Firmware? No, the OS will have to explicitly choose the mode the CPU operates in. The PC, under "normal" legacy BIOS, always boots to 16 bit mode (like in OP).

Firmware? No, the OS will have to explicitly choose the mode the CPU operates in. The PC, under "normal" legacy BIOS, always boots to 16 bit mode (like in OP). From there on, you manually have to change the processor operating mode to 32 bits (protected mode) or 64 bits (long mode), by setting some privileged registers and doing a jump to 32/64 bit code. In the case of UEFI, it may be possible to explicitly ask for a certain operating mode at boot, but I am not familiar with that (yet).