Motorola 68012

Motorola 68000 family

The Motorola 680x0/m68k/68k/68K is a family of 32-bit CISC microprocessor CPU chips and was the primary competition for the Intel x86 family of chips in personal computers of the 1980s and early 1990s. Although no modern desktop computers are based on the 68k, derivative processors are still widely used in embedded applications.

Family members

Main uses

The 68k line of processors has been used in a variety of systems, from modern high-end Texas Instruments calculators (the TI-89, TI-92, and Voyage 200 lines) to older members of the palm pilot series, and even radiation hardened versions in the critical control systems of the Space Shuttle. However, they became most well-known as the processors powering desktop computers such as the Apple Macintosh, the Commodore Amiga, the Atari ST, and several others. The 68k was also the processor of choice in the 1980s for Unix workstations and servers from firms such as Sun Microsystems, NeXT and Silicon Graphics. There was a 68k version of CP/M.

Today, these systems are either end-of-line (in the case of the Atari), or are using different processors (in the case of Amiga, Macintosh, Sun, and SGI). Since these platforms are now more than a decade old, their original manufacturers are unlikely to support an operating system for this hardware or are even out of business. However, the Linux, NetBSD and OpenBSD operating systems still include support for 68k processors.

The 68k processors were also used in the Sega Mega Drive/Sega Genesis and SNK Neo Geo consoles as the main CPU. Other consoles such as the Sega Saturn used the 68k for audio processing and other I/O tasks, while the Atari Jaguar included a 68000 which was intended for basic system control and input processing, but due to the Jaguar's unusual assortment of heterogeneous processors was also frequently used for running game logic.

Microcontrollers derived from the 68k family have been used in a huge variety of applications. For example, CPU32 and ColdFire microcontrollers have been manufactured in the millions as automotive engine controllers.

Architecture

People who are familiar with the PDP-11 or VAX usually feel comfortable with the 68000. With the exception of the split of general purpose registers into specialized data and address registers, the 68000 architecture is in many ways a 32-bit PDP-11.

The instruction set was much more "orthogonal" than those of many processors that came before (e.g., 8080) and after (e.g., x86). That is, it was typically possible to combine operations freely with operands, rather than being restricted to using certain addressing modes with certain instructions. This property made programming relatively easy for humans, and also made it easier to write code generators for compilers.

The 68k instruction set can be divided in the following broad categories:

  • Load and store (Move.B, Move.W, Move.L)
  • Arithmetic (Add, Sub, Mul, Div)
  • Bit shifting (left or right, logical or arithmetical)
  • Bit rotation (ROR, ROL, ROXL, ROXR)

68050 and 68070

There is no 68050; the design that was destined to be the 68050 was eventually released as a version of the 68040. There is also no revision of the 68060, as Motorola was in the process of shifting away from the 68k and 88k processor lines into its new PowerPC business, so the 68070 was never developed. Had it been, it would have been a revised 68060.

Motorola mainly used even numbers for major revisions to the CPU core such as 68000, 68020, 68040 and 68060. The 68010 vas a revised version of the 68000 with minor modifications to the core, and likewise the 68030 was a revised 68020 with some more powerful features, none of them significant enough to classify as a major upgrade to the core.

There was a CPU with the 68070 designation, which was a microcontroller version of the 16/32-bit 68000. This 68070 was used as the main CPU in the Philips CD-i. This CPU was, however, produced by Philips and not officially part of Motorola's 680x0 lineup.

Last generation

The 4th generation 68060 shared most of the features of the Intel P5 architecture. Had Motorola decided to continue the 680x0 series, the next processor (68080) would likely have resembled Intel's P6 architecture.

Other variants

After the mainline 68k processors' demise, the 68k family has been used to some extent in microcontroller/embedded microprocessor versions. These chips include the ones listed under "other" above, i.e. the CPU32 (aka 68330), the ColdFire, the QUICC and the DragonBall.

Competitors

Desktop

During the 80s and early 90s, when the 68k was widely used in desktop computers, it mainly competed against Intel's x86 architecture, which to this day remains the only architecture used in IBM Compatible PCs. Generation 1 68k CPUs primarily competed against the 16-bit 8086/8088 and 80286. Generation 2 competed against the 80386 (the first 32-bit x86 processor), and generation 3 against the 80486. The fourth generation competed with the original Pentium line, but it was not nearly as widely used as its predecessors, since much of the old 68k marketplace was either defunct or nearly so (as was the case with Atari, Amiga and NeXT), or converting to newer architectures (PowerPC for the Macintosh, SPARC for Sun, and MIPS for SGI).

Embedded

There are dozens of processor architectures that are currently successful in embedded systems. Some are microcontrollers which are much simpler, smaller, and cheaper than the 68k, while others are relatively sophisticated and capable of running complex software. Embedded versions of the 68k often compete with processors derived from the PowerPC, ARM, MIPS, and SuperH architectures, among others.

Bibliography

  • Howe, Dennis, ed. (1983). Free On-Line Dictionary of Computing. Imperial College, London. http://foldoc.org. Retrieved September 4, 2007.

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