[Libre-soc-dev] mfocr and mtocrf v3.0B spec ambiguity

[Luke Kenneth Casson Leighton]

apologies, there is another spec discrepancy between IBM POWER9 behaviour
and what is listed in the v3.0B spec.

microwatt's behaviour has been designed to match that of IBM POWER9 by
running unit tests that test exact correspondance.

this because anything other than that results in disastrous binary
incompatibility.

microwatt analyses bit 20 in the XFX Form and if set will perform a one-hot
priority pick of FXM (bits 12 to 19).

if not set then the entire mask is used to select CR registers.

the specification not only says nothing about bit 20 (it is not listed as
being used), it states that "preferred" behaviour is permitted.

a specification that can allow ambiguous behaviour *will* result in
incompatibility.

can i recommend that the specification be updated to reflect actual IBM
POWER9 and microwatt behaviour?

otherwise, applications are guaranteed to break.

l.



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