Re: [stella] Strange RIOT(?) register writes in Star Ship?!?

Subject: Re: [stella] Strange RIOT(?) register writes in Star Ship?!?
From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx>
Date: Thu, 29 Aug 2002 21:34:14 +0200
> While reverse-engineering Star Ship, I found two strange 
> register writes, here they are, with my comments:
> 1: Startup of the console:
>  LDA #$10       ; Useless port direction instructions...
>  STA SWBCNT     ; ...on the 2600 (?)
> 2: Routine handling Screensaver mode
>  LDA gameOffBool      ; A-> 00 v FF
>  BNE SaverColorOk     ; Game running?
>  STA saverColor       ; Y: Savercolor-> 0
> SaverColorOk:
>  EOR #$FF             ; N: 2 Useless...
>  STA SWCHB            ; ...instructions(?)

There are three unused bits in SWCHB. If you set these to output
you can read back whatever you write to SWCHB. So you could consider
them as three bits more RAM, if you will. In the upper example it
seems that the state of "gameOffBool" is stored in bit D4 of SWCHB.
I suppose it's used to check if the game is off at the same time
as the RESET or SELECT switches are checked.

Ciao, Eckhard Stolberg

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