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PDP-10 Archives
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BB-PBQUC-BM_1990
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help/push.hlp
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PUSH command
The PUSH command creates a new level of TOPS-20 inferior to the one
from which you give the PUSH command.
Format
@PUSH (COMMAND LEVEL)
Characteristics
A New Level of TOPS-20
The PUSH command creates an inferior level of the TOPS-20
command processor (EXEC). The system's SYSTEM:COMAND.CMD
file and your login directory's COMAND.CMD file are executed
again, you have a fresh copy of memory and can begin giving
commands as if you had just logged in. However, job-wide
parameters (for example, connected and accessed directories,
logical name definitions, most parameters altered by SET
commands) are unaffected by the PUSH command and retain
their values.
Hints
Creating a Different Copy of an EXEC
You can use the PUSH command to create an inferior level of
an EXEC of your choice. Normally, PUSH creates the EXEC
defined by the system logical name, DEFAULT-EXEC:. Use the
DEFINE command to define a job logical name, DEFAULT-EXEC:,
with the name of the EXEC you want to create each time you
PUSH.
Note that many TOPS-20 programs have their own PUSH
commands. However, only the EXEC and OPR PUSH commands
refer to the job's definition of DEFAULT-EXEC:.
Using CONTINUE STAY With PUSH
You can use the PUSH command to run two programs at once or
to do other work that requires more than one copy of memory.
Simply use the CONTINUE /STAY or CONTINUE /BACKGROUND
command to continue execution of your current program before
using PUSH. After PUSH you can run another program or
otherwise alter memory without affecting memory for the
first program. See Example 2. But see also Warning, below.
Use of Multiforking Instead of PUSH
The PUSH command allows you to run several programs at once
by running the programs at different EXEC levels. The
EXEC's multiforking feature allows you to run multiple
programs at the same EXEC level. Working from a single EXEC
makes multiprogramming easier to monitor and manage. For
information on multiforking see the KEEP command or the
TOPS-20 User's Guide.
Restrictions
Number of Successive PUSH commands
You can give as many pairs of PUSH and POP commands as
necessary to complete your task. Although there is a limit
to the number of times you can give PUSH without giving
intervening POP commands, this limit is large enough
(approximately 24, although smaller for a heavily loaded
system) not to interfere with most applications. There is a
smaller limit (usually 5) on the number of EXECs that can
give Queue-class commands.
Use the INFORMATION SUPERIORS command to learn how many
superior EXEC levels you have created.
Invalid Definition of DEFAULT-EXEC:
If you define DEFAULT-EXEC: with the name of a nonexistent
EXEC, (if, for example, you make a spelling error in your
DEFINE command), the PUSH command ignores the job's
definition of DEFAULT-EXEC: and creates the EXEC defined by
the system definition.
Withheld Log-out Capability
You can usually log out from a lower level of TOPS-20 than
the one to which you logged in. By doing so, you
simultaneously conclude all processes of your job. However,
if a program (such as, PTYCON) has initialized a level of
the TOPS-20 command processor but has withheld log-out
capability from it, you must use the POP command, followed,
if necessary, by a program command to exit from the program
and return to a higher level of TOPS-20, before you can log
out.
Warning
Competition Between Processes
If you have two programs running at once after using
CONTINUE /STAY or CONTINUE /BACKGROUND and PUSH commands
(see Hints, above) they may try to access the same files at
the same time. Or, TOPS-20 commands given at the lower
level may be intercepted by a program running at the higher
level. For a discussion of these possibilities, see the
Restrictions section of the CONTINUE command description.
Effect on Memory and Terminal
The PUSH command preserves your present memory, gives you a fresh
copy of memory, and leaves your terminal at a new TOPS-20 command
level.
Related Commands
CONTINUE /STAY for beginning execution of a program
before giving the PUSH command
INFORMATION SUPERIORS for displaying the number of superior
EXEC levels.
POP for returning to a previous level of
TOPS-20
Examples
1. Give the PUSH command.
@PUSH
TOPS-20 Command processor 7(28)
2. Run a program, and give a CTRL/C to return to TOPS-20 command
level. Give a CONTINUE /STAY command to resume this
program's execution, and then a PUSH command for a new copy
of the TOPS-20 command language. Repeat this process twice;
now you have three programs running at once. In the lowest
(fourth) level of your job, begin editing a file. (Note:
when running more than one program in this way, be sure that
they do not use the same compiler or the same data base;
otherwise, competition among them could cause unpredictable
situations to develop.)
@RUN TESTF1
^C
@CONTINUE /STAY
@PUSH
TOPS-20 Command processor 7(28)
@RUN DMN
^C
@CONTINUE /STAY
@PUSH
TOPS-20 Command processor 7(28)
@RUN PRODUK
^C
@CONTINUE /STAY
@PUSH
TOPS-20 Command processor 7(28)
@EDIT ARTIFI.CTL
3. Define logical name DEFAULT-EXEC: with the name of a
specialized EXEC. Then, run that EXEC with the PUSH command.
@DEFINE DEFAULT-EXEC: SYSTEM:EXTENDED-EXEC.EXE.7
@PUSH
TOPS-20 Command processor 7(6530)