boot

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

What is boot

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.)
• 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 (currently known as just grub), which works on both platforms (EFI and BIOS based) . For more information, check out this page (archived since XNU support has not been mentioned in newer versions of grub docs).

Requirements

To use boot, you need:

• A MBR-formatted HFS+ volume containing a Darwin system
• startupfiletool (source)
• boot-132 (patched (archive) to include the EFI tables the Darwin system expects to be present)

Installation

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

launchctl unload /System/Library/LaunchDaemons/com.apple.diskarbitrationd.plist
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
sync
launchctl load /System/Library/LaunchDaemons/com.apple.diskarbitrationd.plist

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

Troubleshooting

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
sh-3.2# 

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...