This publication provides, for reference purposes, a
detailed definition of the machine functions per­
formed by System/370.
The manual describes each function to the level of
detail that must be understood in order to prepare an
assembly language program that relies on that func­
tion. It does not, however, describe the notation and
conventions that must be employed in preparing
such a program, for which the user must instead
refer to the appropriate assembly language manual,
such as the IBM System/360 Operating System
Assembly Language, GC28-6514.
The information in this publication is provided
principally for use by assembly language program­
mers, although anyone concerned with the functional
details of System/370 will find it useful.
Note that this manual is written as a reference
document and should not be considered to be an
introduction or a textbook for System/370. It as­
sumes the user has a basic knowledge of data proc­
essing systems and, specifically, the System/370,
such as can be derived from the Introduction to
IBM Data Processing Systems, GC20-1684, and
the IBM System/370 System Summary, GA22- 7001. Persons intending to use the information pre­
sented here in preparing computer programs should , also become familiar with the publications dealing
with the programming language to be used. The lan­
guage publications available in the System/370 Sys­
tem Library, as well as all publications relating to
other aspects of the system, are listed and described
in the IBM System/3 70 Bibliography, GC20-0001. All facilities discussed in this manual are not nec­
essarily available on every model of System/370.
Furthermore, in some instances the definitions have
been structured to allow for some degree of extensi­
bility, and therefore certain capabilities may be
described or implied that are not offered on any
model. Examples of such capabilities are the provi­
sions for the number of channel mask bits in the
control register, for the size of the processor ad­
dress, and for the number of CPUs sharing main stor­
age. The allowance for this type of extensibility
should not be construed as implying any intention by
IBM to provide such capabilities. For information
about the characteristics and availability of fea-
tures on a specific System/370 model, use the func­
tional characteristics manual for that model. The
availability of features on System/370 models is
summarized in the IBM System/3 70 System Sum­ mary, GA22-7001. Page of GA22-7000-4 Revised September 1, 1975
By TNL: GN22-0498
Preface
The information presented in this manual is
grouped into 14 chapters and several appendixes:
IBM System/3 70 highlights some of the major
features of System/370--particularly those that con­
stitute advances beyond System/360.
System Organization describes the major group­
ings within the system--the central processing unit,
main storage, and input/output--with some attention
paid to the composition and characteristics of those
groupings.
Program Execution explains the role of instruc­
tions in program execution, looks in detail at instruc­
tion formats, and describes briefly the use of the
program status word (PSW), of branching, and of
interruptions. It also details the aspects of program
execution on one CPU as observed by channels or
another CPU. System Control describes in depth the facilities
for the switching of system status, for program pro­
tection, for special externally initiated operations,
and for certain system enhancements. It deals specif­
ically with CPU states, control modes, the PSW, control registers, protection, monitoring, program­
event recording, timing facilities, resets, store status,
and initial program loading.
Dynamic Address Translation explains the opera­
tion of the machine facility which, coupled with spe­
cial programming support, makes the use of a virtual
storage possible in System/370. Dynamic address
translation (DAT) eliminates the need to assign a
program to a fixed location in real main storage and
thus reduces the addressing constraints on system
and problem programs.
Interruptions details the System/370 mechanism
that permits the CPU to change its state as a result
of conditions external to the system, within the sys­
tem, or within the CPU itself. Six classes of interrup­
tions are identified and described: machine-check
interruptions, program interruptions, supervisor-call
interruptions, external interruptions, input/output
interruptions, and restart interruptions.
Multiprocessing describes the facilities required
for the sharing of main storage by multiple CPUs and associated I/O. System Control Instructions contains detailed
descriptions of all of the instructions, except for the I/O instructions, that are available only to the con­
trol program.
General Instructions contains detailed descrip­
tions of all of the instructions in the standard in­
struction set that are available to all programs.

Page ()f GA22-7000-4 Revised September 1, 1975
By TNL: GN22-0498
Decimal Instructions describes in detail the in­
struetions provided by the decimal feature.
Floating-Point Instructions contains detailed de­
scriptions of the instructions provided by the
floating-point feature and by the extended-precision
floating-point feature. Alachine-Check Handling describes the Syst
1 em/370 mechanism for detecting, correcting,
and reporting machine malfunctions.
Input/Output Operations explains the programmed
control of I/O devices by the channel and by
the CPU. It includes detailed descriptions of the I/O instructions, channel command words, and other I/O control formats.
System Console describes the basic manual func­
tions and controls available for operating and con­
trolling the system.
The Appendixes include: • A list of the System/370 features • A discussion of certain functions that differ
from System/360 • Lists of the instructions arranged in several
sequences • Summaries of important formats and of condition­
code settings • Table of the powers of 2 • Tabular information helpful in dealing with hexa-
decimal numbers • An EBCDIC chart • Information about number representation • Instruction-use examples
Largely because the manual is arranged for refer­
ence purposes, certain words and phrases appear, of
necessity, earlier in the manual than the principal
discussions explaining them. The reader who en­
counters a problem of this sort should refer to the
index, which will indicate the location of the key
description.