The IBM System/370 (S/370) is a range of computers, from entry-level to mainframes, announced as the successors to the System/360 family on June 30, 1970. The series mostly maintains backward compatibility with the S/360, allowing an easy migration path for customers; this, plus improved performance, were the dominant themes of the product announcement. Like its predecessor, the System/370 became a dominant computing platform and was a major commercial success for IBM. Early 370 systems differed from the 360 largely in their internal circuitry, moving from the Solid Logic Technology hybrid integrated circuits containing separate transistors to more modern monolithic integrated circuits containing multiple transistors per integrated circuit, which IBM called Monolithic System Technology, or MST. The higher density packaging allowed several formerly optional features from the 360 line to be included as standard features of the machines, floating-point support for instance. The 370 also added a small number of new instructions. At the time of its introduction, the development of virtual memory systems had become a major theme in the computer market, and the 370 was considered highly controversial as it lacked this feature. This was addressed in 1972 with the System/370 Advanced Function and its associated dynamic address translation (DAT) hardware. All future machines in the lineup received this option, along with several new operating systems that supported it. Smaller additions were made throughout the lifetime of the line, which led to a profusion of models that were generally referred to by the processor number. One of the last major additions to the line in 1988 were the ESA/370 extensions that allowed a machine to have multiple virtual address spaces and easily switch among them. The 370 was IBM's primary large mainframe offering from the 1970s through the 1980s. Its level of success led to competition from plug-compatible manufacturers such as Amdahl Corporation, Hitachi, and others. IBM began a project to introduce a replacement for the 370 that would lock these vendors out, the Future Systems, but this was cancelled in 1975 and the line was instead updated with more powerful CPUs that were similar but added new features. On September 5, 1990, IBM replaced the System/370 line with the System/390, starting with the ES/9000. The S/390 was based on a new ESA/390 architecture, which slightly expanded the ESA/370 architecture.
Evolution The original System/370 line was announced on June 30, 1970, with first customer shipment of the Models 155 and 165 planned for February 1971 and April 1971 respectively. The 155 first shipped in January 1971. System/370 underwent several architectural improvements during its roughly 20-year lifetime. The following features mentioned in the 11th edition of the System/370 Principles of Operation are either optional on S/360 but standard on S/370, introduced with S/370 or added to S/370 after announcement.
Branch and Save Channel Indirect Data Addressing Channel-Set Switching Clear I/O Command Retry Commercial Instruction Set Conditional Swapping CPU Timer and Clock Comparator Dual-Address Space (DAS) Extended-Precision Floating Point Extended Real Addressing External Signals Fast Release Floating Point Halt Device I/O Extended Logout Limited Channel Logout Move Inverse Multiprocessing PSW-Key Handling Recovery Extensions Segment Protection Service Signal Start-I/O-Fast Queuing (SIOF) Storage-Key-Instruction Extensions Storage-Key 4K-Byte Block Suspend and Resume Test Block Translation Vector 31-Bit IDAWs
Initial models When the first System/370 machines, the Model 155 and the Model 165, were introduced, the System/370 architecture was described as an extension, but not a redesign, of IBM's System/360 architecture which was introduced in 1964. The System/370 architecture incorporated only a small number of changes to the System/360 architecture. These changes included:
13 new instructions, among which were MOVE LONG (MVCL); COMPARE LOGICAL LONG (CLCL); thereby permitting operations on up to 2^24-1 bytes (16 MB), vs. the 256-byte limits on the 360's MVC and CLC; SHIFT AND ROUND DECIMAL (SRP), which multiplied or divided a packed decimal value by a power of 10, rounding the result when dividing; optional 128-bit (hexadecimal) floating-point arithmetic, introduced in the System/360 Model 85 a new higher-resolution time-of-day clock support for the block multiplexer channel introduced in the System/360 Model 85. All of the emulator features were designed to run under the control of the standard operating systems. IBM documented the S/370 emulator programs as integrated emulators. These models had core memory and did not include support for virtual storage, as they lacked a DAT (Dynamic Address Translation) box
Logic technology All models of the System/370 used IBM's form of monolithic integrated circuits called MST (Monolithic System Technology) making them third generation computers. MST provided System/370 with four to eight times the circuit density and over ten times the reliability when compared to the previous second generation SLT technology of the System/360.
Monolithic memory On September 23, 1970, IBM announced the Model 145, a third model of the System/370, which was the first model to feature semiconductor main memory made from monolithic integrated circuits and was scheduled for delivery in the late summer of 1971. All subsequent S/370 models used such memory.
Virtual storage In 1972, a very significant change was made when support for virtual storage was introduced with IBM's "System/370 Advanced Function" announcement. IBM had initially (and controversially) chosen to exclude virtual storage from the S/370 line. The August 2, 1972 announcement included:
address relocation hardware on all S/370s except the original models 155 and 165 the new S/370 models 158 and 168, with address relocation hardware four new operating systems: DOS/VS (DOS with virtual storage), OS/VS1 (OS/360 MFT with virtual storage), OS/VS2 (OS/360 MVT with virtual storage) Release 1, termed SVS (Single Virtual Storage), and Release 2, termed MVS (Multiple Virtual Storage) and planned to be available 20 months later (at the end of March 1974), and VM/370 – the re-implemented CP/CMS
Virtual storage had in fact been delivered on S/370 hardware before this announcement:
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