IBM System/370

System/370

Designer	IBM
Bits	32-bit
Introduced	1970
Design	CISC
Type	Register–Register Register–Memory Memory–Memory
Encoding	Variable (2, 4 or 6 bytes long)
Branching	Condition code, indexing, counting
Endianness	Big
Predecessor	System/360
Successor	S/370-XA, ESA/370, ESA/390, z/Architecture
Registers
General-purpose	16× 32-bit
Floating point	4× 64-bit[a]

The IBM System/370 (S/370) is a range of IBM mainframe computers announced as the successors to the System/360 family on June 30, 1970. The series mostly^[b] 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.

Early 370 systems differed from the 360 largely in their internal circuitry, moving from the individual transistors and small-scale integrated circuits to more modern devices using multiple transistors per integrated circuit, which IBM referred to as 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. In September 1990, the System/370 line was replaced with the System/390. The 390, which was based on a new ESA/390 model, expanded the multiple memory concept to include full hardware virtualization^{[disputed (for: It was there earlier)  – discuss]} that allowed it to run multiple operating systems at the same time.
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