NAME
boot procedures - UNIX startup
DESCRIPTION
How to start UNIX. UNIX is started by placing it in core
at location zero and transferring to zero. Since the system
is not reenterable, it is necessary to read it in from disk
or tape.
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
one way to run UNIX is to maintain the system code on a tape
using tp. Caution: the file /usr/mdec/tboot (DECtape) or
/usr/mdec/mboot (magtape) must be present when the tape is
made! 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 sys-
tem on the tape (we use plain `unix'). It is strongly rec-
ommended that a current version of the system be maintained
in this way, even if it is usually booted from disk.
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.
The system can also be booted from a disk file with the aid
of the uboot program. When read into location 0 and exe-
cuted, uboot reads a single character (either p or k for RP
or RK, both drive 0) to specify which device is to be
searched. Then it reads a UNIX pathname from the console,
finds the corresponding file on the given device, loads that
file into core location zero, and transfers to it. Uboot
operates under very severe space constraints. It supplies
no prompts, except that it echoes 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. If it fails to find the file, how-
ever, it jumps back to its start, so another try can be
attempted, starting again with the p or k. Notice that
uboot will only load a file from drive 0, and the file sys-
tem it searches must start at the beginning of the disk.
Uboot itself usually resides in the otherwise unused block 0
of the disk, so it can be loaded by ROM program; mkfs can be
used to put it there when the file system is created. It
can also be loaded from a tp tape as described above.
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The switches. The console switches play an important role
in the use and especially the booting of UNIX. During oper-
ation, the console switches are examined 60 times per sec-
ond, 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 fas-
ter than the bus error timeout (about 10μs) 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 begin-
ning 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)
2
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 $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
FILES
/unix - UNIX code
/usr/mdec/mboot - tp magtape bootstrap
/usr/mdec/tboot - tp DECtape bootstrap
/usr/mdec/uboot - file system bootstrap
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SEE ALSO
tp (I), init (VIII)
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