Visualizing the Mac Startup Process: What Happens When a Mac Boots?
Have you ever wanted to know what’s going on when you startup a modern Mac? You press the power button, you see an Apple logo, and the Mac boots up into MacOS… from the average user perspective it’s about that simple, right? But what is actually happening behind the scenes after you press that power button and you’re booting macOS?
That hidden technical side of the Mac booting process is what an excellent visual diagram from Howard Oakey at EclecticLight helps to demonstrate.
If you’re curious about the technical aspects of the Mac boot sequence, and the variables involved, check out the graphic below from EclecticLight to learn a bit about the underpinnings of the startup process. To better grasp the visualization, be mindful of the colors as described by the graphics creator:
“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”
As you can see, there is quite a bit going on behind the scenes as the Mac performs a variety of initializations and checks on the hardware side of things before loading the actual operating system itself. You’ll also get an idea of what happens (and when) if you hold down the Option key to load the startup manager to change the startup drive or boot from an external drive, or try to start into Recovery mode (or internet recovery), boot in safe mode, or Verbose mode, or use target disk mode, or any of the other startup options.
If you’re interested in learning even more about the Mac boot process from a technical standpoint, there is an excellent ongoing series at eclecticlight.co to detail specifics about the boot events, with the following articles currently available on the topic:
- Before boot.efi: the Mac’s initial self-test routines
- Booting the Mac: loading boot.efi and Secure Boot
- Booting the Mac: EFI ‘firmware’
- Booting the Mac: the kernel and extensions
- How the Mac determines what to boot from (work in progress)
The ongoing series pertains to modern macOS and Mac OS X releases on modern Macs. But if you’re curious about older system software and older machines, from our own archives we have a much older article that discusses the Mac OS X boot process, but being from a different era (circa 2007 of Mac OS X Tiger and Leopard era), it appears that much has changed as the Mac has become more secure, including securing the boot process itself. Nonetheless if you’re interested in how older Macs and older Mac OS X releases booted you may find it a fascinating read. Likewise, Apple developer documentation also details a bit about the Mac boot process here, but it too appears a bit outdated (from 2013 according to the date on the documentation page).
Thanks to our friend (and past writer here at osxdaily!) Keir Thomas at MacKungFu for the great find via Twitter:
Incredible work by Howard Oakey explaining the macOS boot process. This kind of info just isn't easy to find online and essentially impossible to find in digestible form. https://t.co/TSeC0OOP0r
— Mac Kung Fu (Keir Thomas) (@MacKungFuTips) August 25, 2018
If you happen to be on Twitter you can follow @osxdaily over there too. Anyway, enjoy learning more about the Mac bootup procedure!