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bootstrap - Disk boot process for PowerMac GNU/Linux

BOOTSTRAP(8) System Manager's Manual BOOTSTRAP(8)


bootstrap - Disk boot process for PowerMac GNU/Linux


This man page describes the bootstrap process for both OldWorld and NewWorld PowerMacs. OldWorld PowerMacs all have a hardware MacOS ROM and the case is beige in color. NewWorld PowerMacs do not have a hardware MacOS ROM, and are in colored, translucent cases. All G3s in colored cases are NewWorld, as are all G4s and later. This man page is divided into three sections, OLDWORLD, NEWWORLD, and IBM. Please read the section appropriate to your hardware.


The process of booting PowerMac/Linux from a disk starts with Open Firmware loading the boot block from the first bootable partition of the boot disk into memory. The user specifies which device is to be the boot disk by setting the boot-device environment variable to the Open Firmware name of the boot disk, or by giving it as an explicit argument to the Open Firmware boot command. OldWorld PowerMacs typically do not require bootstrap partitions like NewWorld PowerMacs do.
Open Firmware then transfers control to the first-stage bootstrap ( first.b), located at the beginning of the boot block. The boot block also contains the list of block numbers for the second-stage bootstrap. First.b reads these blocks into memory, thus loading the second-stage bootstrap.
The task of the second-stage bootstrap ( second.b) is to load the Linux kernel into memory and pass it any arguments given by the user. Second.b can also be used for loading other programs, such as diagnostic programs or other operating systems, as long as they are present as an ELF binary in an ext2 filesystem.
Second.b gets two string values from Open Firmware, called bootpath and bootargs. Bootpath is the Open Firmware name of the boot disk (i.e., the device that the first-stage bootstrap was loaded from). If Open Firmware auto-booted, or if the boot command was given without arguments, then bootpath and bootargs are set to the values of the boot-device and boot-file variables, respectively. If the boot command was given with arguments, the first argument becomes bootpath and any subsequent arguments are saved in bootargs.
Second.b uses the Open Firmware input and output devices for communicating with the user. By default, the modem port is used for both, but this can be changed by setting the Open Firmware input-device and output-device variables.
Second.b starts by printing a message to indicate that it has started, and then reads the configuration file. By default, the configuration file is /etc/quik.conf(5) on the same partition as the boot block, but this can be overridden with quik(8). The configuration file must be on the same disk as the boot block. The format of the configuration file is described in quik.conf(5).
Then second.b prints its boot: prompt and waits for the user to type a command line. Normally the configuration file specifies a timeout, and if the user does not type anything within that period of time, second.b proceeds using the bootargs value as the command line. If the timeout value is 0, second.b will always use the bootargs value, ignoring anything the user types. This can be useful when a modem is connected to the modem port.
Having obtained a command line, second.b takes the first word (whitespace-separated) as the name of the program to load. Any remaining words on the line become arguments to be passed to the program when it is loaded. If the command line is empty, second.b uses the value of the default keyword in the configuration file, or failing that, the first program specified in the configuration file.
The configuration file can specify several alternative programs to load (referred to as images in the configuration file syntax), along with shorthand labels for them and extra arguments to be prepended to those specified by the user. The program name given in the command line can be either an explicit path name or a shorthand label. If it is a shorthand label, the configuration file gives the corresponding path name.
Path names are of the form
where device is the Open Firmware name of the disk, partno is the (decimal) number of the partition on that disk, and filepath is the path to the file in the ext2 filesystem on that partition. The default for device is bootpath, and the default for partno is the first bootable partition on device. Alternatively, the /filepath section can be replaced by a span of 512-byte blocks to load using the syntax [start-end] where start and end are decimal block numbers.
Second.b will attempt to open the file identified by the path name and load it into memory as an ELF binary. If the file cannot be found, or if it is not an ELF binary, second.b will print an error message and print its boot: prompt again. In this case there is no timeout and second.b does not use the bootargs value.
Once second.b has loaded the program into memory, it transfers control to it, passing it the list of arguments.


