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 21:44:25 -0300
----- Original Message -----
From: Oliver Achten <achten@xxxxxx>
To: <stella@xxxxxxxxxxx>
Sent: Sunday, October 14, 2001 7:04 PM
Subject: RE: [stella] More complicated differences between 6502 and 6507?

> ...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
> Bus through an LS245 Bus driver to the Atari.

Be aware also that when you do a LDA CXM0P (for example) the lines 5 to 0
from the data bus won´t be updated either. I´ve read on this list that  (if
I remember correctly)  Video Pinball checked the value of the whole register
not just bits 7 and 6 for detecting collisions.

> 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.

That´s exactly what happens to me... if you´re going to debug the code have
in mind that maybe the missile is positioned relying in some undocumented
feature/bug. Like, if you do STA RESM0 during HSYNC, the missile would be
positioned two pixels before the start of the visible screen. You know, the
leftmost part of the screen is very problematic ;-)

> > I haven´t seen the problem you describe with dodge'em before. The score
> > 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.

Dodge'em doesn´t do any illegal read, the problem must be elsewhere...



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