Re: [stella] Superchip: What if....

Subject: Re: [stella] Superchip: What if....
From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx>
Date: Fri, 10 Jun 2005 16:37:57 -0400
> It depends on the hardware doing the reading.  On a DEVOS 7800 they read
> as $ff.  On my home made cart dumper I didn't have anything pulling the
> lines high, so they read random values.

If DevOS detects a Superchip game, it skips the Superchip addresses
in each bank. It's the PC software that fills this area in the
binary with $ff, because that's what we use in z26 to autodetect
Superchip ROMs. ;-)

It seems that the 6507/6502 processor is pretty good at
maintaining the state of the data bus. So, if nothing is driving
the data bus during a read cycle, the CPU will read the value
that was last on the data bus. Depending on the addressing mode
used for the read, this is usually the low or high byte of the
address to read. For example if I use DevOS to read the entire
ROM area with no cartridge inserted, the ROM will contain a
series of 256 times the same byte counting up from $40 to $ff.
This is because I read the area from $4000 to $FFFF with 
indirect-y-indexed addressing.

Also, if you read from the TIA collision registers, the TIA
will only drive the two topmost bits of these bytes. The
other six bits on the data bus will be left floating. Still
the values read by the CPU are reliable enough for some
games, like Video Pinball, to rely on the state of all eight

Ciao, Eckhard Stolberg

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