Friday, 15 April 2016

Bit Rot?

I thought I should bring all the C ports up-to-date today; Amiga is there and Neo Geo is all there bar the new (Mick Farrow graphics) sprite tiles being generated (ie. it has the dip-switch setting and the code just needs a simple tile bank offset to be added).

It wasn't all plain sailing though, the Neo Geo port gave me a bit of grief. Not having touched this in months, I just needed to add a few stubs for new palette routines so it would link and fire up MAME. I was a little surprised to be greeted by the cross-hatch screen. I generated a clean build, fired it up again, and same thing. The dreaded bit-rot... hmm...

After checking that the files were all being generated, and grasping at a few other straws, I decided to find out exactly what the cause of a cross-hatch screen is on the MVS. Turns out they're colour-coded, and my blue border meant that a cartridge could not be found! The only thing that could explain that under emulation would be a corrupt P1 (program) ROM.

Loading the P1 ROM into a hex editor revealed immediately that the file was all-but-blank! Time to go back and take a closer look at the compilation output. It's not easy to maintain a clean build on a multi-platform project, especially retro platforms with development toolchains that can be rather old, and as it turns out, the Neo Geo gcc (v2.95.2 circa 1999) complains quite a bit. So much so, that you tend to ignore it after a while. Examining the output more carefully revealed an assembler warning about incorrect attributes being set for .rodata in the newly-added Mick Farrow sprite graphics data arrays.

Without going into too much detail, I have needed to use section attributes to force gcc to place certain data structures into ROM - const just hasn't worked in some instances (yes I know the differences of placement). The confusing thing is, it was never an issue with the original ZX or CPC graphics arrays, and they're not significantly different to the new file. After using some Google Fu it seemed it was a known gcc bug at various times in the past, however the work-around didn't assemble in my toolchain. Again, grasping at straws, having noticed that the existing graphics files had additional arrays defined before the sprite arrays, I instantiated a dummy 1-byte array at the top of the new file and - viola - it worked! And for the record, exactly the same section attributes were still being set for the sprite data!

The whole process was probably a good hour or so.

I'm not about to ponder any further on this issue - it's not broke (anymore) so I won't fix it!

UPDATE: Mick Farrow's graphics on the Neo Geo port.

Mick Farrow's modified graphics on the Neo Geo

Incredibly the game now uses no less than six (6) sprite tile ROM's, not quite full but in the order of 40,000 tiles. In reality, at least half are never used by Knight Lore (ie. the VFLIP'ped versions, with the exception of a few border sprites) but at least the framework is there for the other filmation titles that may use them.

As a result, I had to find a larger host game, this time pspikes2. Basically each sprite tile ROM pair holds the sprites for an entire graphical variation. Note that Mick's panel graphics are not available (yet) as the panel is rendered on the FIX layer. I'd need to rip them and allocate another bank in the FIX ROM, which I will probably do soon.

UPDATE #2: Speaking of bit rot, in attempting to add another set of panel tiles for the Mick Farrow graphics, I've broken what was already there. I can't coax my tool into reproducing the existing panel data, let-alone a new one. Annoying...

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