Languages which don't support low level access to the machine, such as memory-mapped I/Os, often (if not always) have library routines which provides such access. Now, if the language itself does not support such access, how are these library routines implemented?
It really depends on the language. Some languages have an "interop" facility which allows a method written in that language to call a library primitive that can be implemented in another language (C, assembler, etc.).
For example, .NET languages have the P/Invoke facility that allows them to call unmanaged DLLs (that can be written in C, C++, assembler, or any other language that allows exported functions from DLLs), but for internal CLR use there's also the "internal call" modifier which is a direct call into a library primitive implemented inside one of the CLR DLLs.