Well, the jump from real mode (16 bit) at boot to protected mode (32 bit) and protected to long mode (64 bit) are all accomplished by flipping some bits in the

Well, the jump from real mode (16 bit) at boot to protected mode (32 bit) and protected to long mode (64 bit) are all accomplished by flipping some bits in the control registers and doing a “long jump” (a regular jump is implicitly the same segment address/selector, but a long one has an explicit one). Basically, the processor only detects operating mode changes when a long jump is performed. (This allows the OS to set up a process for execution, flip the mode bits if needed, then long jump to the process)