Using multi-tasking at the command line level Three extra commands have been provided to assist in multitasking. These are kill , which stops a named process. The process status is given by ps , which lists all current processes. The wait command is provided to synchronise the com- pletion of commands. One extra control sequence is , which returns control to the keyboard. There are three new special characters. The & character after a command puts that programintoasynchronous, orbackground, mode. The pipecommand isproduced by placing | between two or more commands, to feed the output from the first into the input of the next.
In assign only, the \ backslash character makes the path that follows literal. Alt Ctrl C There are additional error codes and messages relating to the multitasking com- mands. Killing programs Typing sends a signal to the current synchronous (foreground) process, which should then exit with a negative exit code. If all your programs are behaving correctly, should always return control to the command line prompt. Alt Ctrl C Alt Ctrl C Running programs asynchronously You may run any 1616/OS command in the background, as an asynchronous task, by adding an & symbol at the end of the command line. The system will print out the process’s PID, and start it running.
If the process is going to produce output which you want to see, it is best to direct it to a file for later inspection. If you don’t want to see the output, direct it onto the NULL: device. Examples: Typing SSASM myfile.s -l >myfile.lst }myfile.errs & will start up the 1616 assembler, assembling myfile.s . The listing will go to myfile.lst , any error reports will go to myfile.errs and the entire assembly proceeds in the background, permitting you to edit something else. typing type myfile.lst >cent: & will send the file to the printer, in the background, so you can use the machine while it prints. Simple as that.
If a process is started asynchronously, and its standard input is not redirected with the mechanism, the system will connect standard input to the NULL: character device, which always returns end-of-file (error code -18). This ensures that a background process cannot compete with the foreground one for input. Page 58 User Tutorial Manual Multi Tasking