[stella] Alpha version of my game

[emooney@xxxxxxxxxxxxxxxx (Erik Mooney)]

Attached in MIME format is the current state of my game.  It's a Space
Invaders clone, using playfield graphics to display an 11x5 matrix of
invaders instead of the 6x6 of the Space Invaders we all know and hate =)
In its current state, Game Select toggles between single-player and two
player simultaneous modes, and Game Reset begins a game.. the B/W and
difficulty switches are not yet used.  And there's no sound yet.

Try this out and let me know what happens.  It works fine on PC Atari and
on a Supercharger attached to my 4-switch NTSC 2600.  I'd appreciate it if
someone would tell me what it does on PAL systems.  (I'll probably
implement the B/W switch as a PAL/NTSC switch eventually.)

Please don't distribute the game beyond here yet, though.

Tech notes: It uses simple player graphics to display the players, and
repositions them with multiple copies (if necessary) to display the number
of lives remaining (idea stolen from Defender.)  The missiles are used for
the players' missiles, naturally enough... the ball is used for the invader
bombs, and it can be recycled up to 5 times to display 5 bombs
simultaneously (as always, no two bombs in the same vertical zone.)
Everything is displayed every frame - there's no alternating-frames logic
for anything.

Known bugs/limitations:
I don't yet check for collisions between enemy bombs and players.. that's
cause playing the game would be just too hard in its current setup..
suggestions welcome =)

I haven't implemented scores yet either.. I plan to use the playfield to do
that, at the top of the screen.  I'm going to add a saucer, too, as all
Space Invaders games should have =)

When the second player's missile should be disabled, it gets displayed at
the top of the screen instead... this will be fixed when I add score
display.  Does not affect gameplay.

When an invader bomb is due to begin its display on the same scanline as an
invader is being displayed, it gets skipped for that frame.  This is
because there's no time to set RESBL during a scanline in which I'm writing
to the playfield registers six times.  This is hardly noticeable, though.

The bombs change color on different scanlines along with the invaders..
this gives a very nice 2600 feel to it (see Super Breakout), so I don't
plan to change it =)

Each successive wave begins lower down than the previous; there is no
limitation on this, so eventually the waves will start so the bottom row of
invaders is at the level of the players.. this will screw up the scanline
count so the display will lose sync if you play long enough.

There are no "shields" above the players, nor will there be.. they would be
extremely difficult to implement with the setup of my screen kernel.

Attachment: inv.bin
Description: Binary data