BOOT PROCEDURES(VIII) 11/1/73 BOOT PROCEDURES(VIII)
NAME
boot procedures - UNIX startup
DESCRIPTION
The advent of the new system has changed the boot
procedures. These procedures apply only to C-language
systems.
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.
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
space.
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
177346
010040 mov r0,-(r0) / use tc addr for wc
012710 mov $3,(r0) / read bn forward
000003
105710 1: tstb (r0) / wait for ready
002376 bge 1b
112710 movb $5,(r0) / read (forward)
000005
000777 br . / loop; now halt and start at 0
DECtape (drive 0) with search:
012700 1: mov $tcba,r0
177346
010040 mov r0,-(r0) / use tc addr for wc
012740 mov $4003,-(r0) / read bn reverse
004003
005710 2: tst (r0)
002376 bge 2b / wait for error
005760 tst -2(r0) / loop if not end zone
177776
002365 bge 1b
012710 mov $3,(r0) / read bn forward
000003
105710 2: tstb (r0) / wait for ready
002376 bge 2b
112710 movb $5,(r0) / read (forward)
000005
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
172526
010040 mov r0,-(r0) / usr mt addr for wc
012740 mov $60003,-(r0) / read 9-track
060003
000777 br . / loop; now halt and start at 0
RK (drive 0):
012700 mov $rkmr,r0
177414
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
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
FILES
/usr/sys/unix - UNIX code
/usr/mdec/mboot - tp magtape bootstrap
/usr/mdec/tboot - tp DECtape bootstrap
/usr/mdec/uboot - file system bootstrap
SEE ALSO
tp(I), init(VII)