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


Bits	Description
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:

Slots 0-3 are physical slots that have DMA-master capability.
Slots D, E, and F are not actual physical slots, but refer to the onboard direct memory access (DMA), SCSI, Ethernet, and audio controllers. For convenience, these classes of devices are viewed as being plugged into slots D, E, and F.

Note –
Some SBus slots are slave-only slots. Drivers that require DMA capability should use ddi_slaveonly(9F) to determine whether their device is in a DMA-capable slot. For an example of this function, see attach() Entry Point.