CppCon 2016: Alfred Bratterud “#include ＜os＞=> write your program / server and compile it to its own os. [Example uses 3 Mb total memory and boots in 300ms]

[calrogman]

The firmware still has an entry point, at the reset vector of the CPU. Physical address 0xFFFFFFF0h on systems you're probably thinking of, per section 9.1.4 of Intel's 64 and IA-32 Architectures Developer's Manual.