Booting the Mac: Visual Summary

This article provides a simplified visual summary of the various stages which take place when a modern Intel Mac starts up in macOS 10.12 or 10.13, from pressing the Power button through to running the kernel and its extensions.

The sequence starts at the top, with user inputs in green at the left, processes in pink next, actions etc. to the right of the centre, and interactions in blue at the right. Key sources of data are shown in red. Macs equipped with T2 processors are rather different: their main differences are shown in boxes with heavy black outlines.

The upper section of the diagram, down to running boot.efi, is what Apple refers to as BootROM, and boot.efi is known as the OS X booter.

There remains uncertainty as to the exact order in which some of these stages happen, particularly during the BootROM phase. Some hardware support, such as that for Bluetooth keyboards, seems to vary between different models, and to occur at more than one point.

Here it is:



Individual articles in this series include:

  1. Before boot.efi: the Mac’s initial self-test routines
  2. Booting the Mac: loading boot.efi and Secure Boot
  3. Booting the Mac: EFI ‘firmware’
  4. Booting the Mac: the kernel and extensions
  5. How the Mac determines what to boot from

There may well be errors and omissions: if you see any please comment here or let me know so that I can correct both those articles and the diagram above.

(Updated 16 September 2018 with details of screens seen during boot process.)