IBM System/360 Model 91

<h2 id="models">Models</h2>
There were four models of the IBM System/360 Model 91.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a> They differed by their main memory configuration, all using IBM's <a href="/facts/IBM_2395_Processor_Storage/FdE1N81V">2395 Processor Storage</a>.
The 91K had 2 MB, using one 2395 Model 1.
Both the 91KK and the 91L came with 4 MB of main memory: the former used a pair of 2395 Model 1s, the latter a single 2395 Model 2.
The 6 MB KL was equipped with one Model 1 and one Model 2 IBM 2395s.

<h3>Models built</h3>
There were only 15 Model 91s ever produced, four of which were for IBM's internal use.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> After quoting from Pugh et al, William H. Blair says "Many disagree on the number of 360/91s that IBM built or sold. I have read and heard it authoritatively stated that the number was 10, 11, 12, 14, 15, or 20." As for those delivered to customers, "a 360/85 was delivered from when a 91 was ordered until it was ready."<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a><a class="footnote-ref" id="fnref:10" href="#fn:10">10</a>

<h3>Differences from standard System/360 behaviour</h3>
Because of the emphasis on speed, there were some minor differences in the system's behaviour:<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a>

<ol><li>Floating point divide results could differ in the least significant bit.</li>
<li>Some exceptions were imprecise.</li>
<li>The handling of floating point underflow and overflow was different, although arguably better.</li>
<li>Memory stores could occur out of sequence. A memory barrier instruction was added to allow control of this where necessary.</li></ol>
<h2 id="ibm-and-nasa">IBM and NASA</h2>

<a href="/facts/IBM/QszA7nwd">IBM</a> had a long history with <a href="/facts/NASA/TIMCV0yV">NASA</a> including the use of IBM components on <a href="/facts/Human_spaceflight/VgVxsGmq">crewed space flights</a> such as the IBM ASC-15 on <a href="/facts/Saturn_1/5Tp3eQ72">Saturn 1</a>, the IBM ASC-15B on the <a href="/facts/Titan_(rocket_family)/sTTkEDps">Titan Family</a>, IBM GDC on <a href="/facts/Project_Gemini/7PYE3AV4">Gemini</a>, <a href="/facts/Saturn_Launch_Vehicle_Digital_Computer/cPcKqDsI">IBM LVDC</a> on <a href="/facts/Saturn_1B/gpHk3gcW">Saturn 1B/5</a>, <a href="/facts/IBM_System/4_Pi/mV9eBgK4">IBM System/4 Pi</a>-EP on the <a href="/facts/Manned_Orbiting_Laboratory/hx5gC4Ic">MOL</a>, and the IBM System/4 Pi-TC 1 on the <a href="/facts/Apollo_Telescope_Mount/47TjNPP9">Apollo Telescope Mount</a> and <a href="/facts/Skylab/h7v9LWKE">Skylab</a>.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a>
The Model 91 was shipped 9 months late to the <a href="/facts/Goddard_Space_Flight_Center/WfoEKknM">Goddard Space Flight Center</a> in October 1967 and did not begin regular operations until January 1968 after it passed the federal government operations testing.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a>

<h3>IBM System/360 Model 95</h3>
The Model 95 was a variant of the Model 91 with 1 megabyte of <a href="/facts/Thin-film_memory/sRT9kCDp">thin-film memory</a> and 4 megabytes of core memory.<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a><a class="footnote-ref" id="fnref:15" href="#fn:15">15</a> NASA acquired the only two 360/95s ever built.<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a><a class="footnote-ref" id="fnref:17" href="#fn:17">17</a><a class="footnote-ref" id="fnref:18" href="#fn:18">18</a>
The console of the Model 95, for which no Functional Characteristics manuals exist, was identical to that of the 360/91.<a class="footnote-ref" id="fnref:19" href="#fn:19">19</a>

<h2 id="first-internet-connected-server">First internet connected server</h2>
In 1971, <a href="/facts/University_of_California,_Los_Angeles/KwPz7AK1">UCLA</a> used an IBM 360/91 to provide "production computing services" to <a href="/facts/ARPANET/LyMGWurJ">ARPANET</a>. The services it provided included job submittal, a "mailbox" system and <a href="/facts/FTP/THuWgzfw">FTP</a>.<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a>

<h2 id="in-popular-culture">In popular culture</h2>
There is a Model 91 Panel that is currently on display at the <a href="/facts/Living_Computer_Museum/tVm5SXPL">Living Computer Museum</a> in <a href="/facts/Seattle,_Washington/7lYIrCa6">Seattle, Washington</a> that was borrowed and featured in the movie <a href="/facts/Tomorrowland_(film)/IAPFaRzv">Tomorrowland (2015)</a>.

