68k
Encyclopedia : 6 : 68 : 68K : 68k
The Motorola 680x0/0x0/m68k/68k/68K family of CISC microprocessor CPU chips were 32-bit from the start, and were the primary competition for the Intel x86 family of chips.
The 68k family members
- Generation one
- * Motorola 68000 a hybrid 16/32 bit chip (16-bit bus)
- * Motorola 68EC000
- * Motorola 68HC000
- * Motorola 68008 a hybrid 8/16/32 bit chip (8-bit bus)
- * Motorola 68010
- * Motorola 68012
- Generation two (fully 32-bit)
- * Motorola 68020
- * Motorola 68EC020
- * Motorola 68030
- * Motorola 68EC030
- Generation three (fully 32-bit)
- * Motorola 68040
- * Motorola 68EC040
- * Motorola 68LC040
- Generation four (fully 32-bit)
- * Motorola 68060
- Others
- * Motorola CPU32 (aka Motorola 68330)
- * Motorola 68360 (aka QUICC)
- * Motorola ColdFire
- * Motorola DragonBall
Main uses
The 68k line of processors have been used in a variety of systems, from Texas Instruments TI-89 calculators up to critical control systems of the Space Shuttle. However, they have become most well-known as processors powering desktop computers such as the Apple Macintosh, the Commodore Amiga, the Atari ST, and others.
Today, these desktop systems are either end-of-line (in the case of the Atari), or are using different processors (as is the case for the Amiga and the Macintosh). Since these desktops are now more than a decade old, the original manufacturers are either out of business, or no longer provide an operating system for the hardware; however, the Debian, NetBSD and OpenBSD operating systems are still supported for m68k processors.
The 68k processors were also used in the Sega Genesis, SNK Neo Geo and Atari Jaguar consoles as the main CPU. Other consoles such as the Sega Saturn also used the 68k as an audio processor and other IO tasks.
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 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 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
Where did the 68050 go? Was there no -070?
Note that there is no 68050, this is because 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.
There was a CPU with the 68070 designation, which was a microcontroller version of the 16-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.
The next 68k generation
The 4th generation 68060 shared most of the features of the Intel P5 architecture of x86. Had Motorola decided to stick with the 680x0 series, it is very likely that the next processor (68080) would 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, and the DragonBall.
Competitors to the mainstream 68ks
The principal competitors in the microcomputer market for generation one were the x86 architecture 8086/8088 first-generation and 80286 second-generation IA-16 chips. For generation two, it was the 80386 IA-32 chips, and for generation three it was the 80486 IA-32 chips. Generation four did compete against the Pentium IA-32 chips, but to a lesser extent, as much of the hitherto 68k marketplace was shifting over to the PowerPC, sounding the death knell for the 680x0 on the desktop.
- This article was originally based on material from the Free On-line Dictionary of Computing, which is [Foldoc licenselicensed] under the GFDL.
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