http://www.computerhistory.org/atchm/adobe-photoshop-source-code/
With the permission of Adobe Systems Inc., the Computer History Museum is pleased to make available, for non-commercial use, the source code to the 1990 version 1.0.1 of Photoshop. All the code is here with the exception of the MacApp applications library that was licensed from Apple. There are 179 files in the zipped folder, comprising about 128,000 lines of mostly uncommented but well-structured code. By line count, about 75% of the code is in Pascal, about 15% is in 68000 assembler language, and the rest is data of various sorts.
To download the code you must agree to the terms of the license, which permits only non-commercial use and does not give you the right to license it to third parties by posting copies elsewhere on the web.
