Motorola 68000 series

The Motorola 68000 series (also termed 680x0, m68000, m68k, or 68k) is a family of 32-bit complex instruction set computer (CISC) microprocessors. During the 1980s and early 1990s, they were popular in personal computers and workstations and were the primary competitors of Intel's x86 microprocessors. They were most well known as the processors powering the early Apple Macintosh, the Commodore Amiga, the Sinclair QL, the Atari ST, the WeatherStar, the Sega Genesis (Mega Drive), and several others. Although no modern desktop computers are based on processors in the 68000 series, derivative processors are still widely used in embedded systems.

Motorola ceased development of the 68000 series architecture in 1994, replacing it with the development of the PowerPC architecture, which they developed in conjunction with IBM and Apple Computer as part of the AIM alliance.

Family members

• Generation one (internally 16/32-bit, and produced with 8-, 16-, and 32-bit interfaces)
  • Motorola 68000
  • Motorola 68EC000
  • Motorola 68HC000
  • Motorola 68008
  • Motorola 68010
  • Motorola 68012
• Generation two (internally fully 32-bit)
  • Motorola 68020
  • Motorola 68EC020
  • Motorola 68030
  • Motorola 68EC030
• Generation three (pipelined)
  • Motorola 68040
  • Motorola 68EC040
  • Motorola 68LC040
• Generation four (superscalar)
  • Motorola 68060
  • Motorola 68EC060
  • Motorola 68LC060
• Others
  • Freescale 683XX (CPU32 aka 68330, 68360 aka QUICC)
  • Freescale ColdFire
  • Freescale DragonBall
  • Philips 68070

Improvement history

68010:

• Virtual memory support (restartable instructions)
• 'loop mode' for faster string and memory library primitives

68020:

• 32-bit address & arithmetic logic unit (ALU)
• Three stage pipeline
• Instruction cache of 256 bytes
• Unrestricted word and longword data access (see alignment)
• 8× multiprocessing ability
• Larger multiply (32×32 -> 64 bits) and divide (64÷32 -> 32 bits quotient and 32 bits remainder) instructions, and bit field manipulations
• Addressing modes added scaled indexing and another level of indirection
• Low cost, EC = 24-bit address

68030:

• Split instruction and data cache of 256 bytes each
• On-chip memory management unit (MMU) (68851)
• Low cost EC = No MMU
• Burst Memory Interface

68040:

• Instruction and data caches of 4 KB each
• Six stage pipeline
• On-chip floating-point unit (FPU)
• FPU lacks IEEE transcendental function ability
• FPU emulation works with 2E71M and later chip revisions
• Low cost LC = No FPU
• Low cost EC = No FPU & MMU

68060:

• Instruction and data caches of 8 KB each
• 10 stage pipeline
• Two cycle integer multiplication unit
• Branch prediction
• Dual instruction pipeline
• Instructions in the address generation unit (AGU) and thereby supply the result two cycles before the ALU
• Low cost LC = No MMU
• Low cost EC = No MMU & FPU

Feature map

Main uses

The Sega Genesis used a 68000 clocked at 7.67 MHz as its main CPU.

The 68000 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 all of the members of the Palm Pilot series that run Palm OS 1.x to 4.x (OS 5.x is ARM-based), 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 Sinclair QL, the Atari ST, and several others. The 68000 was also the processor of choice in the 1980s for Unix workstations and servers from firms such as Sun Microsystems, NeXT and Silicon Graphics (SGI). There was a 68000 version of CP/M called CP/M-68K, which was initially proposed to be the Atari ST operating system, but Atari chose Atari TOS instead.

Also, and perhaps most significantly, the first several versions of Adobe's PostScript interpreters were 68000-based. The 68000 in the Apple LaserWriter and LaserWriter Plus was clocked faster than the version used then in Macintosh computers. A fast 68030 in later PostScript interpreters, including the standard resolution LaserWriter IIntx, IIf and IIg (also 300 dpi), the higher resolution LaserWriter Pro 600 series (usually 600 dpi, but limited to 300 dpi with minimum RAM installed) and the very high resolution Linotronic imagesetters, the 200PS (1500+ dpi) and 300PS (2500+ dpi). Thereafter, Adobe generally preferred a RISC for its processor, as its competitors, with their PostScript clones, had already gone with RISCs, often an AMD 29000-series. The early 68000-based Adobe PostScript interpreters and their hardware were named for cold war U.S. rockets and missiles: Atlas, Redstone, etc.

