Use a frequency of 4Mhz for the SPI bus. This will allow the Mega8 Atmel AVR chip to communicate over the same SPI bus as the MMC/SD cards will. Have a way to disconnect the SPI pins from the programmable logic chip so that in system programming of the Mega8 part can occur. Use a two byte 'window' for the ISA bus port. This will allow for 6809 assembly instructions such as LDD and STD to work where two bytes can be read/written faster. Have two bytes to determine the ISA address we are going to address. This will allow access to locations on the ISA bus from $0-$FFFF. Could add extra pins for greater address space if necessary.