Subject: RE: [stella] More complicated differences between 6502 and 6507? From: Oliver Achten <achten@xxxxxx> Date: Mon, 15 Oct 2001 00:04:48 +0200 (MEST) |
Hello Nicolás! First: many, many thanks for your detailed response! :-) > In Haunted House, the problem you describe occurs at address $F441 where > this opcode is executed: SBC $0F. Reading from $0F has an undefined > result, > a real atari 2600 will read, and subtract to the accum, #$0F. This happens Sounds like a typing mistake... > because, since the read can´t be done, the data bus isn´t updated, so > whatever the bus has on the previous cicle will be used. Reading other > values instead of #$0F causes problems with the position of the player, in > particular if #$00 is read, the game will enter into an endless loop. ...and that explains everything! The story: My design allows the auto recognition of a cartridge (if a cartridge is inserted, the system recognizes this and executes the cartridge rom instead of the BIOS). To prevent conflicts between the BIOS rom and the cartridge rom on startup, i have to route the Data Bus through an LS245 Bus driver to the Atari. And this may cause the misplacing of the player, because the LS245 will deliver a defined value (probably #$00) during the illegal read. Unfortunately, there is no way to fix this (i will eventually try different types of bus drivers). But there may be the possibility to include a software patch into the BIOS... ;-) > I have the same problem you describe with Berzerk in my emu. I don´t know > if > this happens because the missile isn´t positioned properly, or because it > should have the same color as the background. Well sice you gave me the hint, i will have a more detailed look in the disassembly... On the modified Atari, the missile shows on powerup only. When i start the game (pressing fire), the missile is gone when the playfield finished appearing. After game over, when the game returns to attraction mode, the error does not occur. > I haven´t seen the problem you describe with dodge'em before. The score is > displayed using the playfield, maybe for those lines the playfield is > being > displayed as duplicated... who knows ;-) Again, i will look now into the source more deeply, but i am afraid, that these illegal reads are causing this problem. When i am completely finished with the project (which is nearly done) i will of course publish the schematics, the ROM BIOS and the transfer program on this list. Again, thank you Nicolás! :-) Regards Oliver -- GMX - Die Kommunikationsplattform im Internet. http://www.gmx.net - Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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