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 caches)
        -F (ignore "Kernel Flags" specified in 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)|UUID|"Label"    Specified as a disk/partition pair, an UUID, or a 
                             label enclosed in quotes.
                             
  "Hide Partition"        Remove unwanted partition(s) from the boot menu.
    =partition               Specified, possibly multiple times, as hd(x,y), an
     [;partition2 ...]       UUID or label enclosed in quotes.
    
  "Rename Partition"      Rename partition(s) for the boot menu.
    =partition <alias>       Where partition is hd(x,y), UUID or label enclosed
     [;partition2 <alias2>   in quotes. The alias can optionally be quoted too. 
     ...]

  PciRoot=<value>         Use an alternate value for PciRoot (default value 0).
  
  "Scan Single Drive"     Scan the drive only where the booter got loaded from.
    =Yes|No               Fix rescan pbs 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.
  SystemId=<UUID>         Set manually the system id UUID,
                          SMUUID in smbios config (reserved field) isn't used.
  SystemType=<n>          Set the system type where n is between 0..6
                          (default =1 (Desktop)