68K-based Macs and NuBus Power Macs must have Mac OS installed to load another OS, usually with virtual memory turned off. Since the Old World ROM usually boots to the Toolbox, most operating systems have to be installed using a boot loader from inside Mac OS.

If a hardware problem occurs during the early part of the boot process, the machine will play the Chimes of Death and freeze on some Macs, this will be accompanied by a Sad Mac icon and an error code describing the problem. If no disk to boot from is present, an icon depicting a floppy disk with a question mark in the middle blinks on the screen. If a bootable disk is found, the Happy Mac icon is displayed, and control is handed over to Mac OS. The Toolbox then checks for a disk in the floppy drive, and scans all SCSI buses for a disk with a valid System Folder, giving preference to whatever disk is set as the startup disk in the parameter RAM. The Toolbox does a memory test, enumerates Mac OS devices it knows about (this varies from model to model), and either starts the onboard video (if present) or the option ROM on a NuBus or PCI video card. On all Old World machines, once the Toolbox is loaded, the boot procedure is the same. NuBus Power Macs and all 68K Macs boot directly into the Toolbox.

This can be changed, just as on New World ROM Macs, but with limitations placed on what devices and formats can be used on these machines, particularly the early machines like the Power Macintosh 7200, 7500, 8500, and 9500, the Open Firmware implementation was just enough to enumerate PCI devices and load the Toolbox ROM, and these OF revisions have several bugs which must be worked around by boot loaders or NVRAMRC patches. These machines are set to boot from this device by default, thus starting the normal Macintosh startup procedure. PCI Power Macs with an Old World ROM contain an Open Firmware implementation, and a copy of the Macintosh Toolbox as an Open Firmware device.

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