Editing Harvard Mark I (section)

==Operation==
The Mark I had 60 sets of 24 switches for manual data entry and could store 72 numbers, each 23 decimal digits long.<ref name="wilkes"/>  It could do 3 additions or subtractions in a second. A multiplication took 6 seconds, a division took 15.3 seconds, and a logarithm or a trigonometric function took over one minute.{{sfnp|Campbell|1999|p=43}}

The Mark I read its [[Instruction set|instruction]]s from a 24-channel [[punched tape|punched paper tape]]. It executed the current instruction and then read the next one. A separate tape could contain numbers for input, but the tape formats were not interchangeable. Instructions could not be executed from the storage registers. Because instructions were not stored in working memory, it is widely claimed that the Harvard Mark I was the origin of the [[Harvard architecture]]. However, this is disputed in ''The Myth of the Harvard Architecture'' published in the ''IEEE Annals of the History of Computing'',<ref>{{cite journal |last1=Pawson |first1=Richard |title=The Myth of the Harvard Architecture |journal=IEEE Annals of the History of Computing |date=30 September 2022 |volume=44 |issue=3 |pages=59–69 |doi=10.1109/MAHC.2022.3175612 |s2cid=252018052 |url=https://ieeexplore.ieee.org/document/9779481}}</ref> which shows  the term 'Harvard architecture' did not come into use until the 1970s (in the context of microcontrollers) and was only retrospectively applied to the Harvard machines, and that the term could only be applied to the [[Harvard Mark III|Mark III]] and [[Harvard Mark IV|IV]], not to the Mark I or [[Harvard Mark II|II]].{{Cn|date=March 2025}}

The main sequence mechanism was unidirectional. This meant that complex programs had to be physically lengthy. A program loop was accomplished by [[loop unrolling]] or by joining the end of the paper tape containing the program back to the beginning of the tape (literally creating a [[Closed curve|loop]]). At first, [[conditional branch]]ing in Mark I was performed manually. Later modifications in 1946 introduced automatic program branching (by [[subroutine]] call).<ref>{{cite book |last1=Beyer |first1=Kurt W. |title=Grace Hopper and the Invention of the Information Age |date=2015 |publisher=BookBaby |isbn=9781483550497 |url=https://books.google.com/books?id=mp5XDQAAQBAJ&q=%22subsidiary+sequence+unit%22&pg=PT78 |language=en |pages=78–79 }}{{Dead link|date=July 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{citation |mode=cs1|last=Bloch|first=Richard|date=1984-02-22|title=Oral history interview with Richard M. Bloch|language=en-US|pages=9–10|hdl=11299/107123}}</ref><ref>{{Cite web|url=http://www.cbi.umn.edu/hostedpublications/Tomash/index.htm|title=The Erwin Tomash Library on the History of Computing: An Annotated and Illustrated Catalog|date=1948|website=www.cbi.umn.edu|series=CBI Hosted Publications|at=Image: [http://www.cbi.umn.edu/hostedpublications/Tomash/Images%20web%20site/Image%20files/H%20Images/index_5.htm Harvard.Vol 16.1948.subsiderary sequence mechanism], description: ''H Chapter'', pp. 577–578|language=en|access-date=2018-05-08}}</ref><ref>{{harvp|A Manual of Operation|1946}}: ''subsidiary sequence control'', pp. 22, 50, 57, 73, 91</ref>{{sfnp|Campbell|1999|p=53}} The first programmers of the Mark&nbsp;I were computing pioneers [[Richard Milton Bloch]], Robert Campbell, and [[Grace Hopper]].<ref>Wexelblat, Richard L. (Ed.) (1981). ''History of Programming Languages'', p. 20. New York: Academic Press. {{ISBN|0-12-745040-8}}</ref> There was also a small technical team whose assignment was to actually operate the machine; some had been IBM employees before being required to join the Navy to work on the machine.<ref>{{cite book|url=https://books.google.com/books?id=KKmiw-_2gYIC|last=Williams|first=Kathleen|title=Grace Hopper: Admiral of the Cyber Sea|publisher=Naval Institute Press|date=2012|pages=33–34|access-date=August 7, 2019|isbn=9781612512655}}</ref> This technical team was not informed of the overall purpose of their work while at Harvard.{{Cn|date=March 2025}}

<gallery mode="packed">
File:Harvard Mark I card punch.agr.jpg|Card punch used to prepare programs
File:Harvard Mark I program tape.agr.jpg|Program tape with visible [[Patch (computing)|programming patches]]
File:Harvard Mark I constant switches detail.jpg|Rotary switches used to enter program data constants
File:IBM Automatic Sequence Controlled Calculator Sequence Indicators.jpg|Sequence indicators and switches
File:Harvard Mark I rear.JPG|Rear view of the computing section
</gallery>