Mega II
Encyclopedia
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
.
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.
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
from Apple Computer
Apple Computer
Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...
used in some of their Apple II
Apple II
The Apple II is an 8-bit home computer, one of the first highly successful mass-produced microcomputer products, designed primarily by Steve Wozniak, manufactured by Apple Computer and introduced in 1977...
product line. It was used particularly in the Apple IIGS
Apple IIGS
The Apple , the fifth and most powerful model in the Apple II series of personal computers produced by Apple Computer. The "GS" in the name stands for Graphics and Sound, referring to its enhanced graphics and sound capabilities, both of which greatly surpassed previous models of the line...
microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...
, and an updated version, called the "Gemini" chip, was used in the Apple IIe Card
Apple IIe Card
The Apple IIe Card is a compatibility card which allows compatible Macs to run software designed for Apple II computers...
for the Macintosh LC
Macintosh LC
The Macintosh LC was Apple Computer's product family of low-end consumer Macintosh personal computers in the early 1990s. The original Macintosh LC was released in 1990 and was the first affordable color-capable Macintosh. Due to its affordability and Apple II compatibility the LC was adopted...
. This custom ASIC
Application-specific integrated circuit
An application-specific integrated circuit is an integrated circuit customized for a particular use, rather than intended for general-purpose use. For example, a chip designed solely to run a cell phone is an ASIC...
integrated most of the circuitry from earlier Apple II models onto one 84 pin PLCC
Plastic leaded chip carrier
A plastic leaded chip carrier is a chip carrier with a equiangular plastic housing. It is a reduced cost evolution of the ceramic leadless chip carrier ....
integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
, 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
WDC 65C02
The Western Design Center WDC 65C02 microprocessor is an upgraded CMOS version of the popular NMOS-based MOS Technology 6502 8-bit CPU — the CMOS redesign being made by Bill Mensch of the Western Design Center...
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
Hardware emulation
In integrated circuit design, hardware emulation is the process of imitating the behavior of one or more pieces of hardware with another piece of hardware, typically a special purpose emulation system. The emulation model is usually based on RTL source code, which is compiled into the format...
.
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. When the 16-bit16-bit
-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor. Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. The Intel 8088 was program-compatible with the Intel 8086, and was 16-bit in that its registers were 16...
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
8-bit
The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...
system compatibility.