In the C++ case there are the ISO papers and the “community” papers about drafts for future and present definition of the language.
What are the equivalent sources for the assembly for both X86 and ARM platform ?
I found really hard to keep up with assembly because it’s really fragmented ( even considering only the X86 platform ) and in many case I can’t even find out what a register X refers to, or what CPU set suppports that feature or register.
There isn’t an index of all the registers, isn’t it ?
As mentioned in the comments, the answer to your question is complicated because manufacturers produce manuals for their processors. In addition x86 is furhter complicated because there are generally separate documentation manuals for 32 bit and 64 bit processors (I’m not familiar enough with ARM to comment here).
Intel x86 (both 32 and 64 bit) can be found here: Intel® 64 and IA-32 Architectures Software Developer Manuals
AMD x86 documents: All processors and AMD64 Architecture
and Software Manuals. There are a lot of resources here and you will probably need to filter it a little depending on exactly what you are looking for.
ARM makes reference manuals available by ARM architecture: ARM Architecture