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GA22-7000-4
File No. S/370-01




IBM System/370
Systems Principles of Operation
Page of GA22-70004
Revised September 1, 1975
E:y TNL: GN22-0498




Fourth Edition (September 1974)

This major revision obsoletes GA22-7000-3. The revision includes format changes,
corrections, and additions.
Significant technical changes are indicated by a vertical line to the left of the change.
R'equests for copies of IBM publications should be made to your IBM representative or the
IBM branch office serving your locality.
Changes are made periodically to the information herein; before using this publication in
connection with the operation of the System/370, refer to the latest IBM System/370
Bibliography, GC20-0001, and associated technical newsletter, for the editions that are
applicable and current.
This manual has been prepared by the IBM System Products Division, Product Publica-
tions, Dept. B98, PO Box 390, Poughkeepsie, N.Y., 12602. A form for readers' comments
is provided at the back of this publication. If the form has been removed, comments may
bt: sent to the above address. Comments and suggestions become the property of IBM.
({;) Copyright International Business Machines Corporation 1970, 1972, 1973, 1974
Page of GA22-7000-4
Revised September 1, 1975
By TNL: GN22-0498

Preface

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