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 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.  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 executed, 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, however, 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 system 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.  "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 (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 beginning of a device into core location zero.  The octal words constituting the program are listed on the left. 

DECtape (drive 0) from endzone:
012700mov$tcba,r0
177346
010040movr0,−(r0)  / use tc addr for wc
012710mov$3,(r0)/ read bn forward
000003
1057101:tstb(r0)/ wait for ready
002376bge1b
112710movb$5,(r0)/ read (forward)
000005
000777br./ loop; now halt and start at 0
DECtape (drive 0) with search:
0127001:mov$tcba,r0
177346
010040movr0,−(r0)  / use tc addr for wc
012740mov$4003,−(r0)/ read bn reverse
004003
0057102:tst(r0)
002376bge2b/ wait for error
005760tst−2(r0)/ loop if not end zone
177776
002365bge1b
012710mov$3,(r0)/ read bn forward
000003
1057102:tstb(r0)/ wait for ready
002376bge2b
112710movb$5,(r0)/ read (forward)
000005
1057102:tstb(r0)/ wait for ready
002376bge2b
005007clrpc/ 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:
012700mov$mtcma,r0
172526
010040movr0,−(r0)/ usr mt addr for wc
012740mov$60003,−(r0)/ read 9−track
060003
000777br./ loop; now halt and start at 0
RK (drive 0):
012700mov$rkda,r0
177412
005040clr−(r0)/ rkda cleared by start
010040movr0,−(r0)
012740mov$5,−(r0)
000005
1057101:tstb(r0)
002376bge1b
005007clrpc
RP (drive 0)
012700mov$rpmr,r0
176726
005040clr−(r0)
005040clr−(r0)
005040clr−(r0)
010040movr0,−(r0)
012740mov$5,−(r0)
000005
1057101:tstb(r0)
002376bge1b
005007clrpc

FILES

/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 (VIII)