[hgs3]

I'm being pedantic, but on modern hardware, the ISA is an abstraction over microarchitecture and microcode. It's no longer a 1-to-1 representation of hardware execution. But, as programmers, it's as low as we can go, so the distinction is academic.

[pjmlp]

Still one layer below C, and with plenty of features not available on C source code.

[rramadass]

Compiler intrinsics do give you C/C++ api access to relevant ISA subsets as platform-specific extensions.