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 asthe conditional-swapping feature, extends
the processing capabilities of the system still further.
Along withSystem/360, System/370 has the
capability of addressing a main storage of
16,777,216 bytes, and theSystem/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 aSystem/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 manySystem/370 models,
the speed of accessing main storage is improved by
the use of a cache. The cache is a buffer--not appar
ent tothe 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 ofSystem/370 is the capability provided to
attach a wide variety ofI/O devices through several
types of multiplexing channels. LikeSystem/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 individualSystem/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 ofSystem/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 optionalCPU features and the storage ca- 10 System/370 Principles of Operation
pacity, as well as the quantity and type ofI/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 beunpredictable·, 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 formaterrors
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 onSystem/370 must observe both the pre
ceding limitations and the following three limita
tions:
1. The program must notuse PSW bit 12 as an
ASCII bit, which is a function that is provided
only forSystem/360. 2. The program must not depend on main-storage
locations assigned specifically forSystem/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 allI/O instructions, handles exception
alconditions\ 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 newSystem/370 in
structions, for the advanced reliability, availability,
and serviceabilityfea,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 70 can also operate in the mode of System/360 and run all of the supervisory and appli-
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 processing capabilities of the system still further.
Along with
capability of addressing a main storage of
16,777,216 bytes, and the
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
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
the speed of accessing main storage is improved by
the use of a cache. The cache is a buffer--not appar
ent to
quested from main storage without the delay associ
ated with accessing main storage itself.
Another major aspect of the general-purpose de
sign of
attach a wide variety of
types of multiplexing channels. Like
attachment of unbuffered devices and of a large
number of communications devices. Additionally,
which is particularly well-suited for the attachment
of buffered devices and high-speed cyclic devices.
An individual
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
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
pacity, as well as the quantity and type of
2. The program should be independent of the
relation between instruction execution times,
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
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
9
use of invalid operation codes or invalid com
mand codes.
be run on
ceding limitations and the following three limita
tions:
1. The program must not
ASCII bit, which is a function that is provided
only for
locations assigned specifically for
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
al
tion of multiple programs.
types of supervisory programs. Some of these pro
grams provide support for the new
structions, for the advanced reliability, availability,
and serviceability
provide for system and application programs to op
erate in a virtual-storage environment.
System/3 7