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                        TOPS-20 Beware File


                              7-Feb-80


                            VERSION 4(5)


                             Release 4





COPYRIGHT (C) 1976,1977,1978,1979,1980 BY DIGITAL  EQUIPMENT
CORPORATION, MAYNARD, MASS.


THIS SOFTWARE IS FURNISHED UNDER A LICENSE AND MAY  BE  USED
AND COPIED ONLY IN ACCORDANCE WITH THE TERMS OF SUCH LICENSE
AND WITH THE INCLUSION OF THE ABOVE COPYRIGHT NOTICE.   THIS
SOFTWARE  OR ANY OTHER COPIES THEREOF MAY NOT BE PROVIDED OR
OTHERWISE MADE AVAILABLE TO ANY OTHER PERSON.  NO  TITLE  TO
AND OWNERSHIP OF THE SOFTWARE IS HEREBY TRANSFERRED.

THE INFORMATION  IN  THIS  SOFTWARE  IS  SUBJECT  TO  CHANGE
WITHOUT  NOTICE  AND SHOULD NOT BE CONSTRUED AS A COMMITMENT
BY DIGITAL EQUIPMENT CORPORATION.

DIGITAL ASSUMES NO RESPONSIBILITY FOR THE USE OR RELIABILITY
OF  ITS  SOFTWARE  ON  EQUIPMENT  WHICH  IS  NOT SUPPLIED BY
DIGITAL.

                                                      Page 2





1.0 INTRODUCTION

1.1 Function

This document supplements the TOPS-20  manuals  by  pointing
out  hints  and  potential  problems  discovered at the last
minute.



2.0 TOPS-20 MONITOR AND UTILITIES

2.1 File and Directory Limitations

Due to the increased size  of  the  file  descriptor  blocks
(FDBs)  in Release 4, the following numbers will be somewhat
smaller than their Release 3A values:

     1.  The number of files in a directory

     2.  The number of subdirectories within a directory

     3.  The number of top-level directories in the system



2.2 Programs Running Under SYSJOB

The batch commands  in  PTYCON.ATO  have  been  replaced  by
commands  in  SYSJOB.RUN  and  SYSTEM.CMD.  The programs are
invoked by commands in SYSJOB.RUN, and the commands  to  the
programs  are  in  SYSTEM.CMD,  which  is  read by OPR.  The
defaults are 3 batch streams, 2 line printers,  and  1  card
reader.  If you wish to remove 1 line printer, delete one of
the  "RUN  SYS:LPTSPL"  lines  in  SYSJOB.RUN,  and   change
SYSTEM.CMD  to  read  "START PRINTER 0".  To remove the card
reader, delete from SYSJOB.RUN  "RUN  SYS:SPRINT"  and  "RUN
SYS:CDRIVE", and delete the line in SYSTEM.CMD "START READER
0".



2.3 EXEC Commands

The ^ESET STRUCTURE  commands  have  been  replaced  by  OPR
commands.  (OPR runs under PTYCON.)

The SREMOVE command is replaced by  the  DISMOUNT  STRUCTURE
name:/REMOVE  command.   The  TMOUNT,  SMOUNT, and SDISMOUNT
commands are in Release 4 for compatibility only.  They have
been  replaced  by MOUNT TAPE, MOUNT STRUCTURE, and DISMOUNT
STRUCTURE, respectively, and will be  removed  in  the  next

                                                      Page 3


release.   The  TMOUNT  and  SMOUNT  commands  will behave a
little differently under Release 4.  The TMOUNT command will
work  only  if  the  ENABLE TAPE-DRIVE-ALLOCATION command is
included in the 4-CONFIG.CMD file.



2.4 2020 Card Reader

There is a problem in the  processing  of  the  hopper-empty
condition  by card readers on 2020 systems.  The data on the
last card in the hopper  is  not  processed.   This  can  be
circumvented with the following technique:

For each hopperful of data, append a blank card to the cards
in  the hopper.  This will cause the data on the former last
card to be processed.  The blank card will be passed by  the
reader, but its contents ignored.  Note that this applies to
every physical hopperful of cards, not to logical "decks".



2.5 DDT

User-mode DDT (UDDT) now occupies pages 766 through  777  of
the user address space.  Programs that rely upon using pages
766 and 767 may possibly not operate  correctly  if  DDT  is
merged with them.



