computer systems, networking, data-processing, memory, and data-storage. There are over 1,000 interviews recorded as of 2021, including panel discussions on the origins of the IBM PC and the hard disk drive, and individual interviews with Joanna Hoffman, Steve Chen, Dame Stephanie Shirley, and Donald Knuth. The museum's exhibit "Revolution: The First 2000 Years of Computing," opened to the public on January 13, 2011. It covers the history of computing in 20 galleries, from the abacus to the Internet. The entire exhibition is also available online. The museum has a Liquid Galaxy in the "Going Places: A History of Silicon Valley" exhibit. The exhibit has 20 preselected locations that visitors can fly to on the Liquid Galaxy. Other exhibits include a restoration of an historic PDP-1 minicomputer, two restored IBM 1401 computers, and an exhibit on the history of autonomous vehicles, from torpedoes to self-driving cars. An operating Difference Engine designed by Charles Babbage in the 1840s and constructed by the Science Museum of London was on display until January 31, 2016. It had been on loan since 2008 from its owner, Nathan Myhrvold, a former Microsoft executive. Software The CHM is also home to an extensive collection of software, curated by Al Kossow, a veteran of Apple Computer whom the museum hired in 2006. Kossow is responsible for preservation and accession of software in the museum, as well as for developing CHM's software-themed exhibitions. Kossow was a contributor to the museum long before being hired full-time and is the proprietor of Bitsavers, a large online repository of historical computer manuals and archived software and firmware acquired from his own collection and through donations from his peers. In 2010 the museum began with the collection of source code of important software, beginning with Apple's MacPaint 1.3, written in a combination of Assembly and Pascal and available as download for the public. In 2012 the APL programming language followed. In February 2013 Adobe Systems, Inc. donated the Photoshop 1.0.1 source code to the collection. On March 25, 2014 Microsoft followed with the source code donation of SCP MS-DOS 1.25 and a mixture of Altos MS-DOS 2.11 and TeleVideo PC DOS 2.11 as well as Word for Windows 1.1a under their own license. On October 21, 2014, Xerox Alto's source code and other resources followed. Past exhibits On June 23, 1990, the Walk-Through Computer exhibit opened to help visitors learn how computers work.