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 with the iMac Pro and the 2018 MacBook Pro, bootup is controlled by Apple's new T2 chip, which controls encryption of user data to flash-based drives. There is no longer a startup chime on such newer machines by default, but it may be re-enabled in Terminal with the following command (which is undocumented and unsupported):