DTBOOT.MEM

PROGRAMMING SPECIFICATIONS
 DTBOOT
 PDP-10 DECTAPE BOOTSTRAP
 Date:  10-Mar-71
 File:  DTBOOT.RNO
 Edition:  2
 This document  describes  the  software  as  of
 Version 2.
 The information in this document is subject  to
 change   without   notice  and  should  not  be
 construed as a commitment by Digital  Equipment
 Corporation.
 Actual distribution of the  software  described
 in  this specification will be subject to terms
 and conditions to be announced at  some  future
 date by Digital Equipment Corporation.

TABLE OF CONTENTS
Page
1.0     Purpose of DTBOOT                      3
2.0     Commands to DTBOOT                     3
3.0     Tape Format                            5
4.0     Comparison with TENDMP                 6
5.0     Compatibility with TOPS10 Monitor      6
6.0     Assembly and READIN of DTBOOT          7
Appendix - DTBOOT Summary for Operators        8

1.0  PURPOSE OF DTBOOT
DTBOOT is a bootstrap program intended to transfer  programs
between  PDP-10  core  storage  and  standard  format PDP-10
DECtape files, and to perform other utility  functions  with
such DECtapes.
DTBOOT requires a PDP-10 central processor and core  memory,
the PDP-10's console typewriter, a TD10 DECtape control, and
at least one DECtape transport.  DTBOOT can be assembled for
any memory size.
DTBOOT is primarily intended to load and run executive  mode
programs   (monitors,   diagnostics)   from   DECtape.   The
following functions are available:
1.  Clear core, load a program, and start it.
2.  Load a program without clearing other core areas.
3.  Clear core, load a program, but do not start it.
4.  Type out a short form of the DECtape's directory.
5.  Delete a file from the DECtape.
6.  Initialize  an   empty   tape        directory,   either
retaining  its  existing  tape  identifier  word or
supplying a new one.
6.  Declare a starting address to be associated with  a
program to be dumped or started.
8.  Dump core onto a DECtape file.
9.  Go to the current starting address.
A simple command string, with handy defaults,  is
typed  by the operator to control DTBOOT.

2.0  COMMANDS TO DTBOOT
The commands for DTBOOT are best described  by  showing  the
longest form and then showing the simplifications provided.
The full command to load the file  TOPS10.SAV  from  DECtape
unit DTA3 is:
DTA3:TOPS10.SAV/L
The command terminator may be carriage return, or any  other
control character or ALTMODE.
The unit specifier DTA3:   is  actually  examined  only  for
numeric  characters,  so it can be abbreviated to simply 3:.
If a series of commands is  performed  addressing  the  same
unit,  the  unit  specifier  can  be  omitted  on successive
commands (unless an  error  has  occurred,  causing  a  BELL
typeout).
The file extension is assumed to be  .SAV  unless  specified
otherwise,  and  can  thus  be omitted in the usual case.  A
blank extension can be specified by typing just the  period;
e.g., 3:NAME./L.
The filename can be omitted,  and  will  then  be  defaulted
according to the following rules:
1.  For the tape-writing command, the file  name  CRASH
is assumed.
2.  For tape reading, if exactly one file exists on the
tape  with  the extension .SAV, that file's name is
assumed.   Otherwise,   the  file  name  SYSTEM   is
assumed.
The particular action to be performed by DTBOOT is  selected
by  a switch, which is an octal number or single letter that
follows a slash (/) at the end of the command.  In the first
example above, the L is the command switch, meaning Load the
program into core from tape.
The most common function of DTBOOT is to load  and  start  a
program  (e.g.,  the timesharing system), and this action is
the default if no switch is typed at all.
The non-blank switches, and the actions they cause,  are  as
follows:
/L  Clear core, load the program from tape, and set the
starting address from the tape.
/M  Same as /L, but do  not  clear  core  first,  i.e.,
merge the file into core.
/n  (where n is  an  octal  number)  Set  the  starting
address to n.
/D  Dump  core  on  tape  with  the  current   starting
address.
/F  Type the file directory.
/K  Kill (delete) the specified file from the tape.
/Z  Zero the directory of the tape.  To  set  the       tape
identifier   to   TAPE99,   for   instance,     type
3:^TAPE99/Z.     The  uparrow  precedes   the   tape
identifier, as in FILEX.
Obviously, not all of these commands require all  arguments.
For  example,  a  filename  would  be  ignored  with  the /Z
command.

