Editing IBM System/370 (section)

===Virtual storage===
In 1972, a very significant change was made when support for virtual storage 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.<ref name="Pugh91">{{cite book|first=E.W.|last=Pugh |author2=L.R. Johnson |author3=John H. Palmer|title=IBM's 360 and early 370 systems|publisher=MIT Press|isbn=0-262-16123-0|url=https://archive.org/details/ibms360early370s0000pugh|url-access=registration|location=Cambridge|year=1991}}</ref>{{rp|479–484}}<ref>{{cite web|url=http://www.feb-patrimoine.com/histoire/english/information_technology/information_technology_3.htm|title=Information technology industry timeline, 1964–1974}}</ref> The August 2, 1972 announcement included:
* address relocation hardware on all S/370s except the original models 155 and 165
* the new S/370 models 158 and 168, with address relocation hardware
* four new operating systems: [[DOS/VS]] (DOS with virtual storage), [[OS/VS1]] ([[OS/360 and successors|OS/360]] [[OS/360 and successors#OS/MFT|MFT]] with virtual storage), OS/VS2 (OS/360 [[OS/360 and successors#OS/MVT|MVT]] with virtual storage) Release 1, termed [[OS/VS2 (SVS)|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 (operating system)|VM/370]] – the re-implemented [[CP/CMS]]

[[File:Hist. Rechenzentrum.png|thumb|300px|right|System/370-145 3D Rendering]]
[[File:IBM System 370-145 und Bandlaufwerke 2401.png|thumb|300px|right|3D Rendering of computer center with IBM System/370-145 and IBM 2401 tape drives]]
[[File:IBM 370-145 2.png|thumb|300px|right|System/370-145 3D Rendering]]
[[File:IBM 370-145.png|thumb|300px|right|System/370-145 system console.]]

Virtual storage 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 &ndash; since this would have signaled that the S/370 was about to receive address relocation technology).<ref>{{cite conference|url=http://www.leeandmelindavarian.com/Melinda/25paper.pdf|last=Varian|first=Melinda|title=VM and the VM community, past present, and future|conference=[[SHARE (computing)|SHARE]] 89 Sessions 9059-9061|year=1997|page=29}}</ref> The S/370-145 had an associative memory<ref name=SY24-3581-1>{{cite book
 | title     = IBM Maintenance Library 3145 Processing Unit Theory - Maintenance
 | edition   = Second
 | url       = http://www.bitsavers.org/pdf/ibm/370/fe/3145/SY24-3581-1_3145_Processing_Unit_Theory-Maintenance_Oct71.pdf
 | id        = SY24-3581-1
 | date      = October 1971
 | publisher = IBM
 | pages     = CPU 117–129}}</ref><ref name=SY24-3581-4>{{cite book
 | title     = IBM Maintenance Library 3145 Processing Unit Theory - Maintenance
 | edition   = Fifth
 | url       = http://www.bitsavers.org/pdf/ibm/370/fe/3145/SY24-3581-4_3145_Processor_Theory_Maintenance.pdf
 | id        = SY24-3581-4
 | publisher = IBM}}</ref>{{rp|CPU 117-CPU 129}} used by the [[microcode]] for the DOS compatibility feature from its first shipments in June 1971;<ref name=SY24-3581-1/> the same hardware was used by the microcode for DAT.<ref name=SY24-3581-4 />{{rp|CPU 139}} Although IBM famously chose to exclude virtual storage from the S/370 announcement, that decision was being reconsidered during the completion of the 145 engineering, partly because of virtual memory experience at CSC and elsewhere. The 145 microcode architecture simplified the addition of virtual storage, 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 storage 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. The Reference and Change bits of the Storage-protection Keys, however, were labeled on the rollers, a dead giveaway to anyone who had worked with the earlier 360/67. 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 &ndash; which was possible, but not common because of the limited amount of main storage available on the S/370-145).

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.<ref name="S370VM">"IBM's Virtual Memory 370s," Datamation, September 1972, p.58-61</ref> 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.<ref>{{cite journal | author = A. Padegs | title = System/360 and Beyond | journal = IBM Journal of Research & Development | volume = 25 | issue = 5 | pages = 377–390 |date=September 1981 | publisher = IBM | doi = 10.1147/rd.255.0377}} &ndash; tables include model characteristics (Table 1) and announcement/shipment dates (Table 2). The S/370-155-II and -165-II are listed under the former but not the latter, because the upgraded systems were not formally announced as separate models. The "System/370 Advanced Function" announcement, including the -158 and -168, was the main public event.</ref> 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 [[Lease|leasing]] company.<ref name="S370VM" /> 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<ref>"155, 165 Owners Angry with IBM," Datamation, August 1973, p.76-86</ref><!--

Introducing DAT box for 155&165 slowed adoption of MVS?! Quite the opposite is true: the lack of DAT box in initial 155&165 slowed MVS adoption. [But that is the point: the lack of virtual memory in the original release meant that customers were soon asked to pay for an expensive and unexpected upgrade; many said no. It was the NEED for the upgrade, rather than its availability, that caused the problem.]

--> in the slow adoption of OS/VS2 MVS, not only by customers in general, but for many internal IBM sites as well.