Re: packing/unpacking 4-octet longs

[Richard Earnshaw <rearnsha@arm.com>]

> 
> > I'm not aware of this affecting the ARM (except in that FPA format doubles 
> > and long doubles always have the word with the exponent at the lowest 
> > address, but there's nothing in the IEEE FP specs that says this is 
> > invalid).  In particular, storing a word, or multi-word, at an unaligned 
> > address does not change the order of bytes in memory, so
> > 	memcpy(unaligned_address, aligned_address, sizeof(some_word))
> > does not require diddling with the internal order (or have I misunderstood 
> > the problem?)
> 
> That was a useful manual :-)  See 5-21 where it explains that a 
> misaligned 32bit access gets rotated before it is stored :-/

You're talking about the ARM manual I posted?  If so, please read it 
again, more carefully.  Rotation is *never* done before a store, only on a 
load: it's a side effect of the byte-lane steering used for reading bytes; 
and it's also useful for fetching half-words from memory on those 
machines, since the effect of the rotation means that a ldr (32-bit load) 
from a mis-aligned address will always result in the desired bits being 
placed in bits 0:15 (little-endian mode) or bits 16:31 (big-endian mode) 
of the target register.

Anyway, the issue isn't relevant, since the ARM ABI never makes use of 
this behaviour for un-aligned (packed) objects, the memcpy rule I 
described above applies.

R.