boot procedures - UNIX startup


     How to start UNIX.   UNIX is started by placing it  in  core

     starting  at  location zero and transferring to zero.  There

     are various ways to do this.  If UNIX is still intact  after

     it  has  been  running, the most obvious method is simply to

     transfer to zero, but this is not recommended if the  system

     has crashed.

     The tp command places a bootstrap program on  the  otherwise

     unused  block zero of the tape.  The DECtape version of this

     program is called tboot,  the  magtape  version  mboot.   If

     tboot  or  mboot  is  read  into  location zero and executed

     there, it will type `=' on the console, read in a  tp  entry

     name, load that entry into core, and transfer to zero.  Thus

     the next easiest way to run UNIX is  to  maintain  the  UNIX

     code  on  a  tape  using  tp.  Then when a boot is required,

     execute (somehow) a program which reads in and jumps to  the

     first  block  of  the  tape.  In response to the `=' prompt,

     type the entry name of the system on the tape (we use  plain

     `unix').   It is strongly recommended that a current version

     of the system be maintained in this way, even if  the  first

     or third methods of booting the system are usually used.

     The standard DEC ROM which loads DECtape  is  sufficient  to

     read  in  tboot,  but  the  magtape ROM loads block one, not

     zero.  If no suitable ROM is available, magtape and  DECtape

     programs are presented below which may be manually placed in

     core and executed.

     A  third  method  of  rebooting  the  system  involves   the

     otherwise  unused  block zero of each UNIX file system.  The

     single-block program uboot will read a  UNIX  pathname  from

     the  console,  find the corresponding file on a device, load

     that file into core location zero, and transfer to it.   The

     current   version  of  this  boot  program  reads  a  single

     character (either p or k for RP or  RK,  both  drive  0)  to

     specify  which  device  is  to  be searched.  Uboot operates

     under  very  severe  space  constraints.   It  supplies   no

     prompts,  except  that  it  echos a carriage return and line

     feed after the p or k.  No diagnostic  is  provided  if  the

     indicated  file  cannot  be found, nor is there any means of

     correcting typographical errors in the file name  except  to

     start  the  program  over.   Uboot  can reside on any of the

     standard file systems or may be loaded from  a  tp  tape  as

     described above.

     The standard DEC disk ROMs will load and execute uboot  from

     block zero.

     The switches.   The console switches play an important  role

     in  the  use  and  especially  the  booting of UNIX.  During

     operation, the console switches are examined  60  times  per

     second,  and  the  contents  of the address specified by the

     switches are displayed in the display  register.   (This  is

     not  true on the 11/40 since there is no display register on

     that machine.)  If the switch address is even,  the  address

     is  interpreted  in  kernel  (system)  space;  if  odd,  the

     rounded-down address is  interpreted  in  the  current  user


     If any diagnostics are produced  by  the  system,  they  are

     printed  on  the  console only if the switches are non-zero.

     Thus it is wise to have a non-zero value in the switches  at

     all times.

     During the startup of the system, the  init  program  (VIII)

     reads  the  switches  and  will  come  up single-user if the

     switches are set to 173030.

     It is unwise to have a non-existent address in the switches.

     This  causes a bus error in the system (displayed as 177777)

     at the rate of 60 times per second.  If there is a  transfer

     of  more  than  16ms  duration  on a device with a data rate

     faster than the bus  error  timeout  (approx  10us)  then  a

     permanent disk non-existent-memory error will occur.

     ROM programs.   Here are some programs  which  are  suitable

     for  installing  in read-only memories, or for manual keying

     into core if no ROM is present.  Each program  is  position-

     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.

     DECtape (drive 0) from endzone:

        012700      mov     $tcba,r0


        010040      mov     r0,-(r0)       / use tc addr for wc

        012710      mov     $3,(r0)        / read bn forward


        105710  1:  tstb    (r0)           / wait for ready

        002376      bge     1b

        112710      movb    $5,(r0)        / read (forward)


        000777      br      .              / loop; now halt and start at 0

     DECtape (drive 0) with search:

        012700  1:  mov     $tcba,r0


        010040      mov     r0,-(r0)       / use tc addr for wc

        012740      mov     $4003,-(r0)    / read bn reverse


        005710  2:  tst     (r0)

        002376      bge     2b             / wait for error

        005760      tst     -2(r0)         / loop if not end zone


        002365      bge     1b

        012710      mov     $3,(r0)        / read bn forward


        105710  2:  tstb    (r0)           / wait for ready

        002376      bge     2b

        112710      movb    $5,(r0)        / read (forward)


        105710  2:  tstb    (r0)           / wait for ready

        002376      bge     2b

        005007      clr     pc             / transfer to zero

     Caution: both of these  DECtape  programs  will  (literally)

     blow a fuse if 2 drives are dialed to zero.

     Magtape from load point:

        012700      mov     $mtcma,r0


        010040      mov     r0,-(r0)       / usr mt addr for wc

        012740      mov     $60003,-(r0)   / read 9-track


        000777      br      .              / loop; now halt and start at 0

     RK (drive 0):

        012700      mov     $rkmr,r0


        005040      clr     -(r0)

        010040      mov     r0,-(r0)

        012740      mov     $5,-(r0)


        105710  1:  tstb    (r0)

        002376      bge     1b

        005007      clr     pc

     RP (drive 0)

        012700      mov     $rpmr,r0


        005040      clr     -(r0)

        005040      clr     -(r0)

        005040      clr     -(r0)

        010040      mov     r0,-(r0)

        012740      mov     $5,-(r0)


        105710  1:  tstb    (r0)

        002376      bge     1b

        005007      clr     pc


     /unix - UNIX code

     /usr/mdec/mboot - tp magtape bootstrap

     /usr/mdec/tboot - tp DECtape bootstrap

     /usr/mdec/uboot - file system bootstrap


     tp (I), init (VII)