It's such a simple circuit. Use two of 'em!. One is set to the size and location
of the ROM. The other is set to reserve 256 bytes and addressed anywhere else
as needed. When placing the hardware, watch out for routines that scan for RAM
by writing and reading back. (As IDIOT does) Perhaps a control bit fed to the
688's enable pin, so the hardware cannot be accessed until software enables it.
Then you run the /PQ outputs of the 688's thru a gate so either one will deselect
the RAM, and the one for the hardware can enable the chip selects of the hardware
devices, and the lowest address bits operate the register address bits of the devices.