A Mac with Apple silicon has the boot modes described below.
Mode
Key combo
Description
macOS
From a shutdown state, press and release the power button.
Boot ROM hands off to LLB.
LLB loads system-paired firmware and the LocalPolicy for the selected macOS.
LLB locks an indication into the Boot Progress Register (BPR) that it’s booting into macOS, and hands off to iBoot.
iBoot loads the macOS-paired firmware, the static trust cache, the device tree and the Boot Kernel Collection.
If the LocalPolicy allows it, iBoot loads the Auxiliary Kernel Collection (AuxKC) of third-party kexts.
If the LocalPolicy didn’t disable it, iBoot verifies the root signature hash for the signed system volume (SSV).
Paired recoveryOS
From a shutdown state, press and hold the power button.
Boot ROM hands off to LLB.
LLB loads system-paired firmware and the LocalPolicy for the recoveryOS.
LLB locks an indication into the Boot Progress Register that it’s booting into paired recoveryOS, and hands off to iBoot for paired recoveryOS.
iBoot loads the macOS-paired firmware, the trust cache, the device tree and the Boot Kernel Collection.
If paired recoveryOS boot fails, booting into fallback recoveryOS is attempted.
Fallback recoveryOS
From a shutdown state, double-press and hold the power button.
Boot ROM hands off to LLB.
LLB loads system-paired firmware and the LocalPolicy for the recoveryOS.
LLB locks an indication into the Boot Progress Register that it’s booting into paired recoveryOS, and hands off to iBoot for recoveryOS.
iBoot loads the macOS-paired firmware, the trust cache, the device tree and the Boot Kernel Collection.
Safe mode
Boot into recoveryOS per the above, then hold Shift while selecting the startup volume.
Boots to recoveryOS as per the above.
Holding the Shift key while selecting a volume causes the BootPicker app to approve that macOS for booting, as normal; it also sets an nvram variable that tells iBoot to not load the AuxKC on the next boot.
System restarts and boots to the targeted volume, but iBoot doesn’t load AuxKC.
