provide a commercial instruction set. Joining the
floating-point feature to the standard instruction set
provides a scientific instruction set, which in turn
can be augmented by the inclusion of the
extended-precision floating-point feature. If both
the decimal feature and the floating-point feature are
installed with the standard instruction set, a universal
instruction set is obtained. Adding other features,
such as the conditional-swapping feature, extends
the processing capabilities of the system still further.
Along with System/360, System/370 has the
capability of addressing a main storage of
16,777,216 bytes, and the System/370 translation
feature, used with appropriate programming support,
provides a user with up to this maximum address
space despite the attachment of a lesser amount of
real main storage. This feature and this support per­
mit a System/370 model with limited real main stor­
age to be used for a much wider set of applications,
and they make many applications with requirements
for extensive main storage more practical and con­
venient. Additionally, for many System/370 models,
the speed of accessing main storage is improved by
the use of a cache. The cache is a buffer--not appar­
ent to the user--that often provides information re­
quested from main storage without the delay associ­
ated with accessing main storage itself.
Another major aspect of the general-purpose de­
sign of System/370 is the capability provided to
attach a wide variety of I/O devices through several
types of multiplexing channels. Like System/360, System/370 has a byte-multiplexer channel for the
attachment of unbuffered devices and of a large
number of communications devices. Additionally, System/370 offers a block-multiplexer channel,
which is particularly well-suited for the attachment
of buffered devices and high-speed cyclic devices.
An individual System/370 installation is obtained
by selecting the system components best suited to
the applications from a wide variety of alternatives
in internal performance, functional ability, and
input/ output.
Compatibility
Although models of System/370 differ in implemen­
tation and physical capabilities, logically they are
upward and downward compatible. That is, within
the limitations of compatibility, as described below,
any program gives identical results on any model.
Compatibility allows for ease in systems backup, and
simplicity in education.
The compatibility rule has four limitations:
1. The systems facilities used by the program
should be the same in each case. For example,
the optional CPU features and the storage ca- 10 System/370 Principles of Operation
pacity, as well as the quantity and type of I/O equipment, should be equivalent.
2. The program should be independent of the
relation between instruction execution times, I/O data rates, access times, CCW execution
times, and elapsed time values.
3. The program should not depend on functions
that are identified in this manual as model­
dependent, on results that are defined to be unpredictable·, or on special-purpose functions
that are not described in this manual.
4. The program should not use or depend on un­
assigned fields unless they are explicitly made
available for program use. Additionally, the
program should not be designed to cause inter­
ruptions by means of format errors
9
such as the
use of invalid operation codes or invalid com­
mand codes. System/370 is forward compatible from System/360, and System/360 programs that are to
be run on System/370 must observe both the pre­
ceding limitations and the following three limita­
tions:
1. The program must not use PSW bit 12 as an
ASCII bit, which is a function that is provided
only for System/360. 2. The program must not depend on main-storage
locations assigned specifically for System/370, such as the interruption-code areas, the
machine-check save areas, and the extended­
logout area.
3. The program associated with input/output
operations must take into account the effects
of channel prefetching, command retry, logout
on channel data check, and the operation-code
assignment for HALT DEVICE.
System Program
The system is designed to operate with a supervisory
program that coordinates the use of system resources
and executes all I/O instructions, handles exception­
al conditions\ and supervises scheduling and execu­
tion of multiple programs. System/370 can operate with several different
types of supervisory programs. Some of these pro­
grams provide support for the new System/370 in­
structions, for the advanced reliability, availability,
and serviceability fea,tures, and for the new I/O capabilities. Additionally, some of these programs
provide for system and application programs to op­
erate in a virtual-storage environment.
System/3 7 0 can also operate in the mode of System/360 and run all of the supervisory and appli-
cation programs written for System/360 that satisfy
the conditions described in "Compatibility." Availability
Availability is the capability of a system to accept
and successfully process an individual job. System/370 machine facilities permit increased
availability by (1) allowing a larger number and a
broader range of jobs to be processed concurrently,
thus making the system more readily accessible to
any particular job, and (2) limiting the effect of an
error and identifying more precisely its cause, with
the result that the number of jobs affected by errors
is minimized and the correction of the errors is facili­
tated.
Several design aspects make this possible. A program is checked for the correctness of
instructions and data as the program is execut­
ed, and program errors are indicated separately
from equipment errors. Such checking and re­
porting assists in locating failures and isolating
effects. The protection facility, in conjunction with dy­
namic address translation, permits the protec­
tion of the contents of main storage from de­
struction or misuse caused by erroneous or
unauthorized storing or fetching by a program.
This provides increased security for the user,
thus permitting applications with different se­
curity requirements to be processed concur­
rently with other applications. Dynamic address translation allows isolation of
one application from another, still permitting
them to share common resources. Also, it per­
mits the implementation of virtual machines,
which may be used in the design and testing of
new versions of operating systems along with
the concurrent processing of application pro-
grams. Additionally, it provides for the concur­
rent operation of incompatible operating sys­
tems. Multiprocessing permits better use of storage
and processing capabilities, more efficient
communication between CPUs, and duplica­
tion of resources, thus aiding in the continua­
tion of system operation in the event of ma­
chine failures. Monitoring, program-event recording, and the
high-resolution timing facilities permit the test­
ing and debugging of programs without manual
intervention and with little effect on the con­
current processing of other programs. Emulation is performed under supervisory pro­
gram control, thus making it possible to per­
form emulation concurrently with other appli­
cations. On most models, error checking and correction
(ECC) in main storage, instruction retry, and
command retry provide for circumventing in­
termittent equipment malfunctions, thus reduc­
ing the number of equipment failures . An enhanced machine-check handling mecha­
nism provides model-independent fault isola­
tion, which reduces the number of programs
impacted by uncorrected errors. Additionally,
it provides model-independent recording of
machine-status information. This leads to
greater machine-check handling compatibility
between models and improves the capability
for loading and running a program on a differ­
ent model when a system failure occurs. A small number of manual controls are required
for basic system operation, permitting most
operator-system interaction to take place via a
unit operating as an I/O device and thus reduc­
ing the possibility of accidental operator errors.
IBM System/370 11
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