| Designer | Motorola |
|---|---|
| Bits | 16/32-bit |
| Introduced | 1979 |
| Design | CISC |
| Endianness | Big |
| Registers | |
| General purpose | 8x 32-bit |
The Motorola 680x0/m68k/68k/68K is a family of 32-bit 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. Although no modern desktop computers are based on the 68k, derivative processors are still widely used in embedded applications. As of 1998, the 68K family was the best-selling 32-bit architecture in the world. 79 million 68K chips were sold in 1997, compared to about 75 million Pentium-based PCs.[1]
Contents |
Family members
- Generation one (internally 16/32-bit, and produced with 8-, 16-, and 32-bit interfaces)
- Generation two (fully 32-bit)
- Generation three (pipelined)
- Generation four (superscalar)
- Others
- Freescale 683XX (CPU32 aka 68330, 68360 aka QUICC)
- Freescale ColdFire
- Freescale DragonBall
Improvement roadmap
The 68020 improved this:
- 32-bit address & ALU.
- 3 stage pipeline.
- Instruction cache of 256 bytes.
- Unrestricted word and longword data access (see alignment).
- 8 x multiprocessing capability.
- 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.
- Split instruction and data cache of 256 bytes each
- On-chip MMU (68851).
- Low cost EC = No MMU.
- Instruction and data caches of 4 kilobytes each
- 6 stage pipeline.
- FPU lacks IEEE transcendental functions capability.
- FPU emulation works with 2E71M and later chip revisions.
- Low cost LC = No FPU.
- Low cost EC = No FPU & MMU.
- Integer 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.
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 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 Sinclair QL, 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 two decades 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)
- Logic operations (And, Or, Not, EOr)
- Type conversion (byte to word and vice versa)
- Conditional and unconditional branches (Bra, BCS, BEq, BNE, BHI, BLO, BMI, BPL, etc.)
- Subroutine invocation and return (BSR, RTS)
- Stack management (push, pop)
- Causing and responding to interrupts
- Exception handling
Efficiency
The 68000 CPU uses at least 4 clock cycles per instruction. 68020 uses 3 cycles at minimum. 68030 and 68040 have a more complex instruction processing but require approximately 2.5 to 1.25 cycles. [2]
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 previous 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 80486 wasn't 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 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, 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 architecture. Had Motorola decided to continue the 680x0 series, the next processor (68080) would likely have resembled Intel's P6 architecture.[citation needed]
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 — other than a small minority of Transmeta VLIW processors — 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
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Wikibooks has a book on the topic of |
- Howe, Dennis, ed. (1983). Free On-Line Dictionary of Computing. Imperial College, London. http://foldoc.org. Retrieved September 4, 2007.
- This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
- ^ Turley, Jim (1998). "MCore: Does Motorola Need Another Processor Family?".
- ^ "Tradechess". http://www.tradechess.com.br/artigos.htm. 090428 tradechess.com.br
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