The system will always select the first one found, unless the boot-device is modified to a specific partition id. The system will search all HFS partitions for a blessed folder and a file of type tbxi within it, if none are located, the boot will fail. A 'blessed' folder is simply one with a special attribute (or flag) set on it. The Boot ROM file is simply a stripped ELF executable image wrapped with a forth boot script. All versions of MacOS have a folder known as System Folder that is 'blessed', and contains a MacOS Boot ROM file which is of type tbxi. The final part, 'tbxi', tells the system to look for files of type tbxi. The notation '' tells the system to start searching for 'blessed' folders (directories) starting on the first HFS or HFS+ partition found on the system. The system will automatically convert 'hd' into the appropriate device path, as hd is simply an alias. The default boot device on Apple systems is 'hd:,\:tbxi'. If the auto-boot? variable is flagged as false, the system will halt the boot process and drop to the Open Firmware user interface. If additional information is required the boot-file variable is read. If the auto-boot? variable is flagged as true, the system will then read the value contained within the boot-device variable and attempt to boot from that device. The system will then examine the values of its environment variables. Open Firmware will then probe the system's I/O buses to determine the device configuration and build a device tree, which describes the hardware it has located.

When enough initialization has occurred to execute Open Firmware, the boot chime is played and Open Firmware is loaded. With the new family of iMac systems, additional pre-firmware diagnostics are performed due to the addition of the UniNorth and KeyLargo ICs. These POST tests are run in native PowerPC code and involve checksum testing, memory testing, detection of the manufacturing test pin and test manager support. The boot ROM stored in NVRAM provides power on self test (POST - diagnostics), and initializes enough of the system to load Open Firmware. The boot process starts with the PowerPC processor executing its reset vector as specified by the Hardware Initialization code.

One of the key functions of Open Firmware is to configure the UniNorth and Keylargo ICs, to construct a device tree, probe devices, include any device drivers and finally select a boot device. On the latest revision of Apple systems Open Firmware provides a variety of functions. Open Firmware is part of this 'New World' boot system. The rest of the system code, that on previous Mac systems (old world) resided on a physical ROM (chip) is now loaded from disk or from network into RAM. The approach uses a small ROM that contains sufficient code to initialize the hardware and load an operating system. Open Firmware is then capable of using these drivers to load an operating system from the device.Īpple's Open Firmware is part of their ROM-in-RAM design approach originally used in the first iMac systems. These drivers can either be integrated into the Open Firmware or provided on an external device, thus providing plug and play capabilities for new boot devices. Open Firmware probes the PCI bus for devices and possible Open Firmware device drivers for those devices.

Open Firmware exists to provide a machine independent means of loading operating systems, from a variety of boot devices. Open Firmware is a boot environment developed using the Forth programming language.

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