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Famous Processors: The Motorola 680x0 FamilyFebruary 07, 2008The Motorola 68k is a family of 16/32-bit CISC microprocessors that were first introduced in 1979 and are still in use today, thus being on the market for 28 years. The architecture has been used in many Unix workstations as well as in famous computers such as the Apple Macintosh, the Commodore Amiga and the Atari ST.  The Motorola 68000 had eight 32-bit registers, which made it a true 32-bit processor, and it has a 24-bit address bus, allowing it to address up to 16 MB of physical memory. One of its biggest problem may have been the lack of support for virtual memory, which made it hard using the Motorola 68000 in Unix workstations (although there were workaroungs). In 1982, the Motorola 68010 added true virtual memory support which fixed this problem. The same year Motorola also released the 68008, which featured an 8-bit data bus and a smaller (20 bit) address bus.  The second generation Motorola 68020, released in 1984, finally added a 32-bit ALU and a 32-bit address bus, new instructions and addressing modes, it was available at speeds ranging from 12 MHz to 33 MHz. It also added multiprocessor support which allowed to build systems using up to eight 68020 processors or math processors at a time. But true multiprocessor machines using the 68020 were quite rare, most Unix workstations used a 68020 along with an FPU (68881 or 68882) and an MMU.  The years later, in 1990, the Motorola 68040 was released, it featured an on-chip FPU, larger on-chip caches (2* 4kb), it was fully pipelined and offered speeds of up to 40 MHz. It was used in the Apple Quadra computers, in the Amiga 4000 and in some NeXT workstations. The 68040's FPU was incapable of IEEE transcendental functions, the processor had a lot of heat problems and it didn't scale well, finally it was impossible to clock it higher than 40 MHz and a planned 50 MHz upgrade had to be cancelled. The 68060 was the last 68k chip ever developed, as Motorola abandoned the architecture in favour of the PowerPC RISC processor. It was used in some third party accelerator boards, but Apple didn't use it as they wanted to push the PowerPC and Unix workstations had already switched to RISC designs such as MIPS, SPARC or Alpha AXP for example. Today, the Motorola 68k architecture isn't used as CPU in computers and workstations anymore, but the architecture has survived as core of embedded processors such as the Motorola ColdFire and DragonBall.
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