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.

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.

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 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.

[3][4][5][6][7][8][9] The following features mentioned in the 11th edition of the System/370 Principles of Operation[3] are either optional on S/360 but standard on S/370, introduced with S/370 or added to S/370 after announcement.

These changes included:[12] These models had core memory and did not include support for virtual storage, as they lacked a DAT (Dynamic Address Translation) box All models of the System/370 used IBM's form of monolithic integrated circuits called MST (Monolithic System Technology) making them third generation computers.

In 1972, a very significant change was made when support for virtual storage was introduced with IBM's "System/370 Advanced Function" announcement.

It was not unusual for this to be another four, five or even six years for the more unfortunate ones, and turned out to be a significant factor[29] in the slow adoption of OS/VS2 MVS, not only by customers in general, but for many internal IBM sites as well.

[40] Two, three or four directly attached IBM 3333 disk storage units provided "up to 400 million bytes online."

[41] Options for the 370/135 included a choice of four main memory sizes; IBM 1400 series (1401, 1440 and 1460) emulation was also offered.

The latter was "double the maximum capacity of the Model 135," which "can be upgraded to the new computer's internal performance levels at customer locations.

A portion of the main memory, the "Reloadable Control Storage" (RCS) was loaded from a prewritten disk cartridge containing microcode to implement, for example, all needed instructions, I/O channels, and optional instructions to enable the system to emulate earlier IBM machines.

[49] It included dynamic address translation (DAT) hardware, a prerequisite for the new virtual memory operating systems (DOS/VS, OS/VS1, OS/VS2).

[49] It included dynamic address translation (DAT) hardware, a pre-requisite for the new virtual memory operating systems.

Starting with the E[69] models, and continuing with the J and S models, IBM offered Enterprise Systems Architecture/370[70] (ESA/370), Processor Resource/System Manager (PR/SM) and a set of backward compatible MVS/Enterprise System Architecture (MVS/ESA) software replacing previous products: IBM's offering of an optional vector facility (VF) extension for the 3090 came at a time when Vector processing/Array processing suggested names like Cray and Control Data Corporation (CDC).

[78] The 4361 has "Programmable Power-Off -- enables the user to turn off the processor under program control";[77] "Unit power off" is (also) part of the 4381 feature list.

This low-end system, announced October 7, 1986,[79] was "designed to satisfy the computing requirements of IBM customers who value System/370 affinity" and "small enough and quiet enough to operate in an office environment."

IBM also noted its sensitivity to "entry software prices, substantial reductions in support and training requirements, and modest power consumption and maintenance costs."

Furthermore, it stated its awareness of the needs of small-to-medium size businesses to be able to respond, as "computing requirements grow," adding that "the IBM 9370 system can be easily expanded by adding additional features and racks to accommodate..." This came at a time when Digital Equipment Corporation (DEC) and its VAX systems were strong competitors in both hardware and software;[80] the media of the day carried IBM's alleged "VAX Killer" phrase, albeit often skeptically.

About the same time, Japanese giants began eyeing the lucrative mainframe market both at home and abroad.

Some of the aspects of this architecture are: Some of the optional features are: IBM took great care to ensure that changes to the architecture would remain compatible for unprivileged (problem state) programs; some new interfaces did not break the initial interface contract for privileged (supervisor mode) programs.

Some examples are: Great care was taken in order to ensure that further modifications to the architecture would remain compatible, at least as far as non-privileged programs were concerned.

[citation needed] All have retained essential backward compatibility with the original S/360 architecture and instruction set.

The GNU Compiler Collection (GCC) had a back end for S/370, but it became obsolete over time and was finally replaced with the S/390 backend.

Although the S/370 and S/390 instruction sets are essentially the same (and have been consistent since the introduction of the S/360), GCC operability on older systems has been abandoned.

[97] GCC currently works on machines that have the full instruction set of System/390 Generation 5 (G5), the hardware platform for the initial release of Linux/390.

As part of the DAT announcement, IBM upgraded channels to have Indirect Data Address Lists (IDALs).

Subsequently, FICON became the standard IBM mainframe channel; FIbre CONnection (FICON) is the IBM proprietary name for the ANSI FC-SB-3 Single-Byte Command Code Sets-3 Mapping Protocol for Fibre Channel (FC) protocol used to map both IBM's antecedent (either ESCON or parallel Bus and Tag) channel-to-control-unit cabling infrastructure and protocol onto standard FC services and infrastructure at data rates up to 16 Gigabits/sec at distances up to 100 km.

Fibre Channel Protocol (FCP) allows attaching SCSI devices using the same infrastructure as FICON.