Re: [stella] My First Atari 2600 Game

Subject: Re: [stella] My First Atari 2600 Game
From: Thomas Jentzsch <tjentzsch@xxxxxx>
Date: Mon, 07 Oct 2002 13:55:14 +0200
Christopher Tumber wrote:
>But once I started fooling around with RESP0/RESP1 I got
>hooked on creating the ultimate 2600 version of that classic
>11x5 game...

I had the very same idea a few months ago.

>1) (Most importantly) As mentioned the last time this came up, 
if you align the sprites closest together with RESP0/RESP1 then 
when you remove a sprite, the previous sprite is shifted out of 
alignment to the left. It is possible to eliminate this 
shifting by spacing the spites a little further apart as I have 
done, however if you do that then a row of 11 sprites is 
extremely wide and there's no room for the Invaders to move 
left and right (A full row could move like 2 or 3 pixels back 
and forth). 

Yes, that was a problem for me too. I managed to get a few more 
pixels, but still maybe not enough. At least it where as many 
steps as Inv21 does.

>2) With this wider configuration there aren't very many cycles 
per scanline to process other things (specifically the player's 
shot) - 4 cycles per sprite, plus any loop overhead and pushing 
values into GRP0 and GRP1 doesn't leave a lot left over. I'm 
not saying it'd be impossible but even with mine trimmed down 9 
sprites it's an incredibly tight squeze.

It is possible using selfmodifying code in RAM.

>Heh, heh, only later did I realise that that couldn't be since 
Invaders21 is available from Hozer and I'd read a couple 
reference to how it and Inv30 run &etc. I went "Oooooooh! 
Dhuh!" and had to check the docs to find out which key mapped 
to the reset switch.... At which point I was able to check out 
the source for Inv30 and realise that it was possible to 
eliminate a sprite just by skipping the relevant RESP0 or 

Inv30? Haven't found that in the archives.

>Anyway, this is geting rather lengthy as it is, so I'll wrap 
it up now. Attached is the most recent bin, should run fine on 
any emulator that supports the RESP0/RESP1 trick and real 
hardware (At least it runs fine on my Supercharger, haven't 
tried it on a real cart ROM but no reason why it shouldn't...)

Looking really good. 

And as allways: If you need help in optimizing your code, just 
ask. :-)

Have fun!	

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