Motorola 68000 Architecture.pdf

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The Motorola 68000 (sometimes shortened to Motorola 68k or m68k and usually pronounced "sixty-eight-thousand")[2][3] is a 16/32-bit complex instruction set computer (CISC) microprocessor, introduced in 1979 by Motorola Semiconductor Products Sector.

As one of the first widely available processors with a 32-bit instruction set, large unsegmented address space, and relatively high speed for the era, the 68k was a popular design through the 1980s. It was widely used in a new generation of personal computers with graphical user interfaces, including the Macintosh 128K, Amiga, Atari ST, and X68000. The 1988 Sega Genesis/Mega Drive console is powered by a 68000.

Later processors in the Motorola 68000 series, beginning with the Motorola 68020, use full 32-bit ALUs and have full 32-bit address and data buses. The original 68k is generally software forward-compatible with the rest of the line despite being limited to a 16-bit wide external bus.[4]

Formally introduced in September 1979,[23] initial samples were released in February 1980, with production chips available over the counter in November.[24] Initial speed grades are 4, 6, and 8 MHz. 10 MHz chips became available during 1981,[25] and 12.5 MHz chips by June 1982.[24] The 16.67 MHz "12F" version of the MC68000, the fastest version of the original HMOS chip, was not produced until the late 1980s.

In 1982, the 68000 received a minor update to its instruction set architecture (ISA) to support virtual memory and to conform to the Popek and Goldberg virtualization requirements. The updated chip is called the 68010.[27] It also adds a new "loop mode" which speeds up small loops, and increases overall performance by about 10% at the same clock speeds. A further extended version, which exposes 31 bits of the address bus, was also produced in small quantities as the 68012.

To support lower-cost systems and control applications with smaller memory sizes, Motorola introduced the 8-bit compatible MC68008, also in 1982. This is a 68000 with an 8-bit data bus and a smaller (20-bit) address bus. After 1982, Motorola devoted more attention to the 68020 and 88000 projects.

Several other companies were second-source manufacturers of the HMOS 68000. These included Hitachi (HD68000), who shrank the feature size to 2.7 µm for their 12.5 MHz version,[24] Mostek (MK68000), Rockwell (R68000), Signetics (SCN68000), Thomson/SGS-Thomson (originally EF68000 and later TS68000), and Toshiba (TMP68000). Toshiba was also a second-source maker of the CMOS 68HC000 (TMP68HC000).

Encrypted variants of the 68000, being the Hitachi FD1089 and FD1094, store decryption keys for opcodes and opcode data in battery-backed memory and were used in certain Sega arcade systems including System 16 to prevent piracy and illegal bootleg games.[28]

The later evolution of the 68000 focused on more modern embedded control applications and on-chip peripherals. The 68EC000 chip and SCM68000 core remove the M6800 peripheral bus, and exclude the MOVE from SR instruction from user mode programs, making the 68EC000 and 68SEC000 the only 68000 CPUs not 100% object code compatible with previous 68000 CPUs when run in User Mode. When run in Supervisor Mode, there is no difference.[31] In 1996, Motorola updated the standalone core with fully static circuitry, drawing only 2 µW in low-power mode, calling it the MC68SEC000.[32]

Motorola ceased production of the HMOS MC68000, as well as the MC68008, MC68010, MC68330, and MC68340 in on June 1, 1996,[33][34] but its spin-off company Freescale Semiconductor was still producing the MC68HC000, MC68HC001, MC68EC000, and MC68SEC000, as well as the MC68302 and MC68306 microcontrollers and later versions of the DragonBall family. The 68000's architectural descendants, the 680x0, CPU32, and Coldfire families, were also still in production. More recently, with the Sendai fab closure, all 68HC000, 68020, 68030, and 68882 parts have been discontinued, leaving only the 68SEC000 in production.[35]

IBM considered the 68000 for the IBM PC but chose the Intel 8088 because the 68000 was not ready; Walden C. Rhines wrote that thus "Motorola, with its superior technology, lost the single most important design contest of the last 50 years".[37] (IBM Instruments briefly sold the 68000-based IBM System 9000 laboratory computer systems.) The 68k instruction set is particularly well suited to implement Unix,[38] and the 68000 and its successors became the dominant CPUs for Unix-based workstations including Sun workstations and Apollo/Domain workstations.

