Technical Q&A HW81

A SCSI little secret

Q: On a Blue & White G3 if I call SCSIBusInquiry for bus 0 (the internal bus) I get back what appears to be valid information. Yet there is no built-in internal or external SCSI connector. Is there a built-in SCSI bus on the B&W G3 1999? Can I use it? How can I tell?

A: The Blue & White G3 uses a custom I/O controller. Inside of this custom ASIC is a MESH SCSI controller cell, so there is indeed a SCSI bus of sorts. This is located in the Name Registry node "Devices:device-tree:pci:pci-bridge:mac-io:scsi".

Note, however, that the SCSI cell is not connected to the outside world and is therefore unusable.

Calling SCSIBusInquiry (for bus 0) does return noErr and information about the motherboard bus. The scsiControllerFamily field is set to "Mesh-NULL " indicating that while there is a MESH controller there, it's unusable. (Note that the string "Mesh-NULL " is a total of 16 bytes long, padded with space characters (0x20), and has a null (0x00) terminator.)

Here're result codes you can expect when accessing bus 0 on the Blue & White G3:

The MESH SCSI controller was left in the B&W G3s to avoid breaking software that assumes because SCSI Manager 4.3 is available, there must be a SCSI bus.

As a developer you should not assume that SCSI Manager 4.3 exists. If it does exist do not assume that there is a SCSI bus.