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BOOT(8) 										  BOOT(8)

NAME
       boot - 2.11BSD bootstrap proceedure

DESCRIPTION
       The  2.11BSD  system  is started by a two-stage process.  The first is a primary bootstrap
       (limited to 512 bytes) which is able to read in relatively small stand-alone programs; the
       second (called boot) is used to read in the system itself.

       The  primary bootstrap must reside in block zero of the boot device (the disklabel resides
       in block one).  It can be read in and started by standard ROM cold boot	routines  or,  if
       necessary,  by  keying  in  a  small startup routine.  The primary bootstrap is capable of
       loading only type 0407 executable files (impure (non-shared), non-separate  I&D.)   Copies
       of  the	block  zero bootstraps are kept in the directory /mdec.  Disklabel(8) is normally
       used to place a copy of the appropriate bootstrap in block zero of new file systems.

       The primary bootstrap loads boot from the file system that starts at block 0 of the  drive
       specified  to  the  boot  ROM.  Normally the boot device is automatically used as the root
       filesystem.  This action can be overriden by specifying the -R command to boot.	 If  boot
       is  not	found the system will hang as the primary boot spins in an endless loop trying to
       find boot.  No diagnostic message results if the file cannot be found.

       o      In an emergency, the bootstrap methods described in the paper Installing and  Oper-
	      ating 2.11BSD can be used to boot from a distribution tape.

       The  secondary  boot  program, called boot, actually brings in the system.  When read into
       location 0 and executed, boot sets up memory management, relocates itself into  high  mem-
       ory,  and  types  its  name and a `:' on the console.  If this is an automatic, unattended
       reboot, boot will use a default file specification for the installation, typing the file's
       name after the prompt.  Otherwise, it reads a file specification from the console.  Normal
       line editing characters can be used to make corrections while typing this (see  below  for
       file  specification  format).   If only a carriage return is typed, a default name (/unix)
       will be used.  Boot finds the [specified] file and loads it  into  memory  location  zero,
       sets  up  memory  management  as  required,  and  calls	the program by executing a `trap'
       instruction.

       For the system to boot, /etc/init must exist and be executable; if it is not,  the  kernel
       will  print a message to that effect and loop.  Further, for a single user boot, the files
       /bin/sh and /dev/console must also exist and /bin/sh must  be  executable  (if  either  of
       these is missing, init will attempt multi-user operation).  For a multi-user boot the file
       /etc/ttys must exist (if missing, init will attempt single user operation).

       Init runs the autoconfig(8) program to probe for and initialize devices.  Autoconfig  only
       knows to look in /unix, thus if an alternate kernel name was specified none of the devices
       except /dev/console and the boot disk will be known.

       If autoconfig problems are suspected (or if  you are simply voyeuristic)  the  debug  flag
       can be turned on by specifying -D to boot (see below).

       When  the system is running in single user mode, it starts a single user shell on the con-
       sole which types a `#' prompt.  After doing any file system checks and  setting	the  date
       (date(1))  a  multi-user system can be brought up by typing an EOT (control-d) in response
       to the `#' prompt.

       Boot file specification format: The file specifications used with boot are of the form:

	      device(ctlr,unit,part)path [-aRrDs]

       or

	      -bootcommand

       where

       device is the type of the device to be searched;

       ctlr   is the controller number of the disk

       unit   is the unit number of the disk or tape;

       part   is the partition number of a filesystem on the specified disk or the tape file num-
	      ber  if the device is a tape.  The underlying device driver must support disklabels
	      and a valid disklabel must be present if part is anything except 0.

       path   is the path name of a disk file to be loaded with all mount prefixes  stripped  off
	      (path  must  be  omitted	for tape files.)  Tape files are separated by single tape
	      marks.

       Flags to boot may be specified in either of two places.	At the :  prompt  and  after  the
       file name.  The options are:

       -a     Ask for a kernel name.  This is present for symmetry only because in order to spec-
	      ify this option you already have to be at the : prompt.

       -D     Turn on the autoconfig debug flag.

