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...