Re: [Qemu-devel] Add ARM920T to ARM emulation

[Jamie Lokier <jamie@shareable.org>]

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