For most users, the startup process consists of three stages: pressing the switch button, the appearance of the Apple logo and loading macOS.

It looks pretty simple, but in fact, in the period between pressing the button and starting the operating system is a complex technical process.

Enthusiast from EclecticLight Howard Oakey have created a visual chart of the system start-up.

In the “Map loading”, the process is divided into colors:

  • The green blocks that the user enters.
  • Pink blocks – running processes.
  • Red blocks are the key data sources.
  • Blue blocks – interaction with the user.
  • Yellow blocks – action system.

New Apple computers come with a coprocessor T2, the scheme is different. The major differences are indicated by black contours on the Map.

Upper part of the diagram, up to the launch boot.efi (known as the boot OS X) is what Apple refers to BootROM.

As can be seen from the diagram, the boot time is happening after a large number of validation processes of iron and system.

The chart details all the startup scripts – the usual start, booting in target disk mode, hold the Option (option) to select the boot disk or the Command (⌘) –R –S –V modes.

This scheme describes the processes on the modern Mac computers. Yourself enthusiast says that Apple has changed a lot in the download OS for the last 10 years.


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