Hi !

I am writing assembly code for some ARMv7a and ARMv8a CPU. I know that code has to be 4 bytes aligned, but I saw in several places (uboot/linux) the ".align 4" GCC directive, which will align to 2**4 = 16 bytes.

When writing code that will…

Why are there different encodings of instructions?

What's the idea/background/etc for their co-existence?

Can different encodings be mixed in the code? (Not ARM encodings with Thumb encodings- without ARM/Thumb mode change,

but, like A1 and A2 or T1…

Any idea about instructions marked as UNPREDICTABLE: can it then be UNDEFINED?

In other words: UNDEFINED REQUIRES the instruction to cause UND-exception, but

MAY UNPREDICTABLE do that, or does it have to execute normally except that the result may be…