3.0  TAPE FORMAT
DECtapes read and written by DTBOOT are entirely  compatible
with  the  TOPS10 monitor system, and are fully described in
the TOPS10 documentation.  This section  is  provided  as  a
summary of that material.
DECtape directory information occupies block 144 (octal)  of
the tape.  Words 0 through 122 contain 1105 bytes describing
the allocation of blocks 1 through 1101 of the tape.  Blocks
1, 2, and 144 are allocated as overhead, blocks 1102 through
1105 are allocated as non-existent.   The  remaining  blocks
contain  file  information (or are free).  Words 123 through
150 of the directory contain filenames.  Words  151  through
176  contain  the  corresponding  file extensions and dates.
(Date fields of files written by DTBOOT are zero.) Word  177
contains the (optional) tape identifier.
In each data block, word zero is the link word, allocated as
follows:   bits 00-17 are the link to the next data block of
the file (zero denoting no next block);  bits 18-27  contain
the  block  number of the first data block of this file, and
bits 29-35 contain a word count of the  data  words  in  the
remainder of this block.
A saved core  image,  as  written  by  DTBOOT  (or  TOPS10),
consists  of  blocks  of  data  preceded  by  a pointer word
(minus-count in the left half, address-1 in the right half).
A positive word in the place of a pointer word signifies the
end of the core-image.  The right half of this word  is  the
starting  address  of  the  saved  program.   The  left half
usually contains the machine instruction JRST which  is,  in
fact, a positive number.  This is the method DTBOOT uses.
Blocks of a given file may be in  any  order  on  the  tape.
When written by DTBOOT (and the SAVE command of TOPS10), the
allocation  is  as  follows:   The  lowest  numbered   block
available   for   use  is  allocated  first.   (This  was  a
requirement of TENDMP, but is optional for DTBOOT.)
Succeeding  blocks  are  allocated   with   at   least   one
intervening  block  (whether free or otherwise used) between
successive  blocks  of  a  file.   If  required,  the   same
procedure   is  used  to  allocate  blocks  in  the  reverse
direction, starting at block 1101, should the file  be  long
enough to reach that end of the tape, and any skipped blocks
become used on the  reverse  pass.   DTBOOT  is  capable  of
efficiently  reading  and  writing  files  allocated in this
manner.

4.0  COMPARISON WITH TENDMP
It is intended  that  DTBOOT  supersede  TENDMP.   The  only
advantages  TENDMP  holds  over  DTBOOT  are  that TENDMP is
smaller than DTBOOT, and that TENDMP  may  be  called  as  a
subroutine,  without  typed  commands.   DTBOOT  lacks  this
facility.  TENDMP will remain available for the  rare  cases
where these advantages require its use.
The  advantages  DTBOOT  holds  over  TENDMP   include   the
following:
1.  First and most importantly,  the  ability  to       read
.SAV  files written by TOPS10 which are long enough
to "turn around", i.e., which   have  allocation  in
both forward and backward direction.
2.  Writing files which  can  be  read  efficiently  by
TOPS10,   without  frequent  tape  rocking  due  to
allocation of consecutive blocks  by  TENDMP.    (In
fact, DTBOOT has the same difficulty reading TENDMP
files as TOPS10 does.)
3.  More  complete  DECtape  control   handling;         the
ability to turn around from end-zone flags.
4.  More nearly standard command strings.
5.  Useful defaults.
6.  Ability to set tape-identifier words.
5.0  COMPATIBILITY WITH TOPS10 MONITOR
There are no known incompatibilities with TOPS10, aside from
the failure of DTBOOT to provide dates on output files.
6.0  ASSEMBLY AND READIN OF DTBOOT
As with TENDMP and BOOTS, DTBOOT is  assembled  by  MACRO-10
from a single source file onto a paper tape.
DTBOOT resides at the top of user core,  and  thus  must  be
assembled  for the proper memory size.  The default is for a
64K system.  The parameter CORE can be set to the number  of
words of memory to produce another size of DTBOOT.
To assemble 64K DTBOOT, type the MACRO command string:
*PTP:_dev:DTBOOT
where dev is  the  device  containing  the  source  file  of
DTBOOT.
To assemble 48K DTBOOT, for example, type:
*PTP:_TTY:,dev:DTBOOT
CORE=140000       ;ASSEMBLE FOR 48K
^Z
^Z
DTBOOT is in RIM10B format when on paper tape, and is loaded
via READIN at the PDP-10 console.
The starting address of DTBOOT is 1000 octal less than CORE.
Thus, 64K DTBOOT starts at 177000.
There are three blocks (0, 1, 2) available for  a  bootstrap
at  the  beginning  of  a  PDP-10  DECtape.  These 600 octal
words, minus a readin pointer and a start  word,  leave  576
octal  for the bootstrap.  At present (version 2 of DTBOOT),
DTBOOT is 575 words long, and is intended to  be  stored  on
DECtape  in this area.  As yet, the means for writing DTBOOT
onto a DECtape's bootstrap area are not yet available.

APPENDIX  DTBOOT SUMMARY FOR OPERATORS
DTBOOT is loaded from paper tape via the  READIN  switch  on
the  PDP-10.   DTBOOT  starts  at  77000(32K),  137000(48K),
177000(64K), 237000(80K), etc.
COMMANDS
DTA3:name.ext/L or 3:name.ext/L
SWITCHES
none      = load and start program
/L        = load, set starting address, do NOT start
/M        = merge (load without clearing core first)
/number   = set starting address to number (octal)
/D        = dump core and starting address on tape
/F        = type file directory
/K        = kill (delete) file
/G        = go to starting address
/Z        = zero the tape directory (3:tapeid/Z sets ID)
DEFAULTS
dev:, initially DTA0:. Otherwise, same as on previous
command (unless the bell was rung).
name=CRASH for /D. Otherwise, if only one .SAV file on
tape, use its name. If more than one .SAV file, use
SYSTEM, unless specified.
ext = .SAV unless specified.
ERRORS
The bell will be rung if any error occurs. Possible errors
are: tape unit not in
remote, not exactly one unit dialed as
dev:, parity error on tape, requested file not in directory
(try /F), or tape filled up while writing out (/D).