Motorola 68000

The Motorola 68000, also referred to as the 68000 or 68k, is a 16/32-bit CISC processor that was produced by Motorola's Semiconductor Products Sector. The 68000 was used in the Apple Lisa and early models of Amiga, Atari ST, and Macintosh computers.

History
Development on the 68000 began in 1976 as the Motorola Advanced Computer System on Silicon (MACSS) project to create a new design to replace the Motorola 6800, which had been surpassed by competitors such as the MOS 6502. It contained a 32-bit instruction set which ran at high speeds for the era, a 24-bit address bus which could address up to 16 MB of RAM, and 16-bit external data bus.

The original 68000 was announced in September 1979 and was fabricated using a HMOS process with a 3.5 µm feature size. Initial samples were released in February 1980, with production chips becoming publicly available in November.

The performance improvements of the 68000 over previous microprocessors led to a 5MHz version being used in the Apple Lisa to help drive its graphical user interface. In January 1983, Motorola provided custom batches, marked "SC87839L", to Apple's Macintosh development team. These units have appeared in early Macintosh prototypes. An 8 Mhz version (clocked to 7.8336 MHz) would be used in the original Macintosh 128K.

Mac models with 68000 processors
Dual in-line package (DIP) version: Quad flat package (QFP) version:
 * Macintosh 128K (7.8336 MHz, January 24, 1984)
 * Macintosh 512K (7.8336 MHz, September 10, 1984)
 * Macintosh XL (5 MHz, January 1, 1985)
 * Macintosh Plus (7.8336 MHz, January 16, 1986)
 * Macintosh 512Ke (7.8336 MHz, April 14, 1986)
 * Macintosh SE (7.8336 MHz, March 2, 1987)
 * Macintosh Classic (7.8336 MHz, September 14, 1992)

Variants
The 68000 design was revised many times and used in subsequent Macintosh models:
 * Motorola 68HC000 - a low power CMOS version, used in the Macintosh Portable and PowerBook 100 at 16MHz each.
 * Motorola 68010 - a modestly updated version, which was not used in any Macintosh model.
 * Motorola 68020 - a fully 32-bit version with a 3-stage pipeline, used in the Macintosh II and Macintosh LC.
 * Motorola 68030 - an update to the the 68020 which incorporated a memory management unit, used in early PowerBooks and the rest of the Macintosh II series, which added built-in support for virtual memory.
 * Motorola 68040 - a high-performance version with a 6-stage pipeline, used in the Centris, Quadra, and PowerBook 500 series.
 * Motorola 68050 - an update to the 68040 with a planned 64-bit bus and built-in graphics acceleration that was cancelled to redirect resources to the 68060 and a low-power version of the 68040.
 * Motorola 68060 - a faster superscalar version that was never adopted by Apple Computer as it had instead migrated to PowerPC processors for its Power Macintosh line. A DayStar accelerator had been announced, but was never released.