Space Invaders Multiplexing

[Glokenpop]

Hi Sorry if this has been asked before I couldn't find it any where.

 

I'm trying to understand how Space Invaders works on the Atari 2600.

 

It has two player object that it can repeat - which gives us the 6 Aliens in a row. 2 * 3 and you can show 2 of three by varying the space and only repeating it once.

 

But how do they explode without flicker? I can't see how that isn't three different player objects in a row. When I look at it in debug is shows one of the player objects changing in the same horizontal position. So it is an alien and further down on the same line it's an explosion.

[alex_79]

The game updates the players' graphics as they're drawn, so that each copy can be different. (if you step through the code in the debugger, you'll see several writes to the GRP0 and GRP1 registers during the scanline)

In "Pepsi Invaders" (which is an hack of this game) the "aliens" are all different in each row, even when there's not an explosion.

Edited June 21, 2022 by alex_79

[Glokenpop]

Ah thanks - I thought that was what must happened but I didn't think the Atari could do that. I assume it was able because of the simplicity of the aliens - they don't move quickly or have much logic so some extra code to change them is possible.

[Thomas Jentzsch]

5 hours ago, Glokenpop said:

Ah thanks - I thought that was what must happened but I didn't think the Atari could do that. 

The usual 6 digits score display code is very similar, just even more condensed.