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The boot project contains the bootloader that is used to boot the system on generic hardware through BIOS. 

boot is a series of files that collectively act as bootloader for BIOS-based hardware.
  • boot (sometimes referred to as "boot2") HFS+ startup file, lives in the HFS+ file system structure (not a regular file). (Could also be used as a regular file; using a special version of boot1h that would no longer need it to be HFS+ the startup file.)
  • boot0 Master Boot Record (MBR), lives in the first 440 bytes of the disk (not identical to the MBR from other OSes)
  • boot1h stage 1 booter (bootsector) for HFS+ partitions, lives in the first 512 bytes of the partition
  • boot1u (not used by PureDarwin)
  • boot1u0 (not used by PureDarwin)
  • cdboot booter for CD-ROMs (only works with valid Extensions.mkext due to a bug)
  • chain0 (tbd; probably not used by PureDarwin)
Since Intel-based Macintosh computers use EFI instead of BIOS, they need to use a different bootloader, efiboot.

Another alternative to the above two bootloaders, is the new XNU loading support in grub2, which works on both platforms (EFI and BIOS based) . For more information, check out this page.


To use boot, you need:
  • A MBR-formatted HFS+ volume containing a Darwin system
  • startupfiletool (source)
  • boot-132 (patched to include the EFI tables the Darwin system expects to be present)


The disk needs to contain a HFS+ volume, and it must not be mounted.

launchctl unload /System/Library/LaunchDaemons/
startupfiletool ${RDEVICE}s1 boot
sleep 3
dd if=boot1h of=${RDEVICE}s1 bs=512 count=1
sleep 3
dd if=boot0 of=${DEVICE} bs=440 count=1
launchctl load /System/Library/LaunchDaemons/

The chameleon boot loader (a fork of boot-132) has a greatly simplified installation procedure, since it does NOT need the disk to be unmounted, and it does NOT need startupfiletool:

fdisk -f boot0 -u -y /dev/rdiskX    # Install boot0 to the MBR
dd if=boot1h of=/dev/rdiskXsY       # Install boot1h to the partition's bootsector
cp boot /Volumes/XXX                # Install boot to the volume's root directory


Non-System Disk

Problem: You get the error
Boot0: MBR
Boot0: Done
Non-System Disk

Solution: You need to set the partition bootable ("boot flag", "set partition active"), e.g., using fdisk:
sh-3.2# fdisk -e /dev/rdisk1
Enter 'help' for information  
fdisk: 1> f 1
Partition 1 marked active.
fdisk:*1> w
Device could not be accessed exclusively.
A reboot will be needed for changes to take effect. OK? [n] y
Writing MBR at offset 0.
fdisk: 1> q

Wrong system booted by default

Problem: On a system with multiple OSes, boot wants to boot the wrong (non-PureDarwin) one by default. This happens when a non-HFS+ partition (e.g., FAT32 or NTFS) has the bootable ("active") flag and comes before the HFS+ partition that contains PureDarwin.

Solution: Modify boot so that it uses the first HFS+ partition instead of the first partition with the bootable ("active") flag:

In boot-132/i386/lib/libsaio/sys.c, comment the following line:

    for ( bvr = chain; bvr; bvr = bvr->next ) 
        if ( bvr->flags & kBVFlagNativeBoot ) bvr1 = bvr; 
        // if ( bvr->flags & kBVFlagPrimary )    bvr2 = bvr; 

To be continued...