On Wed, 25 Jul 2001 ncoplin@orbeng.com wrote:
> When configured for games running in Ultimax mode, the /IO2 line is often
> found to be connected to the EXROM line. I can only decipher this to mean
> that access to $DF00 will map in the ROM to $8000. But what holds it there??
> Would it not disappear without a capacitor or something to hold the EXROM
> line low?
This sounds like wired-and operation. In NMOS logic, the neutral state is
logical '1'. Pull-up resistors (either on-chip or on the circuit board)
keeps inactive lines in the '1' state. A '0' state is actively driven.
Connecting two signals together forms an implicit 'and' operation between
them, since whenever either line is pulled low, also the result will be
pulled low.
So, I'd guess that such a configuration maps the external ROM at
$9f00-$9fff to $df00-$dfff in the I/O area.
Marko
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