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 |
| Registers | |
| General purpose | 16 |
| Floating point | 4 64-bit |
| IBM S/370 registers | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
above, i.e., the most significant (leftmost) bit is designated as bit number 0. |
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The IBM System/370 (S/370) was a model range of IBM mainframe computers announced on June 30, 1970 as the successors to the System/360 family. The series mostly maintained 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. Improvements over the S/360 first released in the S/370 model range included:
The original System/370 line underwent several architectural improvements during its roughly 20-year lifetime.
The first System/370 machines, the Model 155, the Model 165, and the Model 145, incorporated only a small number of changes to the System/360 architecture. These changes included:
They did not include support for virtual memory.
In 1972, a very significant change was made when support for virtual memory 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:
Virtual memory had in fact been delivered on S/370 hardware before this announcement:
Shortly after the August 2, 1972 announcement, DAT box (address relocation hardware) upgrades for the S/370-155 and S/370-165 were quietly announced, but were available only for purchase by customers who already owned a Model 155 or 165. After installation, these models were known as the S/370-155-II and S/370-165-II. IBM wanted customers to upgrade their 155 and 165 systems to the widely sold S/370-158 and -168. These upgrades were surprisingly expensive ($200,000 and $400,000, respectively) and had long ship date lead times after being ordered by a customer; consequently, they were never popular with customers, the majority of whom leased their systems via a third-party leasing company. This led to the original S/370-155 and S/370-165 models being described as "boat anchors". The upgrade, required to run OS/VS1 or OS/VS2, was not cost effective for most customers by the time IBM could actually deliver and install it, so many customers were stuck with these machines running MVT until their lease ended. 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 in the slow adoption of OS/VS2 MVS, not only by customers in general, but for many internal IBM sites as well.
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