ARM do not make uProcessors: they make core IP, which they licence to anyone who wants to pay to use it in their designs - and an damn good design it is too! Arm architecture is most definitely not open-source! :laugh:
What that means is that the "basics" are the same, but the "wrapper" round the ARM core is created by each company using it. Each individual ARM core version - and there are lots now, have a look at the ARM website - is different, and will have different instructions, and each processor which contains an ARM core will "wrap" stuff round that.
Seriously, we can't even explain all the differences for one "family" of ARM-based processors like PowerPC, let alone start to explain all the different manufactures variants! Not in a little tiny textbox - you need a manual for each...:laugh: And there are hundreds of such books:
Google[
^]
Start with the ARM site - it's pretty helpful - and then focus on the manufacturer you will be using.