What happens in the Mac OS X boot process?
Long gone are the days of OS 9, watching our Macs boot up with a series of extensions and control panels that we could always identify. Today with the Unix underpinnings of OS X, many users are entirely unaware of what is going on behind the scenes. So what exactly happens during the Mac OS X boot process? A segment at KernelThread carefully lists the sequence of events, from start to finish. It is fairly thorough and worth a read. It is repeated below for the inquisitive Mac OS X users out there.
Note: As a reader pointed out, PPC uses OF, i386 uses EFI
You turn on your Mac, and this is what happens:
* Power is turned on.
* OF or EFI code is executed.
* Hardware information is collected and hardware is initialized.
* Something (usually the OS, but also things like the Apple Hardware Test, etc.) is selected to boot. The user may be prompted to select what to boot.
* Control passes to /System/Library/CoreServices/BootX, the boot loader. BootX loads the kernel and also draws the OS badges, if any.
* BootX tries to load a previously cached list of device drivers (created/updated by /usr/sbin/kextcache). Such a cache is of the type mkext and contains the info dictionaries and binary files for multiple kernel extensions. Note that if the mkext cache is corrupt or missing, BootX would look in /System/Library/Extensions for extensions that are needed in the current scenario (as determined by the value of the OSBundleRequired property in the Info.plist file of the extension’s bundle.
* The init routine of the kernel is executed. The root device of the booting system is determined. At this point, Firmware is not accessible any more.
* Various Mach/BSD data structures are initialized by the kernel.
* The I/O Kit is initialized.
* The kernel starts /sbin/mach_init, the Mach service naming (bootstrap) daemon. mach_init maintains mappings between service names and the Mach ports that provide access to those services.
From here on, the startup becomes user-level:
* mach_init starts /sbin/init, the traditional BSD init process. init determines the runlevel, and runs /etc/rc.boot, which sets up the machine enough to run single-user.
During its execution, rc.boot and the other rc scripts source /etc/rc.common, a shell script containing utility functions, such as CheckForNetwork() (checks if the network is up), GetPID(), purgedir() (deletes directory contents only, not the structure), etc.
* rc.boot figures out the type of boot (Multi-User, Safe, CD-ROM, Network etc.). In case of a network boot (the sysctl variable kern.netboot will be set to 1 in which case), it runs /etc/rc.netboot with a start argument.
/etc/rc.netboot handles various aspects of network booting. For example, it performs network and (if any) local mounts. It also calls /usr/bin/nbst to associate a shadow file with the disk image being used as the root device. The idea is to redirect writes to the shadow file, which hopefully is on local storage.
* rc.boot figures out if a file system consistency check is required. Single-user and CD-ROM boots do not run fsck. SafeBoot always runs fsck. rc.boot handles the return status of fsck as well.
* If rc.boot exits successfully, /etc/rc, the multi-user startup script is then run. If booting from a CD-ROM, the script switches over to /etc/rc.cdrom (installation).
* /etc/rc mounts local file systems (HFS+, HFS, UFS, /dev/fd, /.vol), ensures that the directory /private/var/tmp exists, and runs /etc/rc.installer_cleanup, if one exists (left by an installer before reboot).
* /etc/rc.cleanup is run. It “cleans” a number of Unix and Mac specific directories/files.
* BootCache is started.
* Various sysctl variables are set (such as for maximum number of vnodes, System V IPC, etc.). If /etc/sysctl.conf exists (plus /etc/sysctl-macosxserver.conf on Mac OS X Server), it is read and sysctl variables contained therein are set.
* syslogd is started.
* The Mach symbol file is created.
* /etc/rc starts kextd, the daemon process that loads kernel extension on demand from kernel or client processes.
* /usr/libexec/register_mach_bootstrap_servers is run to load various Mach bootstrap based services contained in /etc/mach_init.d
* portmap and netinfo are started.
* If /System/Library/Extensions.mkext is older than /System/Library/Extensions, /etc/rc deletes the existing mkext and creates a new one. It also creates one if one doesn’t exist.
* /etc/rc starts /usr/sbin/update, the daemon that flushes internal file system caches to disk frequently.
* /etc/rc starts the virtual memory system. /private/var/vm is set up as the swap directory. /sbin/dynamic_pager is started with the appropriate arguments (swap filename path template, size of swap files created, high and low water alert triggers specifying when to create additional swap files or delete existing ones).
* /etc/rc starts /usr/libexec/fix_prebinding to fix incorrectly prebound binaries.
* /etc/rc executes /etc/rc.cleanup to clean up and reset files and devices.
* /etc/rc finally launches /sbin/SystemStarter to handle startup items from locations such as /System/Library/StartupItems and /Library/StartupItems. A StartupItem is a program, usually a shell script, whose name matches the folder name. The folder contains a property list file containing key-value pairs such as Description, Provides, Requires, OrderPreference, start/stop messages etc. You can run SystemStarter -n -D as root to have the program print debugging and dependency information (without actually running anything).
* The CoreGraphics startup item starts the Apple Type Services daemon (ATSServer) as well as the Window Server (WindowServer).