System 1, also known as Macintosh System Software was the original Macintosh operating system. It is a part of the Classic Mac OS family. It ran on the 68K processor architecture of the time. It was released on January 24, 1984 along with the original Macintosh (aka Macintosh 128K) and replaced by System 1.1 on May 5, 1984.
Due to the limited RAM of early computers, there was no multitasking with multiple applications, however there was small applications that could run while an application was loaded. Also, the Trash lost the contents when the computer was shutdown or an application was loaded (closing Finder).