illumos gate

The SPARC startup code checks, with the iam_positron() function, if we're being booted on a positron.

This was an early PCI development system. Two decades on, we shouldn't need to check for this, even if it's possible that one or two instances escaped into the wild.

There are a couple of references to positron elsewhere, in audio and pci code. Without further detailed knowledge of the platform and any bugs subsequently fixed (or not) in firmware, it's unclear whether we can do anything about those.