低级编程：操作系统如何启动新的线程/进程？

Whenever the bootloader loads the operating system there is presumably only ONE program flow active, right? This would mean, one processor holds the instruction pointer and executes the commands it founds at the position the EIP register points to. At which point and how does the system start to exploit more processes and/or threads (no userland threads, but cpu threads)?Whenever the bootloader loads the operating sys