Stupid game code that works by coincidence

[CFou!]

Originally Posted by StingRay

Not necessarily since what a0 points to can be modified in the vertical blank for example. Let me guess, this is a delay loop?

yes, you are found

i use as a frame counter.

In VertibalBlankInterupt code, the adress in (a0) is incremented by 1 at each VBI


ps: it's of course a delay loop

[CFou!]

In Dragon Lair protection code STOP cpu instruction by:
move.l #copperlistadr,$dff080
clr.w $dff088 ; it's not exctly code but it's to give to you an idea
STOP
; after crypter datas

in fact CPU is freezed but a very long copperlist (in my memory more of 40ko!!!)
decrypts datas by many blitter accesses!!!

when copperlist is finished, 'STOP' is decrypted by good instruction and following code are decrypted..

It's only game in my knowledge using this decryption method

in fact it is more complicate because this little code is decrypted by a code using access faults, SMC and decrypt codes using A7 register as data register (monitor don't like this )

[StingRay]

There were some other games which decrypted code using the copperlist too (Pango & Shinobi f.e.). I quite liked such tricks since they weren't too common.

[Galahad/FLT]

Originally Posted by CFOU!

in my memory for old 68000 games as Double Dragon 2, coder used for DMA registers by example:
- FFF180, EFF180 or DFF380 for DFF180 !!

it's no really stupid code because i suppose coder want hid real code to crackers

but on 68020 and higher, that crashed

;--------------------------
a little game now:

in futur Ghosts'n gobblins trainer i use following lines of code:

Clr.l (a0)
.enc
cmp.l #25,(a0)
blo .enc

is it a stupid code ?

Bloody tell me about it, the tables I had to write to fix every single bloody wrong register address..... the tables are bigger than the code I used in the slave!

For three separate versions!!!!!

[MethodGit]

Originally Posted by IFW

It's been a while... but I think Spaceport had a wrong protection check, that only ever worked because the check for the result was also incorrect

Similarly, Carlos' copy protection doesn't account for much as it'll actually write the correct result for you (or I seem to recall at least... it's been several months since I last browsed through that game's code) before doing the check.

I think this is an appropriate enough thread to discuss here btw..... I'd like to point out an audio issue with Lethal Weapon.

I suppose many of you are aware of how pretty cool the music in this game is, right? Particularly the "night life"-style synth moodiness of the mission hub section at the start?

Well, what many people probably won't be aware of from playing it is that said tune also has this awesome electric guitar playing throughout (it kicks in at 0:23 of [ Show youtube player ]) - and they won't be aware because of a silly sound clash issue which prevents said guitar from ever kicking in at all the moment you jump. At any point. In a platformer.

I tested this thoroughly under all emulators. It doesn't matter if you jump at the very beginning of the track, or during the guitars, or several minutes into the music, once your character's jumping grunt has played, something within the code overwrites/destroys/erases the guitar from existence, leaving you with only the keyboards to listen to, and it'll remain this way until you give the game a reason to restart the music from scratch (say, quitting and starting a new game, or completing the next mission).

So, all in all, you're free to enjoy listening to the tune as fully intended, as long as you keep your feet firmly planted on the lower floor - which in turn restricts you to the first two missions.

I haven't tried to really look into the code to explain this further as I wouldn't really know where to start, but I'm sure one of the experts here can give a more technical lowdown. Either way, it's obviously a programming glitch.

Strange how, from recollection, none of the other tunes in the game lose channels like this...

[BippyM]

Kinda the wrong thread Methodgit, as quite clearly you're explaining "stupid game code that doesn't work"

However this is interesting and I can only think that the grunt uses the same sound channel as the guitar playing, which doesn't restart as it hasn't been instructed to do so.