... "Id I .... II.) \Q 01:' c: t1 CD H td ... =- ... C3 =- " C3 W .... ..... t1 0 t+ c: UJ PI .... .. EO t+ t+ CD 'f • t'" CD 0 lit \Q ..... n \l1li' PI at P- I:' t1 PI CD (II I'd (II t1 0 1-3 t:r t1 ..... lit CD t:I • EO ..... t:=- Ilt CD t+ t+ .... CD 0 t1 t:I • .... t:I lit t+ .... 0 t:I • I C3 0 ..... c: • CD .... Virtual Address • LOCATE THE SEGMENT TABLE
Segment Table Register ICR 1) I Segme.nt Table I • Ongm • 8 25 26 31
31
Page Table • • LOCATE PAGE
TABLE • USE AS INDEX TO PAGE TABLE
ENTRY
Real Address Translate Virtual Addres' 00080424 to Real Address
/
( I I I I I I I I I I Virtual Address "..--;---+-----1 1 J • Locate the appropriate Segment Table entry . The eighth entry in
the Segment Table at location 014440 This entry points to the Page Table. ...- 014440 I I I I ;' II • Locate the appropriate Page Table entry -/
The 13th entry in the Page Table at _ __ I r-I-..I.-----r--'----. 012 Real Address location 014440. This entry contains ,''- ____ --'- _____ -' the real block number. / Block Displacement / • The block number in the Page __ J Table entry and the displacement in
Number
the Virtual Address combine to provide the Real Address

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 Vft/370 performance options in use by one or more virtual machines. I • The degree of access to ftSS 3330V volume. Perforllance of any virtual machine may be improved up by the choice of hardware, operating system, and Vft/370 topics discussed in this section address:
to some limit options. The
1. The performance options available in V!/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 2roblem state.
Any privileged instruction issued by the virtual machine causes a real
privileged instruction exception interruption. The 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. When planning for the virtual machine environment, consideration
should be given to the number and type of privileged instructions to be
executed by the virtual machines. Any reduction in the number of
privileged instructions issued by the virtual machine's operating system will reduce the amount of extra work CP must do to support the machine. CP Introduction 1-25