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[[File:IBM_magnetic_disk_drives_3330+3333.png|thumb|The IBM 3330 <br>Direct Access Storage Facility, <br>code-named Merlin, was introduced in June 1970 for use with the IBM System/370. Its removable disk packs could hold 100 MB]] ▼[[File:IBM System370 model 145 (I198005) (2).jpg|thumb|3 months after the announcement of <br>the 370/155 (and 165)<br> came the Model 145.<ref>{{cite web|url=https://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP3145.html|title=System/370 Model 145|website=IBM Archives|date=23 January 2003 |publisher=IBM}}</ref><br>Unlike the 155 (or 165), <br>the 145's lack of virtual memory could be rectified with a simple [[microcode]] update from a floppy disk, vs the others needing a hardware upgrade.]]
▲[[File:IBM_magnetic_disk_drives_3330+3333.png|thumb|The IBM 3330 <br>Direct Access Storage Facility, <br>code-named Merlin, was introduced in June 1970 for use with the IBM System/370. Its removable disk packs could hold 100 MB]]
The '''IBM System/370 Model 155''' (and the [[IBM System/370 Model 165|Model 165]])<ref name=IBMarc.165>{{cite web|url=https://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP3165.html|title=System/370 Model 165|website=IBM Archives|date=23 January 2003 |publisher=IBM|archive-url=https://web.archive.org/web/20230729090329/https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP3165.html|archive-date=2023-07-29|url-status=dead}}</ref>
were jointly announced Jun 30, 1970<ref name=IBMarc.155>{{cite web|url=https://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP3155.html|title=System/370 Model 155|website=IBM Archives|date=23 January 2003 |publisher=IBM|archive-url=https://web.archive.org/web/20230516082850/https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP3155.html|archive-date=2023-05-16|url-status=dead}}</ref> as "designed for ... the Seventies." That same day [[IBM]] announced the 370/195.<ref>{{cite groupweb|url="NB">coming about 14 months after the announcement of the 360https:/195/www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP3195.html|title=System/370 BothModel 195|website=IBM machinesArchives|date=23 wereJanuary withdrawn Feb2003|publisher=IBM|archive-url=https://web. 9, 1977archive. see org/web/20230729090333/https://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP2195mainframe_PP3195.html|archive-date=2023-07-29|url-status=dead}}</ref> They were the first three models of the [[IBM System/370]] line of computers.
and https://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP3195.html</ref> They were the first three models of the [[IBM System/370]] line of computers.
Three months later a fourth IBM System/370, the [[IBM System/370 Model 145|Model 145]], was announced. Since none of them came with [[virtual memory]], "which was to be a hallmark of the 370 line",<ref name="what-course-for-the-3081">{{cite news|newspaper=Computerworld|date=November 24, 1980|page=34|url=https://books.google.com/books?id=cKmJJd7R8PIC&pg=PA34|title=What Course for the 3081?}}</ref> some said about these early members of the [[IBM System/370]] family, especially about the 165155 &and 155165, that they were not "the real 370 line."<ref groupname="NB">The 195 was noted as "at what-course-for-the time of its introduction, ... IBM's most powerful computing system-3081" and the 145's [[microcode]] could be upgraded from a floppy disk. By contrast, the 155 & 165 needed a hardware addition priced at $200,000 and $400,000 respectively</ref>
Three months later a fourth IBM System/370, the [[IBM System/370 Model 145|Model 145]], was announced.
==Limitations==
Some said about these early members of the [[IBM System/370]] family, looking back, that they were not "the real 370 line" because
"neither offered virtual storage capability, which was to be a hallmark of the 370 line."<ref name="what-course-for-the-3081">{{cite news|newspaper=Computerworld|date=November 24, 1980|page=34|url=https://books.google.com/books?id=cKmJJd7R8PIC&pg=PA34|title=What Course for the 3081?}}</ref>
==Virtual memory==
The 370/155 was described as able to "run under [[DOS/360 and successors|DOS]]." Both the 155 and the larger 370/165 could "run under [[OS/360 and successors|OS/360]]." Being members of the System/370 family, the Model 155 and Model 165 were compatible with each other. Neither machine, as announced, could run a virtual memory operating system. ▼The initially announced System/370 Models 155 and 165 systems did not support virtual memory.
