Date   : Thu, 16 Mar 2006 00:45:29 +0000
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: SASI/SCSI <>IDE, distracting thoughts.

>Message-ID: <4417EEC6.2070404@...>
 
Jules Richardson <julesrichardsonuk@...> wrote:
> Problem AIUI is that ADFS just won't cope with anything other than 256 byte
> sectors at the lowest level (although isn't the ADFS filesystem format
> permanently fixed at 256 bytes/sector too?)
 
The ADFS filesystem uses 21-bit logical sector numbers.
Theoretically, it is irrelavent what size physical sectors those
logical sectors are stored in. RISC ADFS D used 1024-byte sectors
with four logical sectors in each physical sector.
 
To get 8-bit ADFS to do that would require a humoungous rewrite to
fundamentally change the way the disk access code works to buffer
physical sectors somewhere in memory in order to access the
smaller logical sectors.
 
> Out of interest, how's this been addressed with the various IDE projects out
> there? Presumably IDE drives have the same sorts of problems, and are
probably
> fixed at 512 bytes/sector without a LLF? Is half the disk being thrown away,
> or has ADFS been patched so it can address a high/low half of a 512 byte
> sector as a 256 byte one?
 
IDE devices don't use 512 8-bit bytes, they use 256 16-bit words.
So, an 8-bit filing system just doesn't bother to read or write
half the word (usually, the high half).
 
Means you lose half the actual drive capacity, but when you can
pick up 4G IDE drives by standing next to the bin, it's a minor
matter.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
The most perfect world is an imperfect world as the imperfections give
people a reason to strive to change it.