D670 - Unimplemented calls answer a negative placeholder

Status: decided Date: 2026-09-10

An unimplemented call answers the placeholder base 0xF7FF_0000 plus its reason code. The value is negative, so a guest's own rc < 0 check catches it, and it lies outside the 0x80xx_xxxx range every measured platform error uses; fault lookup also recognises its sign-extended 64-bit form.

Why: every error the platform has been measured returning sets the high bit, so guests test the sign. A positive placeholder reads as success to a guest that checks correctly, and a guest that believed one built pointers from what followed; with the high bit set, guests name the failing call themselves. Keeping clear of 0x80 stops the placeholder being read as firmware behaviour, and keeping the low half lets a reader recognise it on sight.

Rejected:

• 0x7FFF_0000: positive, so a guest reads a refusal as success.
• A 0x80xx_xxxx code: indistinguishable from a measured platform error.
• Answering success: plausible output no guest can detect.