Today, these systems are either end-of-line (in the case of the Atari), or are using different processors (in the case of Macintosh, Amiga, Sun, and SGI). Since these platforms had their marketshare peak in the 1980s, their original manufacturers are unlikely to support an operating system for this hardware or are even out of business. However, the GNU/Linux, NetBSD and OpenBSD operating systems still include support for 68000 processors.

The 68000 processors were also used in the Sega Genesis (Mega Drive) and SNK Neo Geo consoles as the main CPU. Other consoles such as the Sega Saturn used the 68000 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. Many arcade boards also used 68000 processors including boards from Capcom, SNK, and Sega.

Microcontrollers derived from the 68000 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.

It had a more orthogonal instruction set 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 68000 instruction set can be divided in the following broad categories:

• Load and store (MOVE)
• Arithmetic (ADD, SUB, MULS, MULU, DIVS, DIVU)
• Bit shifting (ASL, ASR, LSL, LSR)
• Bit rotation (ROR, ROL, ROXL, ROXR)
• Logic operations (AND, OR, NOT, EOR)
• Type conversion (byte to word and vice versa)
• Conditional and unconditional branches (BRA, Bcc - BEQ, BNE, BHI, BLO, BMI, BPL, etc.)
• Subroutine invocation and return (BSR, RTS)
• Stack management (LINK, UNLK, PEA)
• Causing and responding to interrupts
• Exception handling
• There is no equivalent to the x86 CPUID instruction to determine what CPU or MMU or FPU is present.

68050 and 68070

There was no 68050, though at one point it was a project within Motorola. Odd-numbered releases had always been reactions to issues raised within the prior even numbered part; hence, it was generally expected that the 68050 would have reduced the 68040's power consumption (and thus heat dissipation), improved exception handling in the FPU, used a smaller feature size and optimized the microcode in line with program use of instructions. Many of these optimizations were included with the 68060 and were part of its design goals. For any number of reasons, likely that the 68060 was in development, that the Intel 80486 was not progressing as quickly as Motorola assumed it would, and that 68060 was a demanding project, the 68050 was cancelled early in development.

There is also no revision of the 68060, as Motorola was in the process of shifting away from the 68000 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, likely with a superior FPU (pipelining was widely speculated upon on Usenet).

Motorola mainly used even numbers for major revisions to the CPU core such as 68000, 68020, 68040 and 68060. The 68010 was 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 licensed and somewhat slower version of the 16/32-bit 68000 with a basic DMA controller, I²C host and an on-chip serial port. 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 microarchitecture.

Other variants

The Personal Computers XT/370 and AT/370 PC-based IBM-compatible mainframes each included two modified Motorola 68000 processors with custom microcode to emulate S/370 mainframe instructions.^[11][12]

After the mainline 68000 processors' demise, the 68000 family has been used to some extent in microcontroller and 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 1980s and early 1990s, when the 68000 was widely used in desktop computers, it mainly competed against Intel's x86 architecture used in IBM PC compatibles. Generation 1 68000 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 P5 Pentium line, but it was not nearly as widely used as its predecessors, since much of the old 68000 marketplace was either defunct or nearly so (as was the case with Atari and NeXT), or converting to newer architectures (PowerPC for the Macintosh and Amiga, SPARC for Sun, and MIPS for Silicon Graphics (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 68000, while others are relatively sophisticated and capable of running complex software. Embedded versions of the 68000 often compete with processors derived from the PowerPC, ARM, MIPS, and SuperH architectures, among others.

References

Bibliography

• Howe, Dennis, ed. (1983). Free On-Line Dictionary of Computing. Imperial College, London. http://foldoc.org. Retrieved September 4, 2007.
• This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

External links

Wikibooks has a book on the topic of: 68000 Assembly

• BYTE Magazin, September 1986: The 68000 Family