This question is so irrelevant compared to the task at hand. BTW: why would you write an OS in assembly? This was given up in the senventies of the last century, I.e. C was invented to re-write UNIX since without it the work results would not have scaled...
Almost everything you can achieve by assembler can be achieved by decent C cross-compiler - some rare exceptions apply.
Everything you can achieve by C can be achieved by a decend C++ cross-compiler - probably no exceptions apply. ;-)
Is there some I-write-my-own-OS fever breakout? E.g. see also a resent similar request
Expectations on "OS" may vary, though.