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

Subject: RE: [stella] More complicated differences between 6502 and 6507?
From: Nicolás Olhaberry <nolh@xxxxxxxxxxxxx>
Date: Tue, 15 Oct 1996 15:46:49 -0300
----- Original Message -----
From: Oliver Achten <achten@xxxxxx>
To: <stella@xxxxxxxxxxx>
Sent: Sunday, October 14, 2001 12:32 PM
Subject: [stella] More complicated differences between 6502 and 6507?

> Hello guys!


> 6502 and the 6507. I am currently working on my "Super VCS" Project, that
> i have developed a small board which plugs into the 6507 Socket, which
> consists of 8K BIOS ROM, 32KB SRAM, a 6502 CPU and a 6522 VIA.

Woah! Cool!

> which behave strange. In Dodge'em, the scoreboard is messed up (reversed
> digits, wrong SOund effects), in Haunted House, the Player is misplaced on
> left side (the game is not Playable), and in Berzerk, the PLAYER missile
> visible on the left side in the opening screen ((C) 1982 ATARI ). This
> also, when i plug in the original cartridge (the external Adress lines
> disabled). Since every complicated kind of game is working (e.g. Solaris,
> Attack), i question myself if there are any internal differences between
the 6502
> and the 6507. Theoretically, there shouldn´t. I´m using a original MOS6502
> CPU from a 1541 Floppy. HAs anyone information about this?

>From my experience and what I´ve read on this list, there aren´t any
internal differences between both. Those problems are more likely showing up
because of the TIA, which behaves very different from what is expected on
several situations.

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
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.

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.

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 ;-)



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