The Mega II is a custom chip from

Apple Computer Apple Inc. is an American multinational corporation and technology company headquartered in Cupertino, California, in Silicon Valley. It is best known for its consumer electronics, software, and services. Founded in 1976 as Apple Computer Co ...

that is essentially an entire

Apple II Apple II ("apple Roman numerals, two", stylized as Apple ][) is a series of microcomputers manufactured by Apple Computer, Inc. from 1977 to 1993. The Apple II (original), original Apple II model, which gave the series its name, was designed ...

system on a chip, computer-on-a-chip. At least three products from Apple made use of the chip between 1986 and 1995. It was most predominantly used in the Apple IIGS microcomputer, and the basis for a modified variant, called the "Gemini" chip, which was later used in the Apple IIe Card for the Macintosh LC. This custom

application-specific integrated circuit An application-specific integrated circuit (ASIC ) is an integrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in a digital voice recorder or a high-efficienc ...

(ASIC) integrated most of the circuitry from earlier Apple II models onto one 84-pin PLCC

integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

, 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, plus ROM and RAM, provided full support for legacy (8-bit) Apple II software in the Apple IIGS. 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

memory management unit A memory management unit (MMU), sometimes called paged memory management unit (PMMU), is a computer hardware unit that examines all references to computer memory, memory, and translates the memory addresses being referenced, known as virtual mem ...

(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 was used for IIe emulation on the Apple IIGS and Macintosh LC with plug-in card, which used a similar all-in-one IC based on it. It was also used as support circuitry on the Apple II Video Overlay Card.

History

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.Apple II History, Chapter 10 - The Apple IIGS
/ref> When the

16-bit 16-bit microcomputers are microcomputers that use 16-bit microprocessors. A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two ...

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 In computer architecture, 8-bit integers or other data units are those that are 8 bits wide (1 octet). Also, 8-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers or data bu ...

system compatibility.

References

{{Reflist Apple II family