Technical Q&A FW04

gestaltFWVMBackingStore

Q: I noticed that with the latest version of the FireWire 2.2 SDK that there is a new Gestalt attribute bit defined, gestaltFWVMBackingStore. Does this mean that FireWire 2.2 supports FireWire hard disks as a VM backing store?

A: Unfortunately, no, it does not. FireWire 2.2 is not able to be on the page fault path, so it cannot control a FireWire disk which contains the VM backing store. This bit was incorrectly set in FireWire 2.2.

However, now that the cat’s out of the bag, it is obviously an important future direction for FireWire hard disks to be a fully functional part of the system. Becoming able to be on the page fault path and control the VM backing store is an important first step towards this.

The solution is to ignore this bit completely. It has been deprecated and will never be defined to be anything meaningful. In future versions of FireWire.h, gestaltFWVMBackingStore will be renamed gestaltFWBogus to make sure that your compiler forces you to not check this bit (at least not without first flagging it as an error).

Look for a new bit being defined, gestaltFWVMBacking, which will be correctly set once FireWire is able to be on the page fault path. Currently gestaltFWVMBacking will return 0 on all versions of FireWire, including version 2.2.

However, once FireWire is able to be on the page fault path, it does not guarantee that the FireWire disk driver can be. It is possible for a FireWire disk driver to be written in such a way as to not be able to be on the VM page fault path (however, that seems unlikely). It will not be known what drivers are capable of supporting the VM backing store until FireWire itself is made capable of being on the VM page fault path.