IBM System/370

The IBM System/370 (S/370) is a model range of IBM mainframe computers announced on June 30, 1970, as the successors to the System/360 family. 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. In September 1990, the System/370 line was replaced with the System/390. 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 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 The first System/370 machines, the Model 155 and the Model 165, 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.