RE: [stella] Space Instigators - Final Version (?)

Subject: RE: [stella] Space Instigators - Final Version (?)
From: Christopher Tumber <christophertumber@xxxxxxxxxx>
Date: Thu, 17 Oct 2002 16:35:01 -0400
Eric wrote:

>Don't know if it is intentional or not, but you can move before you start
>the game.

Heh, was wondering if someone would notice. It's an homage to the original 2600 version...

>I've also seen some times where it seems like some additional scanlines are
>happening (z26).  The screen also jumps between levels.  No phantom lines
>that I saw.

Hrm, I'll assume you were looking at Sp16.bin, so I'll wait until some feedback on Sp17.bin comes back before I go fiddle. It seems okay on this end now...

>From my experience with Skeleton, the only way to keep your kernal stable
>is to cycle count the hell out of it.  

Yeah, I have a HUGE kernal and the part of the kernal handling display of the Instigators involves quite a bit of computation. The actualy display routine is held in RAM (and there are 10 different routines, which one gets copied into RAM depends upon the horizontal position of the formation but that copying is done in the Vertical Blank) and in preperation for displaying each row, the kernal has to go in and turn on/off RESP0/RESP1 statements depending upon which Instigators are "alive" or "dead". It also has to handle the exceptions where 0 or 1 copies of either the Player1 or Player2 as well as when there are not Instigators at all on that row (so that blank rows don't overlap the houses, or cause the game to end) (If there was more RAM, I could pre-calculate a different routine for each row, but, with the RAM restraints they need to re-use the same routine so the RESP0/RESP1's need to be reset for every row. Plus the exception handler...)

The problem was the early branch once "many Instigators" or "0 or 1 Instigators" has been determined (and it has to do this twice, once for Player1 sprites and once for Player2 sprites). These remaining "routines" were unbalanced, leading to adding/droping scalines once it was down to displaying a couple Instigators 

>The animation of the 2nd & 3rd rows of aliens doesn't seem quite right.
>I'd have to load up MAME & check, but I thought each of those rows had
>unique aliens.

Obviously it's not going to be pixel perfect but check MAME. Maybe you're thinking of the original 2600 version?

>It took me a moment to remember to zap the aliens from outside in rather
>than bottom to top.

Actually, there are a couple strategies - Take out the center columns and then park yourself in the middle of the screen to get a clear shot at the Saucer (Until the formation gets too low, anyway). When there were more scanlines per row, it actually made sense to take out the bottom rows first because it gave you a lot more clearance (it's still really helpfull to take out that bottom row, just not as much...)


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