Subject: Re: [stella] a thing that doesn't let me sleep... From: emooney1@xxxxxxxx (Erik K Mooney) Date: Sat, 23 Aug 1997 11:35:47 -0400 |
>...how could they put seven aliens per row in Atari Galaxian? Short answer: lots and lots of code :) It's an 8k game... disassembling the code shows about half of the second bank to be mostly the same section of code repeated over and over, except with small differences. May even be as much as a different routine for each possible combination of aliens present on any particular row.... either that or a different routine for each possible number of pixels for the leftmost alien to be offset from the left edge of the screen. All the aliens are done with player 0, leaving player 1 to draw the aliens that have left the formation and are attacking the player, and to draw the explosions when the player hits an alien. Looks like the ball is used for the player's missile within the alien formation, and the invaders cannot drop missiles until they are below the lowest line of the formation. Here's a snippet from Distella: LF2DB: LDX #$FC ;2 STA WSYNC ;3 LDA ($80),Y ;5 STA GRP0 ;3 LDA ($82),Y ;5 STA GRP1 ;3 LDA $86 ;3 STA $0110 ;4 this is just STA RESP0 but adds an extra 1-cycle delay STA NUSIZ0 ;3 Player 0 first gets positioned at pixel 74 from WSYNC LDA $87 ;3 NOP ;2 STA RESP0 ;3 pixel 107 STA NUSIZ0 ;3 LDA $88 ;3 INY ;2 STA RESP0 ;3 pixel 140 STA NUSIZ0 ;3 LDA $89 ;3 NOP ;2 STA RESP0 ;3 pixel 177 STA NUSIZ0 ;3 After playing the game for about 20 seconds and exiting into PC Atari's debugger, the values at $86, $87, $88, and $89 were 2, 3, 3, and 0 respectively. So, apparently, hitting RESP0 every 11 cycles and writing different values to NUSIZ allows multiple copies of objects (?) The vertical delay is also set, if that matters. -- Archives updated once/day at http://www.biglist.com/lists/stella/archives/ Unsubscribing and other info at http://www.biglist.com/lists/stella/stella.html
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