IBM System/370 Model 155

The IBM System/370 Model 155 (and the Model 165),[2] were jointly announced Jun 30, 1970[3] as "designed for ... the Seventies." That same day IBM announced the 370/195.[NB 1] They were the first three models of the IBM System/370 line of computers.

IBM System/370 Model 155
ManufacturerInternational Business Machines Corporation (IBM)
Product familySystem/370
Release dateJune 30, 1970 (1970-06-30)
DiscontinuedDecember 23, 1977
WebsiteOfficial website IBM Archives

Three months later a fourth IBM System/370, the Model 145, was announced. Since none of them came with virtual memory, "which was to be a hallmark of the 370 line"[4] some said about these early members of the IBM System/370 family, especially about the 165 & 155, that they were not "the real 370 line."[NB 2]


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."[4]

The 370/155 was described as able to "run under DOS." Both the 155 and the larger 370/165 could "run under 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.

Growth path

The initially announced systems were in many ways merely improved IBM 360 systems. Both were announced as running 360 Operating Systems.[NB 3] No mention was made of virtual memory or new operating systems.

The IBM System/370's basic architecture was described as having been "extended, but not redesigned" from that of IBM System/360.[2]

Upgrade option

In 1972 an upgrade option was announced "to provide the hardware necessary to operate in a virtual memory mode."[5]

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.[6]

Physical memory

Although the joint 155/165 announcement[2] 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 256K to 2 MB; the 165: five possibilities, from 512K to 3 MB. Both models were described as having "a very high-performance buffer storage backed by a large" main memory.


Two byte multiplexer channels could be installed on a 155.

See also


  1. coming about 14 months after the announcement of the 360/195. Both 195 machines were withdrawn Feb. 9, 1977. see and
  2. The 195 was noted as "at the time of its introduction, ... IBM's most powerful computing system" 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
  3. DOS on the 155, MFT or MVT on either machine


  1. "System/370 Model 145". IBM Archives. IBM.
  2. "System/370 Model 165". IBM Archives. IBM.
  3. "System/370 Model 155". IBM Archives. IBM.
  4. "What Course for the 3081?". Computerworld. November 24, 1980. p. 34.
  5. "First IBM DAT Box Installed". Computerworld. August 15, 1973. p. 17.
  6. A. Padegs (September 1981). "System/360 and Beyond". IBM Journal of Research & Development. IBM. 25 (5): 377–390. 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.
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