Editing Sketchpad (section)

==Hardware==
Sketchpad ran on the [[MIT Lincoln Laboratory]] [[TX-2]] (1958) computer at the [[Massachusetts Institute of Technology]] (MIT), which had 64k of [[36-bit computing|36-bit]] [[Word (computer architecture)|words]]. The user drew on the [[computer monitor]] screen with the recently invented [[light pen]], which relayed information on its position by computing at what time the light from the scanning [[cathode-ray tube]] screen is detected. 

To configure the initial position of the light pen, the word <code>INK</code> was displayed on the screen, which, upon tapping, initialised the program with a white cross to continue keeping track of the pen's movement relative to its prior position.<ref name=IES-thesis></ref> Of the 36 bits available to store each display spot in the display file, 20 gave the coordinates of that spot for the display system and the remaining 16 gave the [[Address space|address]] of the n-component element responsible for adding that spot to display.

The TX-2 was an experimental machine and the hardware changed often (on Wednesdays, according to Sutherland<ref name="LookingBack">{{cite journal |last1=Sutherland |first1=Ivan |author1-link=Ivan Sutherland |date=2012 |title=The TX-2 Computer and Sketchpad |journal=Lincoln Laboratory Journal |volume=19 |issue=1 |pages=82–84 |url=https://www.ll.mit.edu/sites/default/files/page/doc/2018-05/LookingBack_19_1.pdf |access-date=6 November 2022}}</ref>). By 1975, the [[light pen]] and the [[cathode-ray tube]] with which it had been used had been removed.<ref name="SequenceChanges">{{cite web |last1=Youngman |first1=James |date=<!-- Undated --> |title=Sequence Changes |url=https://tx-2.github.io/commentary/sequence-changes#crt |website=TX-2 Project |access-date=26 November 2024}}</ref>