Some elementary decoding of the address contained in the NTA register is performed and the result output on two pins OACA and OACB. These outputs are of the open collector type and are pulled low if the address condition is not met. This allows a wire AND function to be used, simplifying the address decoder. The exact function of these pins is different for the upper and lower chips:
In the lower chip, OACA indicates that bits (07:04) are all low. This eases decoding of the low order addresses for a simple slave. OACB indicates that bits (15:08) are all low.
In the upper chip, OACA indicates that bits (23:16) are all low and OACB indicates that bits (29:24) are all low. This allows for external decoding of bits 30 and 31 to determine CSR region.