nRF52832 unaligned memory access

[DanisXZ]

Hi,

Does not nRF52832 support unaligned memory access ?

I had an instruction like this for which the FW was crashing.

Struct A a= *((Struct A*)pSourcePtr);

I replaced the same using a memcpy which will do a byte copy and it does work. 

memcpy(pDstPtr, pSourcePtr, sizeof(Struct A));

[Turbo J]

Yes and no.

If sizeof(struct A) == sizeof(int64_t), it may fail depending on which CPU instructions the compiler generated. See the documentation for the LDRD instruction(s) on arm.com website - these instructions require 32-bit alignment while normal LDR/STR do not.

Other unaligned access is supported by default, but you can disable it with one of the special register bits (forgot the name though).

[DanisXZ]

Hi Turbo J ,

Thank you for your reply. Yes, the size of the struct is 64 bits. 

I checked the disassembly but I do not see the LDRD instruction. I think LDM too does not support unaligned memory access. 

Struct A a= *((Struct A*)pSourcePtr);

If that's the case, how can one make sure that the FW does not use LDM/LDRD with unaligned memory access ? What are the best practices ?

[Turbo J]

Best practise? Call memcpy() or memmove().

One other sort of alignment  excpetions I forgot to mention: FPU access (for float and double) always wants alignment.

[DanisXZ]

Thanks