The Mega II is a custom chip from Apple Computer used in some of their Apple II product line. It was used particularly in the Apple IIGS microcomputer, and an updated version, called the "Gemini" chip, was used in the Apple IIe Card for the Macintosh LC. This custom ASIC integrated most of the circuitry from earlier Apple II models onto one 84 pin PLCC integrated circuit, drastically simplifying design and cost for Apple. The Mega II contained the functional equivalent of an entire Apple IIe computer (sans processor), which, combined with the 65C02 emulation mode of the 65C816 processor, provided full support for legacy Apple II software in the Apple IIGS. The result was one of the earliest single chip examples of full system hardware emulation.
In 1984, after the cancellation of the Apple IIx project, Dan Hillman and Jay Rickard, engineers at Apple, were assigned to lower the cost of the Apple II. They were able to compress the design of almost the entire Apple II onto one chip which they named Mega II. When the 16-bit Apple IIGS project came onto the table, the virtually completed design of the Mega II was a clear starting point for seamless backwards 8-bit system compatibility.