IBM System/370 - Evolution

Evolution

The original System/370 line underwent several architectural improvements during its roughly 20-year lifetime. One very significant change was the introduction of virtual memory, which was first made generally available in 1972 via IBM's "System/370 Advanced Function" announcement. IBM had initially (and controversially) chosen to exclude virtual storage from the S/370 line. The August 2nd, 1972 announcement included:

• Address relocation hardware on all S/370s except the original models 155 and 165
• The new S/370-158 and -168
• 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 memory had in fact been delivered on S/370 hardware before this announcement:

• In June 1971, on the S/370-145 (one of which had to be 'smuggled' into Cambridge Scientific Center to prevent anybody noticing the arrival of an S/370 at that hotbed of virtual memory development – since this would have signaled that the S/370 was about to receive address relocation technology). (Varian 1997:p29) The S/370-145 had relocation hardware (implemented in microcode) from its first shipments in June 1971. Although IBM famously chose to exclude virtual memory from the S/370 announcement, that decision was being reconsidered during the completion of the 145 engineering, due partly to virtual memory experience at CSC and elsewhere. The 145 microcode architecture simplified the addition of virtual memory, allowing this capability to be present in early 145s without the extensive hardware modifications needed in other models. However, IBM did not document the 145's virtual memory capability, nor annotate the relevant bits in the control registers and PSW that were displayed on the operator control panel when selected using the roller switches. Existing S/370-145 customers were happy to learn that they did not have to purchase a hardware upgrade in order to run DOS/VS or OS/VS1 (or OS/VS2 Release 1 – which was possible, but not common because of the limited amount of main storage available on the S/370-145).

Shortly after the August 2nd, 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, majority of which 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.