IBM System/370 is a product of the experience
gained with System/360. It preserves compatibility
forward from System/360--that is, makes it possible
to move from System/360 to System/370 with the
same ease that is possible in moving from a lower
model to a higher model within the system--and at
the same time includes advanced information pro­
c\essing capabilities.
The latest in solid-state and monolithic technolo­
gies is reflected in most phases of System/370 de­
sign. Complementing this design are new facilities
that assist the user in extending his computer appli­
cations: dynamic address translation, channel indi­
rect data addressing, multiprocessing, timing facili­
ties, extended-precision floating point, program­
event recording, monitoring, and the block­
multiplexer 'channel--together with substantially
improved reliability, availability, and serviceability. • Dynamic address translation, a CPU facility
that generally eliminates the need to assign a
program to fixed locations in real main storage
and thus reduces the addressing constraints on
both system and problem programs, provides
greater freedom in program design and permits
a more efficient and effective utilization of
main storage. When one of the operating sys­
tems for virtual storage is employed, dynamic
address translation allows the use of up to
16,777,216 bytes of virtual storage. • Channel indirect data addressing, a companion
facility to dynamic address translation, pro­
vides assistance in translating data addresses
for I/O operations. It permits a single channel
command word to control the transmission of
data that spans noncontiguous areas of real
main storage. • Multiprocessing provides for the interconnec­
tion of CPUs to enhance system availability
and share data and resources. It includes facili­
ties for sharep main storage, for programmed
and special machine signaling between CPUs, and for the programmed reassignment of the 4,096 bytes of real main storage for each CPU. • Timing facilities include a time-of-day clock, a
clock comparator, and a CPU timer, along with (In interval timer that is also available in The time-of-day clock provides a
measure of elapsed time suitable for the indica­
tion of date and time; it has a cycle of approxi­
mately 143 years and a resolution such that the
incrementing rate is comparable to the instruc-
IBM System/370
tion execution rate of the model. The clock
comparator provides for an interruption when
the time-of -day clock reaches a program­
specified value. The CPU timer is a high­
resolution timer that initiates an interruption
upon being decremented past zero. • Extended-precision floating point includes the
facilities for addition, subtraction, and multipli­
cation of floating-point numbers with a frac­
tion of 28 hexadecimal digits. Included in the
feature are instructions for rounding from ex­
tended to long and from long to short formats. • Program-event recording provides program
interruptions on a selective basis as an aid in
program debugging. • Monitoring provides for passing control to a
monitoring program when selected indicators
are reached in the monitored program. It can
be used, for example, in analyzing which pro­
grams get executed, how often, and in what
length of time. • The block-multiplexer channel, which permits
concurrent processing of multiple channel pro­
grams, provides an efficient means of handling I/O devices that transfer data on the I/O inter­
face at a high rate but have relatively long peri­
ods of channel inactivity in between transfers.
To accommodate many of these new functions,
program-addressable control registers and a new CPU mode --extended-control mode --are provided. System/370 provides the capability of running System/360 operating systems, as well as those spe­
cifically designed for its advanced features, with
little or no change in application programs and data.
At the same time, System/370 provides several CPU models at different performance levels, making many
of the new information processing capabilities avail-
able to all System/370 users--from the smallest to
the very large. The wide choice of models and sup­
port, together with the new and advanced features,
makes System/370 one of the most versatile systems
offered today.
General-Purpose Design
Like System/360, System/370 is a general-purpose
system that can readily be tailored for a variety of
applications. A standard instruction set, which is
expanded over that of System/360 and includes the
protection facility, provides the basic processing
capabilities of the system. To this, a decimal feature,
which includes decimal shifting, may be added to
IBM System/370 9

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-