Booting

Booting is the process of starting a computer. For various Macintosh systems, there are very different boot processes, depending on the vintage of the system.

Old World ROM
Starting with the original Macintosh 128K from 1984, classic Macs powered up with a beep and then loaded the Old World ROM, which contained Bill Atkinson's Macintosh Toolbox and was stored in ROM chips on the logic board, which then immediately looked for a startup disk containing the System file. The Macintosh II in 1987 and most later models replaced the startup beep with a chime. During the PowerPC transition, PCI-based Power Macintosh systems from 1995 began transitioning to an Open Firmware (OF) implementation of the Old World ROM.

New World ROM
When the iMac, the "blue and white" Power Mac G3, and the bronze-keyboard PowerBook G3 were introduced in 1998 and 1999 by the newly-returned Steve Jobs, Macintosh computers began using New World ROMs, that switched from relying on the Macintosh Toolbox to only Open Firmware on the logic board. The Mac OS ROM was moved from the logic board into a file in the classic System Folder, which was a necessary step in the transition from classic Mac OS to Mac OS X.

Mac OS X
When power is turned on PowerPC-based systems with Mac OS X, Open Firmware (OF) code is executed. Control is then passed to the bootloader, which is located at  and loads the kernel. Intel-based Macs load from an Extensible Firmware Interface (EFI) ROM which looks for  instead.

macOS
Starting in 2016, Apple removed startup chimes from all new models (except the MacBook Air) by default, but it may be re-enabled in Terminal with the following undocumented (and unsupported) command:

Changes to macOS by Apple caused the above option not to work on 2017 models, but users discovered another undocumented command:

Starting with the iMac Pro and the 2018 MacBook Pro, bootup is controlled by Apple's new T2 chip, which also controls encryption of user data to flash-based drives. The bridgeOS within the T2 chip contains a deeper startup chime that is disabled by default, but can be enabled with the above command. macOS Big Sur is expected to re-enable startup chimes by default when it is released in late 2020.