Subject: [stella] Supercharger internals... From: bwmott@xxxxxxxxxxxx Date: Thu, 7 May 1998 17:35:12 -0400 (EDT) |
Well I got some time to play with the supercharger support for Stella some more today. Here's something I noticed from the previous posts and looking and some source code: Assume $80 = 00, $81 = ff, Y = 0 and X = 5 -- Here's one way to write to RAM: f800 lda $f000,y f803 lda ($80),y f800 lda f801 00 f802 f0 f803 lda f804 80 Now from the cartridge port the following addresses would be seen: f800 f801 f802 f000 So here's the value we're trying to poke $00 f803 f804 0080 0081 ff00 And 5 accesses later is the location to poke $ff00 -- Here's another way to write to RAM: f800 lda $f000 f803 nop f804 sta $ff00 f800 lda f801 00 f802 f0 f803 nop f804 sta f805 00 f806 ff Now from the cartridge port the following addresses would be seen: f800 f801 f802 f000 Here's the value to poke $00 f803 f804 f805 f806 ff00 And here 5 accesses later is the location to poke $ff00 -- Here's some code that I'm not sure if it writes to RAM or not: f800 lda $f000 f804 sta ($80,x) f800 lda f801 00 f802 f0 f803 sta f804 80 Now from the cartridge port the following addresses would be seen: f800 f801 f802 f000 Here's the value to poke $00 f803 f804 0080 0085 0086 ff00 And here 6 :-( accesses later is the location to poke $ff00 So after looking at this I decided to code up the Supercharger support so that it pokes 5 memory ACCESSES after an access to f000 to f0ff. To my surprise all of the Starpath games worked :-) Now, I guess it could be that these games only use the first two code sequences to write to RAM or it could be that this is the way the Supercharger really works. I also fix a bug in the Supercharger emulation so that Bob Colbert's Stell-a-Sketch and Multi game work. I wasn't poking on writes only on reads :-) Later, Brad -------------------------------------------------------------------------- Bradford W. Mott (bwmott@xxxxxxx) Computer Science Department http://www4.ncsu.edu/~bwmott/www North Carolina State University -------------------------------------------------------------------------- -- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/stella.html
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