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…

From the CPU's point of view, how does it distinguish an assembly code is an ARM code or a Thumb code?

Is there some signal bit represent the code is ARM or Thumb code, this is just my guess.