Date   : Fri, 01 Aug 2003 00:47:58 +0100
From   : "Thomas Harte " <thomasharte@...>
Subject: Re: BBC double density disc images

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A while ago I made the argument that the Acorn related emulator formats were
clearly designed from the viewpoint of a BBC Micro user in that the ADFS
formats store things side interleaved meaning that truncating them is highly
inefficient.

Because of this I also saught alternatives as my emulator is of the Electron, 
which is primarily ADFS based. I incorporated non-interleaved implicit disc 
storage into my UEF format (which is rarely used except for tapes) and added 
support in my emulator for UEF and also another file format not directly 
from the Acorn world, FDI. FDI is a format from the world of Amiga emulation 
which can store data at the very lowest level when required. UEF shares the
ability to store lowest level disc encodings, but unlike FDI it is an all
or nothing choice - very low level or very high level encodings only.

Another reason for my incorporation of discs into UEF is the bit multiplexing
feature my emulator supports - which is essentially the same as the 'new
graphics on old games' functionality of many other emulators.

In my opinion the FDI file format is a mess, but source code that can decode 
it is available from the home of disk2FDI, the (DOS only, I'm afraid) program 
that can create FDI images on two drive PCs. See http://www.oldskool.org/disk2fdi/
for FDI information, see particularly http://electrem.emuunlim.com/future/UEFSpecs.htm
for UEF information.

>... and it's easy enough to read DD discs on a PC.

For now, maybe, but not under XP or with the increasingly common USB floppy
drives.

-Thomas

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