Just learn straight machine code instead.
Most assemblers -> slow, shit, niggers, most won't even compile on my machine
Since the only other option for doing assembly is plugging it into your C compiler and to do so requires you to transmogrify it into the single most autistic syntax of any programming language ever conceived, the alternative is to write strings of machines code and run them as function pointers.
Learning MC is easy, just google "<arch> machine code reference" and BLAZOW
!! Easy as that.