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Old World ROM Macintosh computers are the Macintosh models that use a Macintosh Toolbox ROM chip, usually in a socket (but soldered to the motherboard in some models). All Macs prior to the iMac use Old World ROM, while the iMac and all subsequent models are New World ROM machines. In common use, the "Old World" designation usually applies to the first generation of PCI-based Power Macs (and sometimes the very first NuBus-equipped models), but not the older Motorola 68000-based Macs; however, the Toolbox runs the same way on all three types of machines.

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. These machines are set to boot from this device by default, thus starting the normal Macintosh startup procedure. 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 and Power Macintosh 7500, 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. NuBus Power Macs and all 68K Macs boot directly into the Toolbox.

On all Old World machines, once the Toolbox is loaded, the boot procedure is the same. 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. 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.

If a bootable disk is found, the Happy Mac logo is displayed, and control is handed over to Mac OS. 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 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.

Since the Old World ROM usually boots to the Toolbox, most OSes have to be installed using a boot loader from inside Mac OS. 68K-based Macs and NuBus Power Macs must have Mac OS installed to load another OS, usually with virtual memory turned off. PCI Power Macs can be configured to boot into Open Firmware, allowing the firmware to load a boot loader directly, or they can use a specially-prepared floppy disk to trick the Toolbox into loading a kernel (this is used for Linux installation floppy images).

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