Force Start
CP Commands
A force start is required when checkpoint start encounters I/O errors while reading
files, or invalid data. The procedure is the same as for checkpoint start except that
unreadable or invalid files are bypassed.
The CP commands allow you to control the virtual machine from the terminal,
much as an operator controls a real machine. Virtual machine execution can be
stopped at any time by using the 3066 terminal's attention key or the 3270 termi­
nal's ENTER or PAl key. Execution can be restarted by entering the appropriate CP command. External, attention, and device ready interrupts can be simulated on
the virtual machine. Virtual storage and virtual machine registers can be inspected
and modified, as can status words such as the PSW and the CSW. Extensive trace
facilities are provided for the virtual machine, as well a single-instruction mode.
Commands are available to invoke the spooling and disk sharing functions of CP. CP commands are classified by privilege classes. The VM/SP directory entry for
each user assigns one or more privilege classes. The classes are primary system
operator (class A), system resource operator (class B), system programmer (class
C), spooling operator (class D), system analyst (class E), service representative
(class F), and general user (class G). Commands in the system analyst class may
be used to inspect real storage locations, but may not be used to make modifica­
tions to real storage. Commands in the operator class provide real resource control
capabilities. System operator commands include all commands related to virtual
machine performance options, such as assigning a set of reserved page frames to a
selected virtual machine. For descriptions of all the CP commands, see the VM / SP CP Command Reference for General Users and the VM / SP Operator's Guide.
VM/SP 13

Program States When instructions in the Control Program are being executed, the real computer is
in the supervisor state; at all other times, when running virtual machines, the real
computer is in the problem state. Therefore, privileged instructions cannot be exe­
cuted by the virtual machine. Programs running on a virtual machine can issue
privileged instructions; but such an instruction either (1) causes an interruption that
is handled by the Control Program, or (2) is intercepted and handled by the
processor, if the virtual machine assist feature or VM/370 Extended
Control-Program Support is enabled and supports that instruction. CP examines
the operating status of the virtual machine PSW. If the virtual machine indicates
that it is functioning in supervisor mode, the privileged instruction is simulated
according to its type. If the virtual machine is in problem mode, the privileged
interrupt is reflected to the virtual machine. Only the Control Program may operate in the supervisor state on the real machine.
All programs other than CP operate in the problem state on the real machine. All
user interrupts, including those caused by attempted privileged operations, are han­
dled by either the control program or the processor (if the virtual machine assist
feature or VM/370 Extended Control-Program Support is available). Only those
interrupts that the user program would expect from a real machine are reflected to
it. A problem' program executes on the virtual machine in a manner identical to its
execution on a real System/370 processor, as long as the problem program does
not violate the CP restrictions. CP restrictions are documented in the VM / SP Planning Guide and Reference.
14 VM/SP System Programmer's Guide