<h2 id="further-reading">Further reading</h2>
<ul><li><a href="http://www.quadibloc.com/comp/pan05.htm">Information about the Model 91 and the System/360 Family</a>.</li>
<li><a href="/facts/Michael_J._Flynn/BtNQu0WE">Flynn, Michael J.</a>; Low, Paul R. (January 1967). "The IBM System/360 Model 91: Some Remarks on System Development". <a href="/facts/IBM_Journal_of_Research_and_Development/LswQR28R">IBM Journal of Research and Development</a>. 11 (1). <a href="/facts/IBM/QszA7nwd">IBM</a>: 2–7. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1147%2Frd.111.0002">10.1147/rd.111.0002</a>.</li>
<li>Boland, Lawrence J.; Granito, Gerry D.; Marcotte, Alphonse U.; Messina, Benedicto U.; Smith, John W. (January 1967). "The IBM System/360 Model 91: Storage System". <a href="/facts/IBM_Journal_of_Research_and_Development/LswQR28R">IBM Journal of Research and Development</a>. 11 (1). <a href="/facts/IBM/QszA7nwd">IBM</a>: 54–68. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1147%2Frd.111.0054">10.1147/rd.111.0054</a>.</li>
<li>Anderson, David W.; Sparacio, Francis J.; <a href="/facts/Robert_Marco_Tomasulo/E05zkQQh">Tomasulo, Robert Marco</a> (January 1967). "The IBM System/360 Model 91: Machine Philosophy and Instruction-Handling". <a href="/facts/IBM_Journal_of_Research_and_Development/LswQR28R">IBM Journal of Research and Development</a>. 11 (1). <a href="/facts/IBM/QszA7nwd">IBM</a>: 8–24. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1147%2Frd.111.0008">10.1147/rd.111.0008</a>.</li></ul>
<h2 id="external-links">External links</h2>
<ul><li><a href="http://www.columbia.edu/cu/computinghistory/36091.html">The IBM 360/91 at Columbia University</a></li>
<li><a href="http://www.bitsavers.org/pdf/ibm/360/functional_characteristics/A22-6907-2_360-91_funcChar.pdf">IBM System/360 Model 91 Functional Characteristics</a></li>
<li><a href="https://history.nasa.gov/computers/contents.html">Computers used by NASA</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">"IBM Advanced Computing Systems -- Timeline". people.cs.clemson.edu. <a href="https://people.computing.clemson.edu/~mark/acs_timeline.html" target="_blank">https://people.computing.clemson.edu/~mark/acs_timeline.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">"Out of Order Execution of Computer Instructions". <a href="https://researcher.draco.res.ibm.com/researcher/view_page.php?id=6887" target="_blank">https://researcher.draco.res.ibm.com/researcher/view_page.php?id=6887</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">"IBM Archives: System/360 Model 91". IBM. 23 January 2003. Archived from the original on 2023-09-23. Retrieved 28 June 2016. <a href="https://web.archive.org/web/20230923082536/https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP2091.html" target="_blank">https://web.archive.org/web/20230923082536/https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP2091.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">"IBM Archives: System/360 Model 91". IBM. 23 January 2003. Archived from the original on 2023-09-23. Retrieved 28 June 2016. <a href="https://web.archive.org/web/20230923082536/https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP2091.html" target="_blank">https://web.archive.org/web/20230923082536/https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP2091.html</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">"IBM System/360, Model 91 (console)". ed-thelen.org. Retrieved 29 June 2016. <a href="http://ed-thelen.org/comp-hist/vs-ibm-360-91.html" target="_blank">http://ed-thelen.org/comp-hist/vs-ibm-360-91.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Castells-Rufas, David; Saa-Garriga, Albert; Carrabina, Jordi (January 2016). Energy Efficiency of Many-Soft-Core Processors. International Workshop on High Performance Energy Efficient Embedded Systems (HIP3ES 2016). Prague, Czech Republic. arXiv:1601.07133. doi:10.13140/RG.2.1.1276.5042. S2CID 14349505. