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Re: [Qemu-devel] Add ARM920T to ARM emulation
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] Add ARM920T to ARM emulation |
Date: |
Tue, 2 Jun 2009 00:11:15 +0100 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
Paul Brook wrote:
> > I notice that ARMv4T adds the BX instruction, implying that B isn't
> > able to switch to Thumb mode although it can in later architectures (I
> > never saw the point in BX, and I guess ARM ended up the same :-)
> >
> > Is the behaviour defined to _not_ switch to Thumb when using B to an
> > address with the low order bit set, or does it trap, or is it
> > unpredictable?
>
> You need to look again at the B instruction. Hopefully then you'll realise
> why
> you're talking nonsense.
Ach, sorry.
My head's been buried in FDPIC, where BX is only used on ARMvT; on
everything else, LDR/LDM does the job. So I inconveniently forgot
what you cluebricked me about.
> loads into the PC either ignore(v4t) or mode switch (v5) on the low
> address bit. ALU writes to the PC always ignore the low bit in
> Thumb mode. In ARM mode the low bit is either ignored(v6) or used to
> mode switch (v7).
Thanks, it's what I've found too. from reading the 2005 ARM ARM just now.
I'll add that ARM-mode ALU writes to the PC on (v4/v4t/v5) (and loads
on v4/v4t), "bits[1:0] must be 0b00, if they are not, the results are
UNPREDICTABLE". Whereas for (v6), the bits are ignored, and as you
say for (v7) the least-significant bit is meaningful.
-- Jamie