TOPS-20 Commands Reference Manual

home PDP-10 TOPS-20 Commands Reference Manual Top: Chapter 2 Command description

PUSH

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

Give the PUSH command.
```
@PUSH
 TOPS-20 Command processor 7(28)
```
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
```
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)
```