Motorola 68000

Motorola 68000 is a CISC - processor from the 68000er-Familie von Motorola. It possesses internally 32-Bit register, one with 32 bits addressed linear address area, of it 24 bits externally available, eight 32-Bit-Datenregister, eight 32-Bit - address registers as well asa 16-Bit-Datenbus. A variant is Motorola 68008, with which the data bus to 8-bits as well as the address bus are reduced to 20 bits. As if improved version came Motorola 68010 on the market, that around errors was settled andimproved support of virtual memory offers. As Motorola 68012 one offered to 68010 with extended address bus of 31 bits. Later followed still the 68020, 68030, 68040 and 68060.

Ein MC68000, hier von Thomson Semiconductor
A MC68000, here of Thomson Semiconductor

the contents of thatupper 8-bits of an address register of 68000 with the memory access one ignores. It is possible to use these bits freely for example for Meta information to a pointer. At later processor generations from the 68000-Familie address bus external with 32 bits this leads obviouslyto problems. There this kind of programming however on the early Macintosh - computers, later Macintosh versions with 68020-Prozessor without MEMORY management unit (MMU) was very common a special component, had called ADDRESS management unit (AMU), in place of the MMU, that thoseupper 8-bits of addresses optionally out-masked, in order to be able to use old software in a 24-Bit compatibility mode.

The 68000 has only 16 bits ALU and also 16 bits an instruction set, however both with 8, 16 - as also 32-bittigen the data typesto go around can (32bit architecture).

Example of a typical M68000 command: “JSR <I/O> “Jump ton of subroutine

Instruction format: 0100 1110 10mm mrrr

the command is long 16 bits. Three bits (m) are used for the addressing mode, three further bits for that(r) register used and/or. a DISPLACEMENT.

“becomes 0100 1110 1011 1010” in such a way “JSR (d16, PC)”. Since the PC relative addressing with DISPLACEMENT is used here, the following word is used as signed DISPLACEMENT. Other commands, as “MOVEQ are #n, Dx”even completely in 16 bits codes.

The ability of the M68000 with nearly all commands also 32-Bit broad addresses and data to indicate (then naturally with a 16-Bit the bus in several cycles to be read had), simplified the software development uncommonly, there itquasi 32 bits an architecture concerned.

Do not arise like e.g. with 8086 problems to address data records over 64 Kib size. Because of the 16 bits ALU and 16 bits the Datenbuses becomes the 68000 often 16 bitsProcessor mentioned, it implements however complainless 32 bits software. Because of the 16 bits e.g. costs ALU. 32 bits an addition the double time. The circuit for the generation of addresses has however full 32 bits, so that e.g. scanning oneText with 8-bit broad indications is not slower, than from “pure” 16 bitter to would be to be expected. Besides the text can be longer than 64 Kib, without modification of the program. 68000 software is 32 bits software.

As processor alsoNano-code and Microcode are to a certain extent programmable on hardware level by the manufacturer the 68000. That led to as interesting variants as that 68000-360, that directly on the chip a abgespeckte version IBM 360 of command sentence implementing could and for a small360 model one used by IBM. Over the workstations of Apollo Domain and Sun and the Amiga the CCU also their way found 2500 UX into the Unix - world - owing to optional MMU.

In addition this made a simple way possible around variantsto produce the chip. First of it was the 68008 with which only the nano-code was in such a way changed that the data bus was only addressed with 8-bit width. A further variant was substantially better the 68010, with virtualMemory to be operated knew and besides still some detail improvements exhibited. A folder of 68010 was not that long available 68012, which made more address lines available for larger memory. Later variants were usually derived directly from the original CCU, howe.g. the 680EC20 of 68020 (24 instead of 32 address lines) - or the 68EC040 and the 680LC40 of 68040 (it either the FPU or the MMU was then missing). Starting from that 68040 became step by step also first 64-Bit - instructions integrates (forsimple multiplications).

Over three binary coded interrupt lines interrupts can be released with seven different priorities, of it are not maskable the highest. Together with appropriate periphery components are possible up to 256 vectored interrupts.

Furthermore the Prozesser contains control lines, which signal whethernext an instruction or data to be gotten. This makes the structure of computers with separate memory for programs and data (Harvard architecture)

the designation for 68000 possible comes according to a modern legend of the number of transistors, those the processorpossesses. Actually it is however the logical continuation of the type name of the older 8-bits Motorola processor family Motorola 6800.

It among other things used in computers of Apple (Lisa and Macintosh), in the Commodore Amiga, Atari sp and Sinclair QL (68008) as well as in play consoles like the Sega mega drive or the NeoGeo. Also in pocket calculators of Texas Instruments like the TI-89, TI-92 (plus) and Voyage 200 or chess computers (like e.g. in the Fidelity Mach IV as 68020with 20 MHz and Mephisto Amsterdam as 68000 with 12 MHz) the processor one used. In the airbus A320 it steers the elevator aileron computer (ELAC), the part flight controls - system is.

The main operational area of the processor were and are, owing to thatExcellencies interrupt architecture, control computer in the industry, usually with a real time processing system such as OS-9, Nucleus or Linux68k.

The Highend models of the 68000-Familie are the 68040 and 68060 (to 66 and/or. 75 MHz). There Apple after that 68040 on PowerPC - CCUs changedis, became the 68060 only in extension cards for the Amiga and for special applications (e.g. in Embedded of system as in BMW - models of the 1990er ) uses years or high speedable chess computers. With the change also of Amiga on PowerPC was then finally the end reaches - the strategic interests of Motorola were with Apple and IBM.

Meanwhile actual 68000 practically disappeared. Motorola manufactures however a multiplicity of Microcontrollern, which descend from 68000. Motorola Coldfire - family (more abgespeckter 68020and/or. 68040) is further in the Embedded control range much likes and the DragonBall propels still the a riser models of the Palm PDAs and is common in mobile phones far.

A prominent example of a Coldfire based (and still to it Amiga - inspired) system was those Phoenix/Metabox 1000 - SetTopBox of the company Metabox, who however never came on the market.


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