The process of booting so called NewWorld PowerMacs from disk starts with OpenFirmware first attempting to execute the file specified in the boot-device variable. Unlike older versions of OpenFirmware the NewWorld version will not attempt to read a boot sector. By default OpenFirmware attempts to load a file with HFS file type “tbxi” in the “blessed” directory from each partition of each disk OpenFirmware is aware of, the first partition/disk that is found to be bootable is booted immediately.
Ybin(8) configures a bootstrap partition to pass all of OpenFirmware's tests to determine if the partition is considered to be bootable or not. The boot script is given file type “tbxi” and the root directory is marked as “blessed”, the blessing is important because OpenFirmware will immediately consider a partition unbootable if no directory is marked as blessed (you can still manually execute a loader such as yaboot(8) with OpenFirmware even without a blessed directory but it will not happen automatically).
The MacOS System Folder is always marked as blessed, this is required for MacOS as well as OpenFirmware. The MacOS System Folder also contains its own boot loader which has the tbxi file type, this makes installing yaboot(8) onto a MacOS partition is difficult. The only way to install yaboot(8) on a MacOS boot partition is to modify OpenFirmware to boot the CHRP script directly. Given this it is highly recommended that you create a dedicated bootstrap partition for yaboot(8).
Since OpenFirmware boots the first partition it finds to be bootable it is important that the bootstrap partition be first on the disk before any MacOS partition, otherwise MacOS will be booted instead of a dual boot menu used with yaboot(8).
The bootstrap partition should also NOT be mountable by MacOS, the reason is MacOS will (almost always) closely inspect any blessed directories to make sure its real MacOS, if it is not satisfied that the contents are a real copy of MacOS it will unbless the directory, resulting in OpenFirmware no longer considering it bootable. The best way to protect against this is to create the bootstrap partition with the partition type “Apple_Bootstrap” which OpenFirmware accepts as a valid HFS partition, but MacOS will ignore and refuse to mount. The bootstrap partition need not be any larger then 800K. 800K is the minimum size of an HFS filesystem, and is much more then enough for this purpose. You need not, and should not keep kernels on this partition, yaboot(8) will load them from your ext2fs root partition just fine, as well as from any HFS or HFS+ partitions ( yaboot(8) uses OpenFirmware's HFS+ filesystem support).
To create the bootstrap partition, use GNU parted(8) or mac-fdisk(8) to create a partiton of type “Apple_Bootstrap”. This is documented better in mac-fdisks-basics (
The bootstrap need not and should not be mounted anywhere on your filesystem, especially not on top of /boot. Yaboot(8) is able to load the kernels from the ext2fs root partition so that is where they should be kept.
OpenFirmware maintains a hierarchy of all the hardware it is aware of. To access or specify a boot device you must use the OpenFirmware path. For example: the path to a SCSI hard disk partition might look like this: /pci@80000000/pci-bridge@d/ADPT,2930CU@2/@2:2 . The first part, pci@80000000, shows that the target device is accessed through the PCI bus. The next part is the PCI bridge, the next is the name of the SCSI host adapter installed (this name is provided by a BootROM on the card itself), and after that is the SCSI ID number. The colon delimits the device from partition specification, so the last 2 means the second partition of this device. After the partition number we can specify pathnames to files in two ways: lazy and absolute. The “,” delimits the OpenFirmware path from the location of the bootfile. “,\\:tbxi” specifies the file that has a HFS file type of “tbxi” in the blessed directory. If there is not blessed directory this will fail. The second is to specify a absolute pathname to an arbitrary file on the disk, example: 2:,yaboot would load the file named “yaboot” in the root directory of the filesystem. It is possible to load files in subdirectories but OpenFirmware does not always do this reliably, and any special characters such as an embedded space must be expressed like %20 (for a space) the directory separator used by OpenFirmware is the backslash \. Example: 2:,\boot\yaboot. Determining the OpenFirmware path to a given device is unfortunately not a trivial task. If you are using the built in ATA hard disk you can use the alias “hd:”.
Ybin also includes a utility ofpath(8) which can in most cases find the OpenFirmware device path from a unix device node (ie /dev/hda2).
In addition to binary executables OpenFirmware can also execute a CHRP script. This is somewhat similar to a shell script. A CHRP script is useful to create simple boot menus, among other things. CHRP scripts are divided into sections in a way similar to HTML. Here is a basic example of a CHRP script used as a wrapper to yaboot(8) (since OpenFirmware will only load a file with type “tbxi” if it is a CHRP script).
GNU/Linux PowerPC bootloader
boot hd:,\\yaboot
The COMPATIBLE section defines what machines this script is compatible with, if the machine name encoded into the ROM does not match one of these entries OpenFirmware will print out a lot of incomprehensible junk and fail to load the script. The DESCRIPTION is ignored by OpenFirmware as far as I know. The BOOT-SCRIPT section is where arbitrary OpenFirmware Forth commands may go. They are executed the same way as you would enter them on the OpenFirmware command line. The entire script is wrapped with the CHRP-BOOT tags so that such a script may be attached as a header to a binary file. Much more complicated and elaborate CHRP scripts are possible but that is beyond the scope of this document.
Ybin as of version 0.17 includes a more robust script that is automatically configured with the correct OpenFirmware paths based on /etc/yaboot.conf. This new script need not and should not be edited by the user.
If you have G4 hardware then your OpenFirmware may already have a graphical boot selector built in. This selector can be accessed by holding down the option key when booting the machine. You should see a screen with buttons for each bootable partition. The current version (as of ybin(8) 0.13) of ofboot includes a badge icon, the button with a penguin icon is your bootstrap partition. If you decide to use this built in selector you really do not need to use a CHRP script that provides a boot menu. Thanks to Nicholas Humfrey for creating the Badge icon.


IBM hardware such as the RS/6000 require msdos style partition tables. In order to boot from the disk they require a type 0x41 PReP Boot bootstrap partition large enough to hold the bootloader (typically yaboot(8)). The bootloader is copied onto the raw partition as there is no filesystem. This is done either with dd(1) or mkofboot(8).




ybin, and the NEWWORLD, and IBM sections of this man page written by Ethan Benson <>
The OLDWORLD section of this man page was taken from the quik(8) package, which was written by Paul Mackerras.
yaboot was written by Benjamin Herrenschmidt <>.


dd(1), mkofboot(8), ofpath(8), quik(8), quik.conf(5), yaboot(8), ybin(8).
28 April 2001 GNU/Linux PowerPC