2.6 PTY Handling

Prior to Release 4, if a program did a BOUT or a SOUT  to  a
PTY  and  the  PTY's  input  buffer  was  full,  one or more
characters would be lost and a bell would be echoed  in  the
output  stream.  In Release 4, a program trying to output to
a PTY with a full  input  buffer  will  receive  an  illegal
instruction trap with error code IOX33.  The controlling job
can handle the error condition  by  employing  an  ERJMP  or
ERCAL following the BOUT or SOUT.



2.7 Split-speed Terminal Lines

Terminals that send XOFF and XON characters automatically to
prevent  the  system from overflowing their internal buffers
(VT100 for example) may encounter difficulty  if  the  input
baud  rate is set slower than the output baud rate.  This is
because the terminal will not  be  able  to  send  the  XOFF
character   quickly   enough   to   stop   the  system  from
transmitting characters.

                                                      Page 4


2.8 Execute-only Programs

Files that are protected as execute-only cannot be  accessed
under  3A.  Therefore, if you do drop back to 3A, be sure to
reprotect any files that were set to be  execute-only  under
Release 4.

More information is provided in chapter 2 of the  release  4
TOPS20 Monitor Calls Reference Manual.



2.9 DFIN and DFOUT

The DFIN and DFOUT monitor calls now  use  the  normal  KL10
double-precision  floating point format numbers as described
in the System Reference Manual.  The KA10  double  precision
format  mentioned  in  the Monitor Calls Manual is no longer
used.



2.10 The PLEASE Command

The only supported mechanism for invoking the PLEASE program
is  via  the  PLEASE command.  In particular R PLEASE or GET
SYS:PLEASE are not valid and  the  PLEASE  program  may  not
operate correctly if invoked in these ways.


2.11 SETSPD

If  the  POLICY-PROGRAM  clause  is  used  in   the   ENABLE
CLASS-SCHEDULING  command  in  the 4-CONFIG.CMD file, SETSPD
will not process it correctly.  If only the word  POLICY  is
used, the intended result will occur.



3.0 DECNET

Please refer to the file DECNET.BWR on the DECNET  tape  for
"beware" information regarding DECNET.



4.0 BUILDING PROGRAMS FROM SOURCES

4.1 Building the TOPS-20 Monitor

The TOPS-20 monitor must be built using the MACRO  and  LINK
that  are  distributed with Release 4.  Note that in Release
4,  no  PSECT  overlaps  are  permitted  when  building  the
monitor.

                                                      Page 5


4.2 Building RUNOFF

Building RUNOFF generates a copy of SCNMAC.UNV that  is  not
the  correct  version for building other software.  To avoid
any problems, build RUNOFF in a separate directory.



5.0 FRONT-END

5.1 Hardware Field Change Order (FCO)

Because of extensive changes to the terminal  handling  code
in  Release  4 of TOPS-20, FCO 20FE-R-0001 must be installed
in all  2040,  2050,  and  2060  processors  before  running
Release 4.  This is a required FCO, released February, 1979.

The FCO places a DB11-A bus repeater  in  the  first  system
unit  slot  of  the  expansion box in the front end.  Please
check and be sure that the FCO is installed and a DB11-A  is
already in the first system unit.

If  you  require  this  FCO,  contact  your   Regional   FCO
Coordinator immediately (EQ-00920-00).

PLEASE NOTE:  TOPS-20 WILL NOT RUN ON A 2040, 2050, OR  2060
PROCESSOR WITHOUT THIS FIELD CHANGE ORDER.




5.2 Front-End Versions

It is recommended that Release 3A of RSX-20F  be  used  with
Release 3A of TOPS-20, and that Release 4 of RSX-20F be used
with Release 4 of TOPS-20.



6.0 DOCUMENTATION

6.1 WATCH

The data record output feature of WATCH is not supported  in
the  version  of WATCH being released with TOPS20 Release 4.
Note that the record formats are reflected in the new  WATCH
document in the Software Notebooks.  The documentation is to
be used for information only until such time as the  version
of WATCH that supports this feature is released.



6.2 USAGE File Specification

The controller field in the tape mount record is defined  to

                                                      Page 6


be  2  characters.  It is actually 3 characters, which means
all fields after the controller field should be  shifted  by
one.  The total length of the record is 149 characters.