Posted By
Bionic
on 2007-01-08
16:25:05
|  Re: Plus/4 Expansion port...
Actually the circuit does work. It seem to be a pretty neat trick. Without checking the circuit in detail, here is what I believe it does:
-When the button is pressed the flip flop is set.
-When an Irq is asserted (IRQ=LOW) and the flip flop is set it activates OE and pulls A9 low.
-The CPU tries to fetch the IRQ address from $FFFC/$FFFD, but since A9=0 it effectively fetches from $FDFC/$FDFD (unmapped memory space!).
-This sets A15=high, which will active CE of the eprom, and hence map it to the unmapped space.
-The fetched IRQ adress points to somewhere in the eprom and a user program is executed, which will
*Save registers, TED state, Activate module one (C1L=Low)
-C1L=Low resets the flip flow.
-Freezer program is being executed from the eprom.
Really neat, great addition to the virtual SID-Card 
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