Re: [stella] Question about SWCHB

Subject: Re: [stella] Question about SWCHB
From: zu03776 <zu03776@xxxxxxxxxxxxxxxxxx>
Date: Thu, 8 Jul 2004 19:47:32 -0500
> Does this give him an extra bit for RAM or something?

I thought I found the answer to my question in the archives from a similar question asked by Adam Wozniak. http:/ /

Does anyone know if SWBCNT has any use?

Yes, it does. You can store #$34 into SWBCNT and get three extra bits of RAM in the unused bits of SWCHB.

I just played this ROM on Stella 1.2, and the P0 sprite is solid. After compiling MakeWav for the Mac, and putting it on the Supercharger on my Atari 7800, I received a line with holes in it, as expected. So, which does Z26 do?

Basically the line on the left of the screen shows the bits of SWCHB on the screen. If you toggle the A/B switches, B/W color switch, select, and start, you'll see some lines disappear. I'm not terribly fancy with displays.

During the display loop, I count down the scanlines using Y from 192 to 0. For each scanline, I store the current value of Y into SWCHB, then read the value back out of SWCHB and store it into GRP0.

The lines of interest are flagged with ***** MORE BITS ***** comments.

If you wondered, I've been working on a ROM to play various nursery rhymes, and was working to understanding Paul Slocum's music driver by writing my own. The example code was just placed in the video display of this ROM.

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