And that's probably it for Vulgus!

I've decided to put Vulgus to bed.

After a fruitless session, I've exhausted my interest in finishing the RE. Given that I don't have plans to transcode it at all, the ROI for the remining sections is miniscule.

The few remaining 'free-standing' variables are flags and timers for spawning various alien types. I could probably stare at it long enough to come up with some sort of meaningful name for them, but I'm not going to.

The various alien object tables have indeterminate member variables that are almost certainly related to movement, whether they be timers, velocities and/or accelerations, or movement table pointers. I've spent some time trying to nail them down, but the different object table structures for different aliens makes it a nightmare.

Finally, the data structure used to generate and keep track of the background map is missing a whole heap of values, including pointers and offsets into various tables of meta-data, tile codes and colour palettes. I've tried a few times now to decypher it all, but it just leaves me with a headache.

I've thought of simple patching/lifting the source for the purpose of generating the full map, but even that seems overwhelming atm. I think I've simply burnt out on Vulgus.

That's it for Vulgus... for now at least.

I will continue to play it and hopefully (finally) see the titular Vulgus for myself. Part of the quandary I have is that I have no idea how the elements of the final boss look or behave, which makes it difficult to RE the code related to it, and I don't want to spoil the game for myself by cheating or watching a YouTube video.

Let's see what takes my fancy next...

You'll want to collect 'E' as well!

Another session, another few variables decoded (RE'd). Most of the 'free standing' variables have either been decoded, or I have at least some idea of what they do - just not the specifics. A surprising number of seemingly unused variables in the code; initialised but no cross-references to code that reads them. Setting watchpoints in MAME tends to confirm too.

There is a timer for spawning the non-firing or 'kamikaze' aliens in the game (spinning star-discs, black blocks etc) which is initialised after every spawning. However the value from which it is initialised is itself never initialised, other than being zeroed at program initialisation time. As a result, the non-firing aliens tend to come thick and fast in the game!

Related to this; formations (worth big points) consist of either 5 or 6 aliens as specified in a table entry. However the game will 'give up' waiting for 5/6 free slots in the object table after while and just spawn as many aliens as it can given the free slots - which means you may have to kill less aliens to get the bonus formation points! After all these years playing it, I never noticed this!!! 😶

I've finally detailed the result of missing the letter 'E'. Again, the manual suggests an immediate effect when it appears, however it is when you fail to collect it that something happens. In the case of the letter 'E', without going into excruciating detail because it is a little complicated, missing it increases the frequency for spawning of firing aliens (only). Not that it would be possible to increase the frequency of the non-firing aliens because of the abovementioned feature/bug. And like the letter 'D', at some later stage it becomes a moot point because the area will start with maximum frequency anyway. It has no effect on the big/mega aliens either.

Nothing to do with 'E', just eye candy for attention

Aside from the handful of 'free standing' variables mentioned above, it's mostly variables in the map and object tables that haven't yet been fully RE'd. And these of course relate to the two outstanding areas I mentioned in the last post, namely map generation and alien movement.

TBH I'm at the stage now where I just want to be finished - I'm thinking too much about the next project. However I think if I tackle the alien movement next it'll unlock a lot of the outstanding object variables, if not all. That'll leave just mostly map generation. And it would be nice to write something to display the entire map...

Parallax Scrolling!?!

Another quickie; cracked a few more variables in the last session which revealed an interesting if subtle detail to the game.

I had previously deduced the meaning of one variable to be the amount by which the background had scrolled left/right (dy), so it could be used to update, for example, the map and objects superimposed on it. Turns out that whilst I was close, I wasn't quite right.

There are actually two (2) variables updated when the ship is moved left/right; the abovementioned and another variable, updated by a slightly different amount. The other variable is actually dy for the background and ground-based objects and the first variable is dy for airborne objects, such as bullets and aliens.

What this means is there is a subtle parallax scrolling effect when moving the ship left/right! It is hard to notice when playing, but the effect is that your bullets (and certain aliens) will move left/right slightly less than the background moves.

[I should note that I use dy for left/right movement rather than dx because the screen is rotated, and the offset is made to the sprite register that is documented as the 'Y' register. Ultimately it's less confusing to reference the hardware than the physical orientation of the monitor, especially if the hardware is used for multiple games, some horizontal!]

It is getting harder and harder to progress, but I'm yet to finish a session without making at least some progress. A few variables seem to be initialised but never used (like the letter 'S'), but it's not straightforward to prove they're never used. In some cases the best you can do is set a watchpoint in MAME and play the game...

Here are the areas I have visited but have yet to complete:

• Background map generation/rendering; the data appears to be packed into blocks of meta-data
• Alien movement; alien movement is all table-driven
• Planet/Area data that affect difficulty; some of them are simple spawn timers or alien types but I haven't decoded every single parameter as of yet

I think that's about it.