>
>Nick S Bensema boldly stated:
>>
>>IT'S SOFTWARE, I TELL YOU! SOFTWARE!!!!
>>
>>Most games use an EOR or ADC on every write to color registers against a
>>memory location in RAM which holds zero when there's no attract mode, and
>>some pseudorandom number when there is. I've seen the source code to Combat,
>>Defender, and Pitfall, and this is the method used in each of them.
>>
>>Not all games use the color cycling method as an attract mode. Some games
>>have a "demo game" running, while others run a cute animation sequence.
>>See Joust, Ms. Pac-Man, and Moon Patrol.
>
>Thanks for the info. Even after you've told me what to look for, I
>can't find it in the Combat code. I gotta learn 6507 assembly
>someday.
By the way, the B&W switch is also software-based, which is why it only
works on certain games, and has additional functions on other games.
In Pitfall, for example, every color register went through something like this:
LDA ColorToGet
EOR $88
AND $87
STA ColorRegister
If the system is not in attract mode, $88 contained a zero, which meant
no bits were changed. If it is in attract mode, $88 contains just about
any value. Depending on that value, the color either becomes dimmer, or
brighter, or negatived, or otherwise altered.
In $87 there is either a $FF in color mode, or $0F in B&W mode.
This meant that the color was unaffected when in color mode, but in B&W
mode, all the bits that controlled hue were stripped, leaving a grayscale
version of the original color.
--
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
