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832 Part 4½ Family Range, Compatibility, and Evolution
Section 5½ The System/360 and System/370 Family
 
 

4 The System/370 architecture must preserve and extend the open-endedness and generality of design characteristic of the System/360 architecture.


Summary of Architectural Extensions

Table 1 lists the major categories of architectural extension that have been added to the System/3601 architecture to form the System/3702 architecture, including those that were originally introduced on the System/360 Model 85. The extensions arc grouped in terms of architectural facilities, which are mechanisms provided in the machine for performing a specific function. The table also lists the number of new instructions associated with the facility. Note that many of the new facilities have no new instructions associated with them. Table 2 lists all new instructions, which total 40.

Additionally in a number of areas the System/360 architecture was made more specific and predictable within the freedom permitted by the original definition. The following are two examples:

1 The result of a decimal-arithmetic operation is made predictable when an invalid sign code is encountered. This is a common error in source data, and the change permits correction and resumption of the operation.

2 The priority of recognizing program-interruption conditions is specified to achieve repeatability and to make debugging easier.


Compatibility with System/360

Methods of Achieving Compatibility

Major emphasis in the design of the System/370 architecture was placed on defining all changes and extensions so that a valid System/360 program, executed on a System/370 machine, would

obtain the same results as specified in the IBM System/360 Principles of Operation. This compatibility was achieved by four devices:

Restriction. Narrowing System/370 to a more specific operation in areas where the System/360 definition allowed unpredictable

1The System/360 Model 20 is not discussed in the referenced papers nor in this paper, as some of its architectural features are so specialized that it is not convenient to discuss them in the same context.

2This paper covers only those facilities that are described in System/370 Principles of Operation. It does not discuss certain extensions that were made available only on System/360 Models 44 and 67; nor does it describe the following special facilities that are available only on some models: virtual-machine assist (hardware assist for VM/370), extended control-program support (hardware assist for OS/VS1 and for VM/370), APL assist, OS/DOS compatibility, the assist for optical character recognition, emulators for other machines, as well as the System/370 extended facility and recovery extensions first made available on the IBM 3033 Processor Complex.
 
 

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