Re: [Xen-devel] [PATCH v1 3/8] xen/arm: Support sign-extension for every read access

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 29/09/15 14:13, Ian Campbell wrote:
> On Tue, 2015-09-29 at 12:13 +0100, Julien Grall wrote:
>>> Or is it, couldn't we be updating a byte in the middle of the word?
>>
>> *r is a register, the byte/half-word/word... are always living in the
>> lowest bit of the register.
> 
> So just to check:
> 
> ldrs* do support e.g odd offsets, but given a 32 bit MMIO region which
> reads 0xAABBCCDD then when reading from offset 1 at this point *r contains
> 0x000000CC already (and not 0x0000C00) and sign extending the lowest byte
> is therefore correct.
> 
> Right?

Correct. For more details see the pseudo-code to implement the different
ldrs* instruction (for instance A8.8.86 in ARM DDI 0406C.b).

Regards,

-- 
Julien Grall

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