Can variables inside packed structures be read atomically?

Question

I'm writing code for a Cortex M0 (ARM) CPU, and 32-bit reads/writes are atomic. Now I was wondering when I read/write 8bit/16bit variables, are they also guaranteed to be atomic? My instinct says yes, because they are internally aligned to 32-bit sections, so there is no possibility that the CPU needs two separate instructions to read/write them.

But I also happen to store a lot of variables in packed structures to save memory, and there it's possible that variables are not aligned on 32-bit boundaries, so each half of a 16-bit value could be in a different section.

So is it true that I lose atomic operations when I use packed structures?

Answer 1

Using packed structures you will never have read/write atomic operations on fields that overlaps a memory unit boundary. This means that only 8bits operations are guaranteed to be atomic, otherwise it depends on the memory alignment of your fields.