System 3.0 was introduced with the Mac Plus, officially implementing HFS and 800K startup drives and adding support for several new technologies including SCSI and AppleShare and introducing Trash “bulging” (i.e., when the Trash contained files, it would gain a bulged appearance). System 4.0 came with the Mac SE and Macintosh II, which required additional support for the first expansion slots, the Apple Desktop Bus (ADB), internal hard drives and on the Mac II, color, larger displays and the first Motorola 68020 processor.
Source: Article “History of Mac OS”. (2008, December 23). In Wikipedia, The Free Encyclopedia. Retrieved 09:37, December 24, 2008, from http://en.wikipedia.org/w/index.php?title=History_of_Mac_OS&oldid=259775290
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