The boot: prompt waits for you to type advanced startup options.
If you don't type anything, the computer continues starting up normally. It
uses the kernel and configuration files on the startup device, which it also
uses as the root device. 

Advanced startup options use the following syntax:

    [device]<kernel> [arguments]

Example arguments include

 device: rd=<BSD device name>       (e.g. rd=disk0s2)
         rd=*<IODeviceTree path>    (e.g. rd=*/PCI0@0/CHN0@0/@0:1)

 kernel: kernel name                (e.g. "mach_kernel" - must be in "/" )

 flags: -v (verbose)    -s (single user mode), 
        -x (safe mode)  -F (ignore boot configuration file)

 "Graphics Mode"="WIDTHxHEIGHTxDEPTH" (e.g. "1024x768x32")

 kernel flags                       (e.g. debug=0x144)
 io=0xffffffff                      (defined in IOKit/IOKitDebug.h)

Example: mach_kernel rd=disk0s1 -v "Graphics Mode"="1920x1200x32"

If the computer won't start up properly, you may be able to start it up using
safe mode.  Type -x to start up in safe mode, which ignores all cached
driver files.

Special booter hotkeys:
  F5            Rescans optical drive.
  F10           Scans and displays all BIOS accessible drives.
  
Special booter commands:
  ?memory       Displays information about the computer's memory.
  ?video        Displays VESA video modes supported by the computer's BIOS.
  ?norescan     Leaves optical drive rescan mode.

Additional useful command-line options:
  config=<file>             Use an alternate Boot.plist file.

Options useful in the com.apple.Boot.plist file:
  Wait=Yes|No               Prompt for a key press before starting the kernel.
  "Quiet Boot"=Yes|No       Use quiet boot mode (no messages or prompt).
  Timeout=8                 Number of seconds to pause at the boot: prompt.
  "Instant Menu"=Yes        Force displaying the partition selection menu. 

  "Default Partition"       Sets the default boot partition,
    =hd(x,y)                  where 'x' is the disk number, 'y' the partition number.

  GUI=No                    Disable the GUI (enabled by default).
  "Boot Banner"=Yes|No      Show boot banner in GUI mode (enabled by default).
  "Legacy Logo"=Yes|No      Use the legacy grey apple logo (disabled by default).
  
  GraphicsEnabler=Yes|No    Automatic device-properties generation for graphics cards.
  VideoROM=<file>           Use an alternate ROM image (default path: /NVIDIA.ROM).
  VBIOS=Yes|No              Inject VBIOS to device-properties.

  EthernetBuiltIn=Yes|No    Automatic "built-in"=yes device-properties generation
                            for ethernet interfaces.

  USBBusFix=Yes             Enable the EHCI and UHCI fixes (disabled by default).
  EHCIacquire=Yes           Enable the EHCI fix (disabled by default).
  UHCIreset=Yes             Enable the UHCI fix (disabled by default).

  Wake=No                   Disable wake up after hibernation (enbaled by default).
  ForceWake=Yes             Force using the sleepimage (disabled by default).
  WakeImage=<file>          Use an alternate sleepimage file 
                              (default path is /private/var/vm/sleepimage).

  DropSSDT=Yes              Skip the SSDT tables while relocating the ACPI tables.
  DSDT=<file>               Use an alternate DSDT.aml file 
                              (default path: /DSDT.aml /Extra/DSDT.aml)

  SMBIOS=<file>             Use an alternate smbios.plist file 
                              (default path: /smbios.plist /Extra/smbios.plist
                               bt(0,0)/Extra/smbios.plist).

  SMBIOSdefaults=No         Don't use the Default values for SMBIOS overriding
                              if smbios.plist doesn't exist, factory
                              values are kept.

  "Scan Single Drive"       Scan the drive only where the booter got loaded from.
    =Yes|No                 Fix rescan issues when using a DVD reader in AHCI mode. 
  Rescan=Yes                Enable CD-ROM rescan mode.
  "Rescan Prompt"=Yes       Prompts for enable CD-ROM rescan mode.
