The TX-2 Computer & Sketchpad: the foundation for modern graphical computing

by Marius Marinescu / 8 April

TX-2 was an experimental digital computer created at MIT in 1958. It was one of a few first-generation large electronic digital computers in which transistors largely supplanted vacuum tubes. It was designed to facilitate and enhance real-time human-computer interaction. When first implemented, TX-2 had inherited the ferrite-core memory from its predecessor TX-0 (there was no TX-1). It also had two other random-access memory modules that could work concurrently to provide increased computing speeds. TX-2 was an experimental tool to test many techniques and devices, among which were a magnetic-core memory unit and the first thin-magnetic-film memory unit.


William Kantrowitz, a systems programmer on the TX-2 computer, provides the following list of some of the highlights of the TX-2 computer:

• Much of computer graphics [Sketchpad] began on TX-2;

• Early pioneering speech research was carried out on TX-2;

• TX-2 had one of the first, if not the first, two-level memory paging systems;

• Pioneering work in large memories was done with TX-2;

• The Advanced Research Projects Agency network (ARPANet) derived from experiments on TX-2 with a prototype net between TX-2 and a computer at System Development Corporation in California;

• The feasibility of using the ARPANet for packet speech transmission was first demonstrated on TX-2.


The Sketchpad system was the first graphical computer interface. It made it possible for a man and a computer to converse rapidly through the medium of line drawings. Heretofore, most interaction between man and computers had been slowed down by the need to reduce all communication to written statements that could be typed; in the past, we had been writing letters to, rather than conferring with our computers. For many types of communication, such as describing the shape of a mechanical part or the connections of an electrical circuit, typed statements can prove cumbersome. The Sketchpad system, by eliminating typed statements in favor of line drawings, opened up a new area of man-machine communication. It allowed users to visualize and control program functions and became a foundation for computer graphics, computer operating system interfaces and software applications that are used in many facets of modern technology. The currently used graphical user interface, or GUI, was based on Sketchpad.


In 1961, Massachusetts Institute of Technology (MIT) graduate student Sutherland developed a primitive application, Sketchpad, that would run on the TX-2, at MIT’s Lincoln Laboratory. The TX-2 had twice the memory capacity of the largest commercial machines and impressive programmable capabilities. The computer possessed 320 KB (kilobytes) of memory and powered a 23-cm (9-inch) cathode-ray tube (CRT) display. Sketchpad displayed graphics on the CRT display, and a light pen was used to manipulate the line objects, much like a modern computer mouse. Various computer switches-controlled aspects of the graphics such as size and ratio. In 1963, Sutherland published his doctoral thesis, “Sketchpad: A Man-Machine Graphical Communications System.” for which he received the Turing Award in 1988, and the Kyoto Prize in 2012.


Sketchpad’s process for drawing lines and shapes was quite complicated. The system’s functionality was heavily electrical and used electronic pulses shared between the photoelectric cell of the light pen and an electronic gun fired from the CRT. The timing of the pulse displayed a cursor to represent the light pen’s position on the screen and thus converted the computer screen into a sketchpad upon which objects could be drawn. 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 of the n-component element responsible for adding that spot to display.


The clever way the program organized its geometric data pioneered the use of “master” (“objects”) and “occurrences” (“instances”) in computing and pointed forward to object-oriented programming. The main idea was to have master drawings which one could instantiate into many duplicates. If the user changed the master drawing, all the instances would change as well.


Geometric constraints was another major invention in Sketchpad, letting the user easily constrain geometric properties in the drawing – for instance, the length of a line or the angle between two lines could be fixed.


How objects in Sketchpad could be visualized and modeled on a screen became the foundation for modern graphical computing used in advertising, business, entertainment, architecture, and Web design. In 1964, Sutherland collaborated with David Evan at the University of Utah in Salt Lake City to initiate one of the first educational computer-graphics labs. Sketchpad also led to the advanced development of other imaging software, such as computer-aided design programs used by engineers.