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.th "BOOT PROCEDURES" VIII 2/11/75
.sh NAME
boot procedures \*- UNIX startup
.sh DESCRIPTION
.it "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.
.s3
The
.it tp
command places a bootstrap program
on the
otherwise unused block zero of the tape.
The DECtape version of this program is
called
.it tboot,
the magtape version
.it mboot.
If
.it tboot
or
.it mboot
is read into location zero and
executed there,
it will type `=' on the console,
read in a
.it 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
.it 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 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 it is usually booted from disk.
.s3
The standard
DEC ROM which loads DECtape is sufficient to
read in
.it 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.
.s3
The system can also be booted from a disk file
with the aid of the
.it uboot
program.
When read into location 0 and executed,
.it uboot
reads a single character (either
.bd p
or
.bd 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.
.it Uboot
operates under very
severe space constraints.
It supplies no prompts, except that it
echoes a carriage return and line feed after the
.bd p
or
.bd 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, however,
it jumps back to its start,
so another try
can be attempted,
starting again with the
.bd p
or
.bd k.
Notice that
.it uboot
will only load a file from drive 0,
and the file system it searches must start
at the beginning of the disk.
.it Uboot
itself usually resides in the otherwise unused
block 0
of the disk,
so it can be loaded by ROM program;
.it mkfs
can be used to put it there when the file system is created.
It can also be loaded from a
.it tp
tape as described above.
.s3
.it "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.
.s3
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.
.s3
During the startup of the
system,
the
.it init
program (VIII)
reads the switches and
will come up single-user
if the switches are set to 173030.
.s3
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
(about 10\*us)
then a permanent disk non-existent-memory error will occur.
.s3
.it "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.
.s3
.ne 5
.nf
DECtape (drive 0) from endzone:
.if n .ta 3 11 15 23 38
.if t .ta .3i 1i 1.4i 2i 3.5i
	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	\fB.\fR	/ loop; now halt and start at 0
.s3
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
.s3
.fi
Caution: both of these DECtape programs will (literally) blow a fuse
if 2 drives are dialed to zero.
.s3
.nf
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	\fB.\fR	/ loop; now halt and start at 0
.s3
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
.s3
.ne 11
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
.dt
.sh FILES
/unix \*- UNIX code
.br
/usr/mdec/mboot \*- \fItp\fR magtape bootstrap
.br
/usr/mdec/tboot \*- \fItp\fR DECtape bootstrap
.br
/usr/mdec/uboot \*- file system bootstrap
.sh "SEE ALSO"
tp (I), init (VIII)