       -R     force the kernel to use its compiled in root device rather  than	adapting  to  the
	      boot device.

       -s     tell init to enter single user state rather than bringing the system all the way up
	      to multi-user mode.  -r mount the root filesystem read-only.  This is not currently
	      supported by the kernel mostly because pipes are implemented in the filesystem.

       Commands (-bootcommand) to boot are:

       -bootflags N	   where N is a decimal number.

       -bootflags flag	   where flag is from the list above.

       -bootdebug N	   where N is a decimal number.  This is a general purpose flag word used
			   by boot and is not passed to the loaded program or kernel.

       Device is one of the following

	    xp	 RM02/03/05, RP04/05/06, DIVA, SI Eagle, CDC 9766, Fuji 160
	    rp	 RP03
	    rk	 RK05
	    hk	 RK06/7
	    rl	 RL01/2
	    si	 RM05, CDC 9766
	    ra	 RA60/80/81, RX50, RD51/52/53, RC25
	    ht	 TU/TE16
	    tm	 TU/TE10
	    ts	 TS-11

       The stand alone tape drive unit number is specially encoded to specify  both  unit  number
       and  tape density (BPI).  Most tape subsystems either automatically adjust to tape density
       or have switches on the drives to force the density to a particular setting, but for those
       which  don't  the following density select mechanisms may be necessary.	The ts only oper-
       ates at 1600BPI, so there is no special unit density encoding.  The  ht	will  operate  at
       either  800BPI  or  1600BPI.   Units  0 through 3 correspond to 800BPI, and 4 through 7 to
       1600BPI on drives 0 through 3 respectively.  The standard DEC tm only supports 800BPI (and
       hence can't be used with the standard  distribution tape), but several widely used tm emu-
       lators support 1600BPI and even 6250BPI.  Units	0  through  3  correspond  to  800BPI,	4
       through 7 to 1600BPI, and 8 through 11 to 6250BPI on drives 0 through 3 respectively.

       For  example,  to  boot	a  system  from unit 0 on an RK07, type "hk(0,0)unix" to the boot
       prompt.	The specification "ra(1,0)unix" indicates an MSCP disk, unit 1.   The  specifica-
       tion  "ra(1,0,0)unix" indicates an MSCP disk, unit 0 but on controller 1.  And finally the
       specification "ts(0,3)" would cause the fourth file on a tape threaded on `ts' tape  drive
       0 to be loaded and executed.

       Cold boot loaders: The following programs to load and execute the primary bootstrap may be
       installed in read-only memories or manually keyed into main memory.  Each program is posi-
       tion-independent but should be placed well above location 0 so it will not be overwritten.
       Each reads a block from the beginning of a device into  core  location  zero.   The  octal
       words constituting the program are listed on the left.

       RK (drive 0):
	  012700      mov     $rkda,r0
	  177412
	  005040      clr     -(r0)	     / rkda cleared by start
	  010040      mov     r0,-(r0)
	  012740      mov     $5,-(r0)
	  000005
	  105710  1:  tstb    (r0)
	  002376      bge     1b
	  005007      clr     pc

       RP (drive 0)
	  012700      mov     $rpmr,r0
	  176726
	  005040      clr     -(r0)
	  005040      clr     -(r0)
	  005040      clr     -(r0)
	  010040      mov     r0,-(r0)
	  012740      mov     $5,-(r0)
	  000005
	  105710  1:  tstb    (r0)
	  002376      bge     1b
	  005007      clr     pc

       TM (drive 0):
	  012700      mov     $tmba,r0
	  172526
	  010040      mov     r0,-(r0)
	  012740      mov     $60003,-(r0)
	  060003
	  000777      br      .

FILES
       /unix	       system code
       /boot	       system bootstrap
       /etc/init       system process dispatcher
       /mdec/xxuboot   sector 0 boot blocks, xx is disk type

SEE ALSO
       crash(8V), autoconfig(8), reboot(2), disklabel(8), fsck(8), init(8)

3rd Berkeley Distribution		   May 24, 1996 				  BOOT(8)
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