Lpc2000

"Gus" <gus_is_working@...> writes:

> From my understanding, ARM chips don't raise exception when 
> reading/writing unaligned address but it will give invalid results!
> I have 1nt32 pointer and I am reading/writing data through it. How 
> do I catch it if it is pointing to unaligned address?
 
> If an exception is something that can be done, why it is not raising 
> exception on my philips chips?
 
In case it helps, I notice that pp. 127-128 of _ARM Ssytom-on-chip
Architecture_ by Steve Furber mentions that loading a word from a
non-word-aligned address loads the word-aligned word which contains the
addressed byte, but rotated so the addressed byte is put into the LSByte
of the register.  Storing a word to a non-word-aligned address ignores
the 2 LSBits (thus using a word-aligned address).

To check that an address is word aligned, check to see that the two
LSBits are zero.  To leave a register alone if it is already word
aligned or to advance it to the next word aligned address if it is not
word aligned, you can add 3 to the register then force the 2 LSBits to
zero.

I would think that would be necessary only when reading an arbitrary
starting address or an arbitrary offset.  Otherwise, once you have a
word-aligned address, as long as you advance by 4 bytes, it should stay
word aligned.


-- 
Frank