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Double-checking address decoding

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metertech58761:
So I ended up choosing a replacement MPU for my tester project, and along the way, I had to redo the address decoding for the I/O ranges.

Can anyone take a look and see if I did it correctly?

MegaVolt:

--- Code: ---A15 A14 A13 A3 A2
 1                 U4
 0   0   1   1     U2
 0   0   1   0     U3
--- End code ---

If I understand correctly, the choice of chips is correct.
The polarity of signals A00 and BCS is not very clear? Active 0 or 1?

metertech58761:
The 8CS line is the active-low chip select line for the 8279 IC on the display board, and disregarding shadow copies, should correspond to 0x2008-0x200B.

A00 is the mode select line for the 8279; when address 0x2008 is selected, this line should be low, putting the 8279 in data mode. When 0x2009 is selected, this line should be high, putting the 8279 in command mode.

MegaVolt:
Here is the project in the simplest simulator. You can call the truth table and check if it works correctly.

I see the simultaneous selection of BCS and A00 simultaneously with the selection of chips U2...U4. I don't really understand if this should be the case.

123.dig.txt rename to 123.dig

Simulator https://github.com/hneemann/Digital

metertech58761:
I guess I was a bit too optimistic to squeeze the 8279 in like that. :)

I may have to rearrange the coding to move the 8279 down to 0x2800 / 0x2801 instead.

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