- Joined: 28 Sep 1999
- Posts: 1198
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Mask ROM behavior
 Posted: Mon Aug 17, 2009 9:24 pm
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The MB831001 is notably lacking an output enable pin. I ran some tests on the device and figured out the following:
/SLOT-CE=0, /EXM2=0, A14=0, /WR=1 : Drive bus with $0000-$3FFF data
/SLOT-CE=0, /EXM2=0, A14=1, /WR=1 : Drive bus with $4000-$7FFF data
/SLOT-CE=0, /EXM2=1, A14=0, /WR=1 : Drive bus with $8000-$BFFF data
/SLOT-CE=0, /EXM2=1, A14=1, /WR=0 : Accept writes to mapper register.
For any other setting the data bus is tristated. Think of the /WR pin really being R//W as in high to read and low to write. And /EXM2 is really A15.
I have yet to test the MB832011, but if it works differently I'll update this post about it.
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