SVC Interrupt When an SVC interrupt occurs, the SVC interrupt routine is entered. If
the machine is in problem mode, the type of interrupt (if it is other
than an SVC 76 or ADSTOP SVC) is reflected to the pseudo-supervisor
(that is, the supervisor operating in user's virtual machine). Control is transferred to the appropriate interrupt handler for IDSTOP SVCs and all SVC 76s.
If the machine
determined, and a
handler.
is in supervisor mode,
branch is taken to the
External Interrupt
the SVC interrupt code is aPFropriate SVC interrupt
If a timer interrupt occurs, CP processes it according to type. The
interval timer indicates time slice end for the running user. The clock
comparator indicates that a specified timer event occurred, such as
midnight, scheduled shutdown, or user event reached.
The external console interrupt invokes CP processing to switch from
the 3210 or 3215 to an alternate operator's console.
Synchronous Interrupts In an Attached Processor System Generally, when synchronous interrupts (such as program and SVC interrupts) occur in an attached processor system, the first-level
interrupt handler (FLIH) atte.pts to gain the system lock before
proceeding. If it is already in use, the interrupt status is stacked
and deferred. The interrupted processor then attempts to run a user. Real I/O Interrupts
In an attached processor configuration. only the main processor can
receive real 1/0 interrupts. To ensure this, the channel masks in
control register 2 on the main processor are initialized to ones to
enable for interrupts fro. any available channel. On the attached
processor, the channel masks in control register 2 are initialized to
zeros.
Part 2. Control program (CP) 87

Performance Guidelines General Information
The performance characteristics of an operating system, when it is run
in a virtual machine environment, are difficult to predict. This
unpredictability is a result of several factors: • The System/370 model used. • The total number of virtual machines executing. • The type of work being done by each virtual machine. • The speed, capacity, and number of the paging devices. • The amount of real storage available. • The degree of channel and control unit contention, as well as arm
contention, affecting the paging device. • The type and number of VM/370 performance options in use by one or
more virtual machines. I • The degree of MSS 3330 volume use.
Performance of any virtual machine may be improved up
by the choice of hardware, operating system, and VM/370 topics discussed in this section address:
to some limit
options. The
1. The performance options available in VM/370 to improve the
performance of a particular virtual machine.
2. The system options and operational characteristics of operating
systems running in virtual machines that will affect their
execution in the virtual machine environment.
The performance of a specific virtual machine may never equal that of
the same operating system running standalone on the same System/370, but
the total throughput obtained in the virtual machine environment may
equal or better that obtained on a real machine.
When executing in a virtual machine, any function that cannot be
performed wholly by the hardware causes some degree of degradation in
the virtual machine's performance. As the control program for the real
machine, CP initially processes all real interrupts. A virtual machine
operating system's instructions are always executed in state.
Any privileged instruction issued by the virtual machine causes a real
privileged instruction exception interruption. 1he amount of work to be
done by CP to analyze and handle a virtual machine-initiated interrupt
depends upon the type and complexity of the interrupt.
The simulation effort required of CP may be trivial, as for a
supervisor call (SVC) interrupt (which is generally reflected back to
the virtual machine), or may be more complex, as in the case of a Start I/O (SIO) interrupt, which initiates extensive CP processing.
88 IBM VM/370 System Programmer's Guide