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">William H. Blair. "The 360/91 and associated machines". hercules-390 (Mailing list). <a href="https://hercules-390.yahoogroups.narkive.com/ritmdhO6/the-360-91-and-associated-machines#post4" target="_blank">https://hercules-390.yahoogroups.narkive.com/ritmdhO6/the-360-91-and-associated-machines#post4</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Pugh, Emerson W.; Johnson, Lyle R.; Palmer, John H. (1991). IBM's 360 and early 370 systems. Cambridge, Mass.: MIT Press. p. 394. ISBN 0262161230. <a href="0262161230" target="_blank">0262161230</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">gah. "The 360/91 and associated machines". hercules-390 (Mailing list). <a href="https://hercules-390.yahoogroups.narkive.com/ritmdhO6/the-360-91-and-associated-machines#post1" target="_blank">https://hercules-390.yahoogroups.narkive.com/ritmdhO6/the-360-91-and-associated-machines#post1</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">William H. Blair. "The 360/91 and associated machines". hercules-390 (Mailing list). <a href="https://hercules-390.yahoogroups.narkive.com/ritmdhO6/the-360-91-and-associated-machines#post3" target="_blank">https://hercules-390.yahoogroups.narkive.com/ritmdhO6/the-360-91-and-associated-machines#post3</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">"IBM System/360 Model 91 Functional Characteristics" (PDF). Systems Reference Library. IBM. Retrieved 31 May 2024. <a href="http://www.bitsavers.org/pdf/ibm/360/functional_characteristics/A22-6907-2_360-91_funcChar.pdf" target="_blank">http://www.bitsavers.org/pdf/ibm/360/functional_characteristics/A22-6907-2_360-91_funcChar.pdf</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">"Spaceborne Digital Computer Systems" (PDF). NASA Government Archive. National Aeronautics and Space Administration. March 1971. Retrieved 29 June 2016. <a href="https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710024203.pdf" target="_blank">https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710024203.pdf</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Pugh, Emerson W.; Johnson, Lyle R.; Palmer, John H. (1991). IBM's 360 and early 370 systems. Cambridge, Mass.: MIT Press. p. 394. ISBN 0262161230. <a href="0262161230" target="_blank">0262161230</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">"IBM Archives: System/360 Model 95". IBM. 23 January 2003. Archived from the original on 2023-07-20. <a href="https://web.archive.org/web/20230720031116/https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP2095.html" target="_blank">https://web.archive.org/web/20230720031116/https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP2095.html</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">Pugh, Emerson W.; Johnson, Lyle R.; Palmer, John H. (1991). IBM's 360 and early 370 systems. Cambridge, Mass.: MIT Press. p. 394. ISBN 0262161230. <a href="0262161230" target="_blank">0262161230</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">"IBM Archives: System/360 Model 95". IBM. 23 January 2003. Archived from the original on 2023-07-20. <a href="https://web.archive.org/web/20230720031116/https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP2095.html" target="_blank">https://web.archive.org/web/20230720031116/https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP2095.html</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
<li id="fn:17">Chuck Boyer (April 2004). The 360 Revolution (PDF). IBM. One model of the System/360, the 95, was built especially for NASA, and only two units were made. One went to Goddard Space Flight Center in Maryland; the other to the Goddard Institute for Space Studies on upper Broadway in Manhattan. <a href="https://www.vm.ibm.com/history/360rev.pdf" target="_blank">https://www.vm.ibm.com/history/360rev.pdf</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></li>
<li id="fn:18">Pugh, Emerson W.; Johnson, Lyle R.; Palmer, John H. (1991). IBM's 360 and early 370 systems. Cambridge, Mass.: MIT Press. p. 394. ISBN 0262161230. <a href="0262161230" target="_blank">0262161230</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></li>
<li id="fn:19">William H. Blair. "The 360/91 and associated machines". hercules-390 (Mailing list). <a href="https://hercules-390.yahoogroups.narkive.com/ritmdhO6/the-360-91-and-associated-machines#post3" target="_blank">https://hercules-390.yahoogroups.narkive.com/ritmdhO6/the-360-91-and-associated-machines#post3</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></li>
<li id="fn:20">Braden, Robert T. (1977). "A server host system on the ARPANET". Proceedings of the fifth symposium on Data communications - SIGCOMM '77. Snowbird, Utah, United States: ACM Press. pp. 4.1 – 4.9. doi:10.1145/800103.803337. <a href="/wiki/Bob_Braden" target="_blank">/wiki/Bob_Braden</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></li>
</ol>

IBM System/360 Model 91 open-in-new

IBM System/360 Model 91