Chapter 51 ½ Architecture of the IBM System/370 831
the eventual need and was well suited to the extension, since it depended on base registers that were already 32 bits wide. The interruption mechanism and the I/O control formats, however, did not have the required extensibility. (We knew in 1962 that this was the case, but the immediate cost and performance consequences outweighed the need to meet the eventual long-term requirements.) More importantly, the operating systems and compiler-produced application programs had used the extra bits in address words for control purposes and hence required extensive modifications.
Operating Systems
Machine architecture must be developed in conjunction with changes and extensions to existing operating systems. Whereas the original System/360 architecture was developed to provide a good basis on which a completely new operating system could be built, extensions to that architecture have to consider the specific usages and capabilities of the available operating systems.
Architecture Control
The design and control of system architecture must be an ongoing function that can never be considered complete. We found ourselves well into the 1970s making changes in the architecture of System/360 to remove ambiguities and, in some cases, to adjust the function provided.
Objectives of System/370
Motivation
The motivation to extend the System/360 architecture for the new series of machines came from two main sources:
1 The experience with the System/360 architecture in writing application programs, in designing and using operating systems, and in debugging and maintaining both software and hardware had identified a number of bottlenecks and limitations in the efficiency of system use and had pointed out areas where additional machine functions were desirable.
2 The general lowering of the cost of technology for main storage and logic circuitry in relation to the overall system cost made it possible economically to include functions that did not appear justified in the original System/360 architecture.
Specific Objectives
The following were the specific objectives of the System/370 architecture:
1 Improving the level of detail, precision, and predictability of the System/360 architecture. These improvements were made primarily in the areas of interruptions, system control, and the order of storage references. They were motivated largely by reliability and serviceability considerations.
2 Adding new instructions to enhance the performance of frequent functions in application programs. A total of 17 new unprivileged instructions were introduced in the System/370 architecture.
3 Extending the architecture to improve system reliability, availability, and serviceability. Extensions were included to assist diagnostics and recovery by software after a hardware failure (machine-check extensions), to assist in debugging software (program-event recording, monitoring, status storing), and to facilitate formation of multiprocessing systems with multiple CPUs sharing common main storage.
4 Adding new facilities to enhance the performance and function of the operating system and to introduce uniform machine-implemented protocols in the system. Dynamic address translation, timing facilities, and a number of privileged instructions were the main extensions provided for this purpose.
Constraints on System/370
The System/370 architecture was developed subject to the following main constraints:
1 Within the limitations described in the IBM System/370 Principles of Operation, the architecture must be upward compatible with System/360 architecture as far as user programs are concerned; that is, user programs written for System/360 must run efficiently on System/370 models with no modification to these programs. These limitations are that the systems have the same or equivalent facilities and that the programs have no time dependence, use only model-independent functions defined in the Principles of Operation, and not use unassigned formats and operation codes. These limitations essentially mean that compatibility applies only to valid programs.
2 It must be possible to run certain System/360 operating systems unmodified on System/370 models. Even though such operating systems could not fully benefit from the new functions available in System/370, and new support was planned, the ability to execute them was needed for the transition period.
3 It must be possible to attach and operate most types of System/360 I/O devices on System/370.