Writing Device Drivers

Physical SBus Addresses

The SBus has 32 address bits, as described in the SBus Specification. The following table describes how the Ultra 2 uses the address bits.

Table A–2 Ultra 2 SBus Address Bits



0 - 27 

These bits are the SBus address lines used by an SBus card to address the contents of the card. 

28 - 31 

Used by the CPU to select one of the SBus slots. These bits generate the SlaveSelect lines. 

This addressing scheme yields the Ultra 2 addresses shown in Table A–1. Other implementations might use a different number of address bits.

The Ultra 2 has seven SBus slots, four of which are physical. Slots 0 through 3 are available for SBus cards. Slots 4-12 are reserved. The slots are used as follows: