I`ve made the expansion board which will be placed on the top of the mainboard. This board has a 74LS30 IC which makes NAND function over A8-A15 lines. With addition of 74HCT00 they make an additional I/O address decoder for video board and other I/O devices (both on the expansion slots and on the mainboard). The whole thing was assebmled and tested. From this point, the architecture of the system requires the programmer to use BC register to address I/O devices and video adapter. The hardware takes advantage of this addressing mode and uses it to distinguish between the video board and regular I/O devices address space.
The board has six 40 + 3 pin expansion headers. The bus includes all CPU signals, 5V and 3.3V power supply and additional signals from I/O address decoder. There are also 4 jumpers for terminating not-used CPU signals (/INT, /NMI, /BUSRQ, /WAIT)
Here are schematics and board layout design in EAGLE:
The expansion board mounted on top of the mainboard:
The whole setup up to this moment looks like this:
A demo program was rewriten for the board. It now enables to do simple text editing on the LCD.