Abstract: An extended addressing system for allowing use of existing circuit boards but obtaining more address space is disclosed. If given bits in the address value indicate that a former system board address is being presented previously unutilized higher order bits are used as a slot identifier that slot's AEN signal is made low, while all the remaining AEN signals to each slot are made high to disable operation. All the AEN lines remain low as an existing circuit board address is presented or a memory operation is occurring. All the AEN lines go high during a DMA operation.