gcc compiler and byte / word alignment

On 2005-10-18, Fred <fred@nowhere.com> wrote:

>> A word is 32 bits in the ARM world and must be word-aligned. A half-word
>> is 16 bits and must be half-word aligned. The LPC2294 is an ARM7tdmi
>> MCU and thus doesn't support unaligned accesses in hardware.
>>
> The half-word is half-word aligned albeit on an 8 bit bus.

I haven't used the LPC2994, but in general that's OK on the
other parts with which I'm familiar.

> Temporarily I have used byte access and concatenated the
> bytes.  However this really isn't an ideal and is a slow
> solution.  I've been looking through the datasheets and can't
> see any proviso that a half-word read won't access a variable
> on an 8 bit bus. 

Try it and see. :)

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"Grant Edwards" <grante@visi.com> wrote in message 
news:11la1a7jqkh3ib4@corp.supernews.com...
> On 2005-10-18, Fred <fred@nowhere.com> wrote:
>
>>> A word is 32 bits in the ARM world and must be word-aligned. A half-word
>>> is 16 bits and must be half-word aligned. The LPC2294 is an ARM7tdmi
>>> MCU and thus doesn't support unaligned accesses in hardware.
>>>
>> The half-word is half-word aligned albeit on an 8 bit bus.
>
> I haven't used the LPC2994, but in general that's OK on the
> other parts with which I'm familiar.
>

I agree therefore my surprise.

I've looked with a 'scope and it seems as if there's a single read.  The 
bigher byte of the half word mirrors the lower byte.

>> Temporarily I have used byte access and concatenated the
>> bytes.  However this really isn't an ideal and is a slow
>> solution.  I've been looking through the datasheets and can't
>> see any proviso that a half-word read won't access a variable
>> on an 8 bit bus.
>

 

On 2005-10-18, Fred <fred@nowhere.com> wrote:

>>> The half-word is half-word aligned albeit on an 8 bit bus.
>>
>> I haven't used the LPC2994, but in general that's OK on the
>> other parts with which I'm familiar.
>
> I agree therefore my surprise.
>
> I've looked with a 'scope and it seems as if there's a single
> read.  The higher byte of the half word mirrors the lower
> byte.

Bummer.  IMO, what you have isn't really an 8-bit bus. It's
half of a 16-bit bus or a quarter of a 32-bit bus.  

I can just hear the marketing guys saying "Can't use 16-bit
operations on the peripherals? That's OK, will just add '8-bit
bus support' to the list of features".

-- 
Grant Edwards                   grante             Yow!  I will SHAVE and
                                  at               buy JELL-O and bring my
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