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
cbeckpoint start except that unreadable or invalid files are bypassed. CP 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 use of the terminal's attention
key (for 3066 and 3270 terminals, the ENTER key is used); it 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 PSi and the CSW. Extensive
trace facilities are provided for the virtual machine, as well as 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 V8/370 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 modifications 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 !£f and the 82 IBM VM/370 System Programmer's Guide

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 executed by the virtual machine. Programs running on a virtual machine can issue Frivileged 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 VK/370 Extended
Control-program Support is enabled and supports that instruction. CP examines the operating status of the virtual machine PSi. If the
virtual machine indicates that it is functioning in supervisor mode, the
privileged instruction is simulated according to its type. II 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 handled by either the control
program or the processor (if the virtual machine assist feature or VK/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
l as
long as the problem program does not violate the CP restrictions. CP restrictions are documented in the Part 2. Control Program (CP) 83