IBM System/370 Guide

SECTION 10: ARCHITECTURE AND SYSTEM COMPONENTS
10:05 ARCHITECTURE DESIGN
The basic design objectives embodied in System/370 Model 165
architecture provide System/360 Model 65 and 75 users and 7000-series
emulator users with a growth system that incorporates improvements
and additions to System/360 architecture. The Model 165 provides new
system capabilities, performance improvements, and features to increase
system reliability, availability, and serviceability. This has been
achieved under the following conditions:
• System/370 Model 165 architecture is upward compatible with that
of System/360 models so that most user programs written for
system/360 will run efficiently on the Model 165 without
modification.
• Programming systems support for the Model 165 is based on that
provided for System/360 models, namely on OS MFT and MVT.
• Most currently announced System/360 I/O devices will operate on
the Model 165. (See Section 20:05 for a list of the I/O devices
that cannot be included in a Model 165 configuration.)
• The open-ended design characteristic of System/360 has been
preserved and extended in System/370.
As a result of the architecture design criteria used for this new
system, all programs written for System/360 (Models 25 and up) will
operate on a System/370 Model 165 with a comparable hardware
configuration, with the following exceptions:
1. Time-dependent programs
2. Programs using machine-dependent data such as that which is
logged in the machine-dependent logout area. (OS SER and MCH
and DOS MCRR error-logging routines for System/360 models will
not execute correctly.)
3. Programs that use the ASCII mode bit in the PSW
4. Programs that depend on the nonusable lower processor storage
area being smaller than 1512 bytes. This area can be reduced
to 512 bytes by moving the CPU logout area. (See Section 50.)
5. Programs deliberately written to cause certain program checks
6. Programs that depend on devices or architecture not implemented
in the Model 165, for example, the native file of the Model
44, relocation implemented in the Model 67, etc.
7. Programs that use model-dependent operations of the System/370
Model 165 that are not necessarily compatible with the same
operations on System/360 models
Note that these are the same types of restrictions that exist for
compatibility among System/360 models.
The major elements of the Model 165 computing system are illustrated
in Figure 10.05.1. Each component and its new features are discussed
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IBM System/370 Specifications

General

Brand	IBM
Model	System/370
Category	Server
Language	English