Matt,
I've done some research on the 91 NSX ECU since I last wrote you on the Acura-Legend forum and I thought I'll contribute some info here.
To move the ROM address from 0x8000 to 0x0000, the MCU mode needs to be changed from from 2 to 1 or 3. Mode 1 or 3 maps the external memory directly to 0x0000. I found on the NSX ECU, R424 which ties Vcc (5v) to MD1 pin on the MCU can be cut and tie to Vss (0v) to set the mode to 1. I think you already know this.
[edit]
However, there are other problems. When your ROM memory address is mapped to 0x0000, you need a 48K of ROM memory space but without additional logic, wiring a 64K ROM to the MCU would overlap the memory space 0x0000-0xFFFF with the I/O controllers at 0xC000 and 0xE000. The combined MCU ROMs only needs 0x0000-0xBFFF.
I only gave this a quick thought but my approach to address this would be to rewire the top address lines (perhaps A12-A15) from the MCU to the decoder (IC407) to accommodate a 512Kbit ROM (64KB) peacefully with the I/O controllers.
I need to do more research though.
Btw, I made this ECU board markup recently and thought it may help.
Eddy
Ps. Btw, your info on the acura-legend wiki helped me got this started much quicker.
