than 256.. but don't want to misunderstand something.

Your images are blank beyond $3F?  Every one I have ever looked at had a 
copy of the $20 byte PD.OPT section there, copied in from from the drive 
tables of the disk it was formatted in.  There is another, differently 
defined byte at $42 (IIRC, see the PD.OPT section of the L2 RBF docs) that 
I have had to change manually, and which reflects the track density 
capability of that drive. Similar, but not identical to the byte at $10. So 
you should have valid data up to and including $5F in LSN0.

If you don't, I think we have a problem.

> If I took that shortcut I would definitely add code on the server side
> to abort caching if non zero was found in the area past 0x3f.  Quick
> look with a hex editor shows nothing but 0 on a couple RBF images
> here, but obviously something could write there, and in the coco world
> that often means somebody does..
> 
> -Aaron
> 
> On Sat, Jan 25, 2014 at 11:05 AM, Aaron Wolfe <aawolfe at gmail.com> wrote:
> > Its often been noticed that sector 0 of an RBF filesystem gets
> > read/written an awful lot when doing nearly any disk I/O.  I've been
> > working on the dw server some this morning and am being reminded of
> > just how much.
> > 
> > Would it be practical to cache this sector somewhere?  256 bytes is
> > not tiny, but also not impossible to find in the typical system page.
> > The logic for a simple write-through cache would not be very complex.
> > 
> >  I guess I'm looking to the experts for reasons this can't work or
> > 
> > should never be done before looking at how to do it :)  Any thoughts?
> > 
> > -Aaron
> 
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Cheers, Gene
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