The Mac OS X booter (boot.efi on EFI-based Macintosh systems) is responsible for
loading the initial parts of the operating system into memory and executing them. Its default
behavior is to start up Mac OS X in graphical mode. However, certain behavior can be
configured using the com.apple.Boot.plist file, as well as NVRAM variables,
EFI boot options, and keyboard "snag keys".
The com.apple.Boot.plist is a standard plist
format Core Foundation property list stored in XML format. Keys are generally strings like Kernel Flags,
with either string or integer values. The following key-values are currently supported:
- Kernel Flags
- [string] This option specifies arguments to be passed
directly to the kernel to change its behavior (although some kernel options are parsed by the
booter as well for correctness). Common options include "debug=0x144" to enable kernel debugging,
"-v" to enable verbose boot, "-s" to boot to single user mode, "cpus=1" to simulate a
single core system, and "maxmem=1024" to cap available memory to 1024 MB RAM. All desired options
should be space-separated within the <string> tag. The default value is the empty string.
- MKext Cache
- [string] This option specifies the mkext file
to be loaded, which contains kernel extensions. The path uses backslash ("\") path delimeters.
The default value is System\Library\Caches\com.apple.kext.caches\Startup\Extensions.mkext.
- Kernel
- [string] This option specifies the kernel file to
be loaded. The path uses backslash ("\") path delimeters. The default
value is mach_kernel.
- Kernel Cache
- [string] This option specifies the prelinked
kernel file to be loaded, which contains both the kernel and kernel extensions, linked
at their final load addresses. The path uses backslash ("\") path delimeters. The
default value is calculated programmatically using
System\Library\Caches\com.apple.kext.caches\Startup\kernelcache_${ARCH}.${CHECKSUM}
as the template, where ${ARCH} is the architecture of the kernel ("i386" or "x86_64") and
${CHECKSUM} is a 32-bit hash of characteristics of the boot volume and machine hardware.
- Kernel Architecture
- [string] This option specifies which kernel
architecture to use when the kernel file is a universal binary and the hardware supports
more than one kernel architecture. It can be either "i386" or "x86_64" to choose the respective
slice of the universal binary. If the kernel is not a universal binary, this option has
no effect. If the hardware only supports the 32-bit kernel, this option has no effect. The default
value is determined programmatically based on the hardware, and whether Mac OS X Server
is being used.
- Root UUID
- [string] This option is rarely used and specifies
to the kernel what block device should be probed as the root filesystem ("/"), and is most
commonly overriden by the OS installation software when installing onto AppleRAID volumes. It can be
either a filesystem volume UUID, as represented by diskutil info, or
a GPT partition UUID. The default is generated programmatically based on which filesystem
the booter itself was loaded from.
Since the com.apple.Boot.plist file exists on the root filesystem, it is tied to that
OS volume, and is no longer honored if Startup Disk or bless
is used to change the boot preference to another volume.
The Kernel Flags options can also be specified via --option
to bless, which encodes
the string in the EFI boot options along with the
OS boot volume preference. The space-separated strings
are merged with kernel flags specified in the
com.apple.Boot.plist and NVRAM.
At boot time, the booter checks to see if certain keys are being pressed, and alters behavior accordingly.
This is in addition to similar functionality that the firmware itself may implement before starting the
booter. Since pressing keys requires physical interaction, they take precendence over preferences
set through other means, including the com.apple.Boot.plist file and NVRAM.
The following key combinations are currently supported:
- Shift
- Boot in Safe Mode. Effectively the same as passing "-x" in
Kernel Flags, and causes most caches to be ignored by the booter.
- Command-S
- Boot in Single User Mode. Effectively the same as passing "-s" in
Kernel Flags, and causes the system to boot to an interactive shell
with no system services started.
- Command-V
- Boot in Verbose Mode. Effectively the same as passing "-v" in
Kernel Flags, and causes the system to boot to verbose text logging before
starting the graphical user interface.
- Command-R
- Boot in Recovery Mode. This causes the system to boot into special Recovery System image, which can be used used to restore the system from Time Machine Backup, re-install Mac OS X, or use Disk Utility to repair or erase disks.
- 3 2
- Boot with the 32-bit kernel. Effectively the same as passing
"arch=i386" in Kernel Flags, and causes the system to prefer the 32-bit
kernel on systems that would otherwise boot the 64-bit kernel.
- 6 4
- Boot with the 64-bit kernel (if supported on this system). Effectively the same as passing
"arch=x86_64" in Kernel Flags, and causes the system to prefer the 64-bit
kernel on systems that would otherwise boot the 32-bit kernel. If the 64-bit kernel is not supported, the option is ignored.
The Mac OS X booter can also be controlled by NVRAM variables, which are analogous
to environment variables. NVRAM variables are key-value pairs. The "boot-args" NVRAM variable
can be used to provide additional arguments to the kernel, and will be merged with
Kernel Flags in the com.apple.Boot.plist file and
EFI boot options set with bless.
Since NVRAM variables are system-wide, they are consulted regardless of what OS volume is
set as the boot preference. NVRAM variables are persistent even when the boot preference
is changed by Startup Disk or bless.