boot —
system bootstrapping procedures
Normally, the system will reboot itself at power-up or after crashes. An
  automatic consistency check of the file systems will be performed as described
  in fsck(8), and unless this
  fails, the system will resume multi-user operations.
A disk-boot program (/usr/mdec/ufsboot) will attempt to
  load netbsd from partition A of the boot device, which
  must currently be an “sd” disk. Alternatively, network boot
  program (/usr/mdec/netboot) will load
  netbsd from the NFS root as determined by the
  procedure described in
  diskless(8).
  - -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 back to the ROM by pressing the
    ‘L1’ and ‘a’ keys at the same time (if the
    console is a serial port the same is achieved by sending a
    ‘break’). If you do this accidentally you can continue
    whatever was in progress by typing ‘c’ followed by the return
    key.
  - /netbsd
- system code
- /usr/mdec/bootxx
- first-level boot block for disks
- /usr/mdec/netboot
- boot program for NFS (diskless) boot
- /usr/mdec/ufsboot
- second-level boot program for UFS disks
- /usr/mdec/installboot
- program to install bootxx on a disk