Surprisingly int 15 AH=89h is unimplemented in both 'legacy' and 'latest' Bochs main BIOSes.
Surely implementing this and other basic functions of the good old IBM AT BIOS should be a priority over newer fancy things like ACPI and deity knows what...
Are the coders possibly too young and not familiar with ISA-architecture ? I don't run Bochs often, and done nothing like systematic testing; yet oftentimes while firing an old program of mine or some 3rd party one, I will stumble upon problems casued by poor conformance. IMO it should be a priority to review the BIOS basics before adding those fancy extensions. This holds true of the video BIOS also - see other bug report.