IBM System/370 Model 135

The IBM System/370 Model 135 was announced March 8, 1971,[1] the only 370 introduced that year. The 135 was IBM's fifth System 370,[2] and it was withdrawn October 16, 1979.

IBM System/370 Model 135
ManufacturerInternational Business Machines Corporation (IBM)
Product familySystem/370
Release dateMarch 8, 1971 (1971-03-08)
DiscontinuedOctober 16, 1979
WebsiteOfficial website IBM Archives

Special features

Although microcode was not a uniquely new feature at the time of the 135's introduction, the ability to upgrade a system's microcode without changing hardware was not common at that time.[3][4] "Later models like the 85 and 25 had writable control stores, the 85 using a volatile SRAM array and the 25 using a part of core memory."[4]

A "reading device located in the Model 135 console" allowed updates and adding features to the Model 135's microcode.[1][5] [6] The 145, introduced the prior year, also had this feature.[7]

Optional features

The Model 135 was the last of the 370s to be introduced without Virtual memory. Four of the five[8] could be upgraded. Unlike the 155 & 165, which required an expensive [9] hardware upgrade to add a DAT box (Dynamic Address Translation), the 135 & 145 [10] could get obtain their virtual memory upgrade from a floppy disk.

Microcode upgrades were also available to add "user-selected options such as

An upgraded Model 135[12] was termed a 370/135-3[13]

Customers of the 370/135 had a choice of four main memory sizes, ranging from 96K to 256K.


  • The 370/135 was introduced as running "under either OS or DOS.[1] Newer versions thereof (DOS/VS and OS/VS1) and Virtual Machine Facility/370 (VM/370) subsequently became available options once the 135's microcode was upgraded to support virtual memory. This was priced at $120,000 and came with "increased reloadable control store in addition to some power units." The upgrade could be done "in the field" and the resultant system was now deemed a 370/135-3.[14]
  • The 135 was "partly developed at Hursley, UK."[15]


See also


  1. "System/370 Model 135". IBM Archives. IBM.
  2. of 11
  3. 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.
  4. QUOTE: control store was Card Capacitive ROS. System/360's used a variety of read only control stores, so changing the microcode meant replacing parts (e.g., with parts that were personalized by punching holes in a card).
  5. The "reading device" was a built-in (read-only) floppy disk drive.
  7. describes it as an 80K 8-inch floppy.
  8. the 135, 145, 155 and 165, but not the 195
  9. $200,000 and $400,000 respectively
  10. IBM Maintenance Library 3145 Processing Unit Theory - Maintenance (Third ed.). IBM. pp. CPU 117–129. SY24-3581-2.
  11. (1401, 1440 and 1460)
  12. Computerworld,July 5, 1976,p.2
  13. and the upgrade could be done "in the field."
  14. "Users May Have Jump on Upgrades". Computerworld. July 5, 1976. p. 2.
  15. "Some Key Dates in IBM's Operations in Europe, the Middle East, Asia" (PDF). IBM.
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