Adobe gave permission to release the source code of Photoshop Version 1.0.1 which released in 1990, which is written in Pascal and Assembler language.
That first version of Photoshop was written primarily in Pascal for the Apple Macintosh, with some machine language for the underlying Motorola 68000 microprocessor where execution efficiency was important. It wasn’t the effort of a huge team. Thomas said, “For version 1, I was the only engineer, and for version 2, we had two engineers.” While Thomas worked on the base application program, John wrote many of the image-processing plug-ins.
Read more and get the code if you want it