Re: [Qemu-devel] [PATCH] SH: Add prefi, icbi, synco

[Paul Brook <paul@codesourcery.com>]

> The issue comes with how the ISA is versioned. It changes across CPU
> families, with different families borrowing off of each other but not
> being strict supersets of previous generations. It is a complicated mess
> in binutils, and if someone has a better idea of how to do that cleanly
> in qemu, then that is certainly something that should be experimented
> with.

> In addition to the CPU subtype, it is necessary to track the CPU family
> that it falls under also, as this is the only way to know properly how
> the instructions have been inherited.

Binutils ends up a mess because it tries to create a hierarchal tree of cpus, 
IIRC because of the way it represents this in the object file. Qemu is quite 
happy to pick and choose features. There's no need to complicate things by 
pretending there's some sort of logic behind the ISA revisions.

> Going be a set of features is possible to some extent, but there are
> fundamental architectural differences in SH-4A compared to SH-4, so the
> feature thing is rather misleading.

ARM and PPC already have support for multiple system models.
I'd guess most of these differences aren't actually part of the CPU core, 
they're part of the supporting devices/peripherals.

Paul