March 30, 1979 Whether you have to press the Attention key once or twice depends on
the terminal mode setting in effect for your virtual machine. This
setting is controlled by the CP TERMINAL command: cp terminal mode va The setting VM is the default for virtual machines; you do not need to
specify it. The VM setting indicates that one depression of the
Attention key sends an interruption to your virtual machine, and that
two depressions results in an interruption to the control program (CP). The CP setting for terminal mode, which is the default for the system operator, indicates that one depression of the Attention key results in
an interruption to the control program (CP). If you are using your
virtual machine to run an operating system otber than CMS, you might wish to use tbis setting. Issue tbe command:
cp terminal mode cp VIRTUAL MACHINE INTERRUPTIONS Wbile a com.and or program is executing, if you press the Attention key once on a 2741 (or the Enter key on a 3270), you have created a virtual machine interruption. The program baIts execution, your terminal will
accept an input line, and you may: • Terminate the execution of the program by issuing an I.mediate
command to halt execution:
hx
The HX command causes the program to abnormally terminate (abend). • Enter a CMS command. The co.mand is stacked in a console buffer and
is processed by CMS when your program is finished executing and the
next virtual machine read occurs. For example:
print abc listing
After you enter this line, tbe program resumes execution. • If the program is directing output to your terminal and you wish only
to halt tbe terminal display, use the Immediate command:
ht
The program resumes execution. Terminal output can also be
suppressed immediately when you enter a command by placing 'HT at the
end of the command line. The logical line end character (I) allows
the Immediate command HT to be accepted; program execution proceeds
without typing. You can, if you want, cause another interruption and request that
typing be resumed by entering the RT (resume typing) command:
rt • Enter a null line; your program continues execution. The null line is
stacked in the console stack and read by as a stacked command
line.
22 IBM VM/370 CMS User's Guide

April 1, 1981 HX, RT, and RT are three of the CMS Immediate commands. They are
"immediate" because they are executed as soon as they are entered. Unlike other commands, they are not stacked in the console buffer. You can only enter an Immediate command following an attention interruption. CONTROL PROGRAM INTERRUPTIONS You can interrupt a program and enter the CP environment directly by the Attention key twice quickly, on a 2741, or pressing the PA1
key on a 3270. Then, you can enter any CP command. To resume the
proqram's execution, issue the CP command:
cp begin
If vour terminal is operating with the terminal mode set to CP, pressing
the Attention key once places your virtual machine in the CP environment. ADDRESS STOPS AND BREAKPOINTS A program may also be interrupted
you set by the CP issue the command:
by an instruction address stop, which
command ADSTOP. For example, if you
cp adstop 201ea an address stop is set at virtual storage location X'201EA'. When your
proqram reaches this address during its execution, it is interrupted 3nd your virtual machine is placed in the CP environment, where you can
issue any CP command, including another ADSTOP command, before resuming your proqram's execution with the CP command BEGIN.
Breakpoints, similar to address
subcommand which you issue
executing a proqram. For example, if
break 1 201ae stops, are set using the
in the debug environment
you issue: DEBUG before Your program's execution is interrupted at this address and your virtual
machine is placed in the debug environment. You can then enter any DEBUG slbcommand. To resume program execution, use DEBUG subcommand GO. If von want to halt execution of the program entirely, use the DEBUG 5ubcommand HX, which returns your virtual Machine to the :MS environment. You can find more information about settinq address stops
and breakpoints in "Section 11. How VM/310 Can Help You Debug Your Proqrams." Section 2. VM/370 Environments and Mode Switching 23