I am currently trying to write a function in assembly and i want to move 128 bits of a string located at the memory address stored in rdx into the xmm1 register.
If i use movdqa xmm1, [rdx], i get a access violation exception while reading at position 0xFFFFFFFFFFFFFFFF.
If i try to use movdqu xmm1, [rdx] instead, i dont get the exception. The problem is if i use movdqu, the order of the bits is inverted.
So i do not know why i get an exception when using movdqa but not when i am using movdqu
Most of this has been said in the comments already, but let me summarise. There are three problems raised by your code/question:
1) MOVDQA requires the addresses it deals with ([rdx] in your case) to be aligned to a 16-byte boundary and will trigger an access violation otherwise. This is what you are seeing. Alignment to a 16-byte (DQWORD) boundary means that, using your example, you should read from e.g. 0xFFFFFFFFFFFFFFF0 rather than 0xFFFFFFFFFFFFFFFF, because the latter number is not divisible by 16.
2) The address you use, 0xFFFFFFFFFFFFFFFF, is almost certainly invalid.
3) Provided you use MOVDQA to read from a valid 16-byte-aligned memory location, the results (in xmm1 in your case) will be IDENTICAL to when you use MOVDQU. The only relevant difference between the two here is that movdqU allows you to read from Unaligned (hence the U) memory whereas movdqA requires a (16-byte) Aligned memory location. (The latter case will often be faster, but I don't think you need to worry about that at this stage.)