In 1972 an upgrade option was announced "to provide the hardware necessary to operate in a virtual memory mode."<ref>{{cite news|newspaper=Computerworld|date=August 15, 1973|page=17|title=First IBM DAT Box Installed|url=https://books.google.com/books?id=T3qky0Z-gc0C&pg=PA17}}</ref> Unlike the [[IBM System/370 Model 145]], which as early as June 1971 couldincluded havethe virtualhardware memorynecessary capabilityto addedsupport tovirtual itmemory, withand afor simplewhich a [[microcode]] update from a floppy disk , adding support for virtual memory, was announced in 1972, the Model 155 and Model 165 needed expensive hardware additions—additions - $200,000 for the 155 and $400,000 for the 165—to165 - to add virtual memory capability , and even this had to wait until 1972, at which time. theirAn upgraded 155 was known as an IBM System/370 Model 155-II.<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}} – 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> ▼
==Growth path==
The initially announced systems were in many ways merely improved [[IBM System/360|IBM 360]] systems. Both were announced as
running 360 [[Operating System]]s.<ref group="NB">DOS on the 155, MFT or MVT on either machine</ref> No mention was made of [[virtual memory]]
or new operating systems.
The IBM System/370's basic [[Computer architecture|architecture]] was described as having been "extended, but not redesigned" from that of IBM System/360.<ref name=IBMarc.165/>
===Upgrade option===
In 1972 an upgrade option was announced "to provide the hardware necessary to operate in a virtual memory mode."<ref>{{cite news|newspaper=Computerworld|date=August 15, 1973|page=17|title=First IBM DAT Box Installed|url=https://books.google.com/books?id=T3qky0Z-gc0C&pg=PA17}}</ref>
▲Unlike the [[IBM System/370 Model 145]], which as early as June 1971 could have virtual memory capability added to it with a simple [[microcode]] update from a floppy disk, the Model 155 and Model 165 needed expensive hardware additions—$200,000 for the 155 and $400,000 for the 165—to add virtual memory capability, and even this had to wait until 1972, at which time their upgraded 155 was known as an IBM System/370 Model 155-II.<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}} – 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>
==Physical memory==
Although the joint 155/165 announcement<ref name=IBMarc.165/> did not have the word virtual, there were multiple references to (physical) memory, storage (both main memory and disk storage), and [[cache memory]] under the name "buffer".
The 155 had seven main memory choices, ranging from 256K256 KB to 2 MB; the 165: five possibilities, from 512K512 KB to 3 MB. Both models were described as having "a very high-performance buffer storage backed by a large" main memory.
Another signs of not being "the real 370 line" is that the main storage in 3155 was not monolithic, it was using the older type of ferrite [[core storage]]. The 3360 storage unit could have 256 or 512 KB of storage, each 3155 could have one to four 3360 units attached. Also, the microcode in 3155 was "hardwired" while later 370 systems loaded the microcode from a [[floppy disk]], enabling easy upgrades. The 3155 also had very limited error logging capability and used an [[IBM 3215]] typewriter console.
==Channels==
Channel 0 was a byte multiplexor channel, channel 1 to 5 were block multiplexor channels. Channel 0 and 1 were standard, channel 2 to 5 were optional upgrades.
Two byte multiplexer channels could be installed on a 155.
==Operating systems==
▲The 370/155 wassupported described as able to "run underboth [[DOS/360 and successors|DOS /360]] ." Both the 155 and the larger 370/165 could "run under [[OS/360 and successors|OS/360]]. "<ref name=IBMarc.155 /> Being members of the System/370 family, the Model 155 and Model 165 were compatible with each other. NeitherLacking virtual memory support, neither machine, as announced, could run a virtual memory operating system.
A 370/155 upgraded to a 370/155-II, with support for virtual memory added, supported [[DOS/VS]] (DOS/360 with virtual storage), [[OS/VS1]] ([[OS/360 and successors#OS/MFT|OS/360 MFT]] with virtual storage), OS/VS2 Release 1 ([[OS/360 and successors#OS/MVT|OS/360 MVT]] with virtual storage), termed [[OS/VS2 (SVS)|SVS]] (Single Virtual Storage), and Release 2, termed [[MVS]] (Multiple Virtual Storage), and [[VM (operating system)|VM/370]].
==See also==
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