The ED/FB level loader
mr216@greenwich.ac.uk (Russell Marks)
----------------------

Games uploaded by Russell Marks
	Outrun
	Turrican
	Midnight Resistance
	Power Drift
	Gauntlet
	Afterburner
	Op. Thunderbolt

Description by Russell Marks:

Hmm. Possibly, but it'd involve a bit of hacking. The .DAT files have
to be loaded on a speccy (or maybe on Z80 with the tape I/F) *with the
original loader routine*. I usually just cut out the relevant bit,
run it from basic on my +3 with a little M/C routine which gives it
the right values and checks the load worked, save the files, and
transfer those back to my PC, where I strip the +3DOS header and
make sure the files are the right length (I think I screwed this up
on Gauntlet BTW, and some of the graphics get a bit corrupted :-( ).

I wrote a fairly lame README when I first added this level loader
hack to xzx, which I uploaded to ijs along with (I think) Gauntlet
and Op. Thunderbolt (or possibly Afterburner). Here it is:


Details of the level loading mechanism
--------------------------------------

The idea is that initially, a 48k snapshot - usually in .sna format - is
loaded containing the game. Then, whenever the game would ordinarily load an
extra section from tape, something like this happens;

...
LD HL,address_to_load_data_at
LD A,level_number
DEFB 0EDh
DEFB 0FBh
...

This section will of course take the place of the original load-from-tape
routine. The emulator then traps the opcode ED/FB, and loads a file at the
specified address. Under xzx, the filenames are generated as follows
(described as an sprintf() for clarity):

	sprintf(filename,"%s%d.dat",prefix,level_number);

...with the level_number being the value the A register had when the
emulator trapped the opcode.

In xzx, the prefix is taken from the 'basename' of the last snapshot loaded,
e.g. "wibble.sna" -> "wibble". There is also provision for the re-loading of
game-in-progress snapshots, such that you can directly supply the prefix
used while specifying a snapshot to load, e.g. "game_name,my_game.sna". This
will then read in game_name1.dat, game_name2.dat etc. as appropriate.