RE: [stella] More complicated differences between 6502 and 6507?

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

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! :-)



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