-a
Prompt for the root file system device, the system crash dump device, and the path to
init(8).
-d
Bring the system up in debug mode. Here it waits for a kernel debugger connect; see
ddb(4).
-q
Boot the system in quiet mode.
-s
Bring the system up in single-user mode.
-v
Boot the system in verbose mode.
Any extra flags or arguments, or the
<boot string> after the -- separator are passed to the boot PROM. Other flags are currently ignored.
At any time you can break to the kernel debugger
ddb(4) (assuming
options DDB was specified in the kernel configuration file) by sending a serial line BREAK character. If you do this accidentally you can continue whatever was in progress by typing ‘c' followed by the return key.