IBM System/370 Model 135

Although microcode was not a uniquely new feature at the time of the 135's introduction, having been used in most System/360 models and in most System/370 models introduced so far, the ability to upgrade a system's microcode without changing hardware, by storing the microcode in read-write memory rather than read-only memory, was not common at that time.{{cite journal |first=A. |last=Padegs |date=September 1981 |title=System/360 and Beyond |journal=IBM Journal of Research & Development |publisher=IBM |volume=25 |issue=5 |pages=377–390 |doi=10.1147/rd.255.0377 |quote=In the initial System/360 models, microprograms resided in read-only storage, but in most later models read-write storage is used.}} - 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.{{Cite newsgroup |title=DOS/360: Forty years |url=https://groups.google.com/g/alt.folklore.computers/c/RZA6FD27Tc0/m/iHuq1j8860IJ |date=May 16, 2005 |newsgroup=alt.folklore.computers |quote=No, you'll find no IMPL button on a System/360 Model 30. {{sic|I|t's}} 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). 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.|access-date=2023-10-02}}

The read-write memory containing the firmware was loaded from a "reading device located in the Model 135 console"; this allowed updates and adding features to the Model 135's microcode. The "reading device" was a built-in (read-only) floppy disk drive.{{Cite web|title=Floppy Disks - CHM Revolution|url=https://www.computerhistory.org/revolution/memory-storage/8/261|access-date=2021-06-09|website=www.computerhistory.org}} The 145, introduced the prior year, also had this feature.http://www.computerhistory.org/timeline/1968/#169ebbe2ad45559efbc6eb357200edaf describes it as an 80K 8-inch floppy.

The Model 135 was the last of the 370s to be introduced without Virtual memory. Four of the five{{efn|the 135, 145, 155 and 165, but not the 195}} could be upgraded. Unlike the 155 & 165, which required an expensive{{efn|$200,000 and $400,000 respectively}}

hardware upgrade to add a DAT box (Dynamic Address Translation), the 135 & 145 {{cite book| url = http://www.bitsavers.org/pdf/ibm/370/fe/3145/SY24-3581-4_3145_Processor_Theory_Maintenance.pdf| title = IBM Maintenance Library 3145 Processing Unit Theory - Maintenance| id=SY24-3581-4| edition = Fifth| publisher = IBM| page = CPU 139}} could obtain their virtual memory upgrades from a floppy disk.

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

• extended precision arithmetic or
• emulation of the IBM 1400 series."{{efn|(1401, 1440 and 1460)}}

An upgraded Model 135{{Cite book |last= |first= |url=https://books.google.com/books?id=pGJK0ayV3S8C&pg=PT1 |title=Computerworld |date=1976-07-05 |publisher=IDG Enterprise |language=en}} was termed a 370/135-3{{efn|and the upgrade could be done "in the field."}}

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. 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.{{cite web |url=https://books.google.com/books?id=pGJK0ayV3S8C|title =Users May Have Jump on Upgrades|date=July 5, 1976|publisher=Computerworld|page=2}}
• The 135 was "partly developed at Hursley, UK."{{cite web|url=http://www-03.ibm.com/ibm/history/documents/pdf/emea.pdf|archive-url=https://web.archive.org/web/20050110112224/http://www-03.ibm.com/ibm/history/documents/pdf/emea.pdf|url-status=dead|archive-date=January 10, 2005|title=Some Key Dates in IBM's Operations in Europe, the Middle East, Asia|publisher=IBM}}

• [https://web.archive.org/web/20060603082249/http://www-03.ibm.com/ibm/history/exhibits/mainframe/images/920912.jpg System/370 Model 135 operator's console]
• [https://web.archive.org/web/20100820214449/http://www-03.ibm.com/ibm/history/exhibits/mainframe/images/p_2423PH3135.jpg 370/135 with some peripherals]

• List of IBM products
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• [http://1.bp.blogspot.com/-EBNFDM7Qzws/URquTsASkXI/AAAAAAAALMc/H5i8X423aBA/s850/sistema 370 /135 Advertisement: "Presentamos el Sistema/370 Model 135"]{{Dead link|date=March 2023 |bot=InternetArchiveBot |fix-attempted=yes }}

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Category:IBM System/360 mainframe line