Editing EDSAC (section)

===System software===
The ''initial orders'' were hard-wired on a set of [[Stepping switch|uniselector switches]] and loaded into the low words of memory at startup. By May 1949, the initial orders provided a primitive relocating [[Assembly language assembler|assembler]] taking advantage of the mnemonic design described above, all in 31 words. This was the world's first assembler, and arguably the start of the global software industry. There is a simulation of EDSAC available, and a full description of the initial orders and first programs.<ref>{{Cite web| title=Edsac Simulator | url=https://www.dcs.warwick.ac.uk/~edsac/ | access-date=2023-05-24 | website=dcs.warwick.ac.uk | publisher=[[University of Warwick]] | location=UK }}</ref>

The first calculation done by EDSAC was a program run on 6&nbsp;May 1949 to compute [[square number]]s.<ref name=":0">{{Cite web |url=http://www.computinghistory.org.uk/det/5484/EDSAC-performed-its-first-calculations/ |title=EDSAC performed its first calculations |website=Computing History |access-date=2018-11-23 |archive-date=26 February 2021 |archive-url=https://web.archive.org/web/20210226185505/http://www.computinghistory.org.uk/det/5484/EDSAC-performed-its-first-calculations/ |url-status=dead }}</ref> The program was written by [[Beatrice Worsley]], who had travelled from Canada to study the machine.<ref>{{Cite web |url=https://www.thecanadianencyclopedia.ca/en/article/beatrice-worsley |title=Beatrice Worsley |last=Raymond |first=Katrine |date=25 October 2017 |website=The Canadian Encyclopedia |language=en |archive-url=https://web.archive.org/web/20180113171457/http://www.thecanadianencyclopedia.ca/en/article/beatrice-worsley/ |archive-date=13 January 2018 |access-date=2018-11-23}}</ref><ref name=":0" />

The machine was used by other members of the university to solve real problems, and many early techniques were developed that are now included in operating systems.

Users prepared their programs by punching them (in assembler) onto a paper tape. They soon became good at being able to hold the paper tape up to the light and read back the codes. When a program was ready, it was hung on a length of line strung up near the paper-tape reader. The machine operators, who were present during the day, selected the next tape from the line and loaded it into EDSAC. This is of course well known today as job queues. If it printed something, then the tape and the printout were returned to the user, otherwise they were informed at which memory location it had stopped. Debuggers were some time away, but a [[cathode-ray tube]] screen could be set to display the contents of a particular piece of memory. This was used to see whether a number was converging, for example. A loudspeaker was connected to the accumulator's sign bit; experienced users knew healthy and unhealthy sounds of programs, particularly programs "hung" in a loop.

After office hours certain "authorised users" were allowed to run the machine for themselves, which went on late into the night until a valve blew – which usually happened according to one such user.<ref>Professor David Barron, Emeritus Professor of the University of Southampton at a Cambridge Computer Lab seminar to mark the 60th anniversary 6&nbsp;May 2009.</ref> This is alluded to by [[Fred Hoyle]] in his novel ''[[The Black Cloud]]''