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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