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From: Florian Weimer <fweimer@redhat.com>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: linuxppc-dev@lists.ozlabs.org,  gcc@gcc.gnu.org,
 libc-alpha@sourceware.org,  Paul E Murphy <murphyp@linux.ibm.com>
Subject: Re: [PATCH Linux] powerpc: add documentation for HWCAPs
References: <20220524093828.505575-1-npiggin@gmail.com>
Date: Tue, 24 May 2022 11:52:00 +0200
In-Reply-To: <20220524093828.505575-1-npiggin@gmail.com> (Nicholas Piggin's
 message of "Tue, 24 May 2022 19:38:28 +1000")
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* Nicholas Piggin:

> +2. Facilities
> +-------------
> +The Power ISA uses the term "facility" to describe a class of instructions,
> +registers, interrupts, etc. The presence or absence of a facility indicates
> +whether this class is available to be used, but the specifics depend on the
> +ISA version. For example, if the VSX facility is available, the VSX
> +instructions that can be used differ between the v3.0B and v3.1B ISA
> +verstions.

The 2.07 ISA manual also has categories.  ISA 3.0 made a lot of things
mandatory.  It may make sense to clarify that feature bits for mandatory
aspects of the ISA are still set, to help with backwards compatibility.

Thanks,
Florian