The Mega II is a custom chip from Apple Computer that is essentially an entire Apple II computer-on-a-chip. At least three products from Apple made use of the chip between 1986 to 1995. It was most predominantly used in the Apple IIGS microcomputer, and an updated version, called the "Gemini" chip, was later 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 processor, plus ROM and RAM, provided full support for legacy (8-bit) Apple II software in the Macintosh LC. The result was one of the earliest single chip examples of full system hardware emulation.
The Mega II has the built-in equivalent of the IOU and MMU chips, video and keyboard ROMs (with support for other display languages) and likely the keyboard encoder found in the IIe. It also has a built-in keyboard and mouse controller (neither were used in the Apple IIGS). Potentially the Mega II could have been used to produce future models of the 8-bit Apple II with a very low chip count (and reduced physical size) but instead used for IIe emulation on the Apple IIGS and Macintosh LC with plug-in card. It was also used as support circuity on the Apple II Video Overlay Card.
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.