In 1981, Motorola introduced the Motorola 68000 Educational Computer Board, a single-board computer for educational and training purposes which in addition to the 68000 itself contained memory, I/O devices, programmable timer and wire-wrap area for custom circuitry. The board remained in use in US colleges as a tool for learning assembly programming until the early 1990s.[39]

At its introduction, the 68000 was first used in high-priced systems, including multiuser microcomputers like the WICAT 150,[40] early Alpha Microsystems computers, Sage II / IV, Tandy 6000 / TRS-80 Model 16, and Fortune 32:16; single-user workstations such as Hewlett-Packard's HP 9000 Series 200 systems, the first Apollo/Domain systems, Sun Microsystems' Sun-1, and the Corvus Concept; and graphics terminals like Digital Equipment Corporation's VAXstation 100 and Silicon Graphics' IRIS 1000 and 1200. Unix systems rapidly moved to the more capable later generations of the 68k line, which remained popular in that market throughout the 1980s.

By the mid-1980s, falling production cost made the 68000 viable for use in personal and home computers, starting with the Apple Lisa and Macintosh, and followed by the Commodore Amiga, Atari ST, and Sharp X68000.

While the adoption of RISC and x86 displaced the 68000 series as desktop/workstation CPU, the processor found substantial use in embedded applications. By the early 1990s, quantities of 68000 CPUs could be purchased for less than 30 USD per part.[citation needed]

The 68000 also saw great success as an embedded controller. As early as 1981, laser printers such as the Imagen Imprint-10 were controlled by external boards equipped with the 68000. The first HP LaserJet, introduced in 1984, came with a built-in 8 MHz 68000. Other printer manufacturers adopted the 68000, including Apple with its introduction of the LaserWriter in 1985, the first PostScript laser printer. The 68000 continued to be widely used in printers throughout the rest of the 1980s, persisting well into the 1990s in low-end printers.

The 68000 was successful in the field of industrial control systems. Among the systems benefited from having a 68000 or derivative as their microprocessor were families of programmable logic controllers (PLCs) manufactured by Allen-Bradley, Texas Instruments and subsequently, following the acquisition of that division of TI, by Siemens. Users of such systems do not accept product obsolescence at the same rate as domestic users, and it is entirely likely that despite having been installed over 20 years ago, many 68000-based controllers will continue in reliable service well into the 21st century.

In a number of digital oscilloscopes from the 80s,[41] the 68000 has been used as a waveform display processor; some models including the LeCroy 9400/9400A[42] also use the 68000 as a waveform math processor (including addition, subtraction, multiplication, and division of two waveforms/references/waveform memories), and some digital oscilloscopes using the 68000 (including the 9400/9400A) can also perform fast Fourier transform functions on a waveform.

The 683XX microcontrollers, based on the 68000 architecture, are used in networking and telecom equipment, television set-top boxes, laboratory and medical instruments, and even handheld calculators. The MC68302 and its derivatives have been used in many telecom products from Cisco, 3com, Ascend, Marconi, Cyclades and others. Past models of the Palm PDAs and the Handspring Visor used the DragonBall, a derivative of the 68000. AlphaSmart used the DragonBall family in later versions of its portable word processors. Texas Instruments used the 68000 in its high-end graphing calculators, the TI-89 and TI-92 series and Voyage 200.

Video game manufacturers used the 68000 as the backbone of many arcade games and home game consoles: Atari's Food Fight, from 1982, was one of the first 68000-based arcade games. Others included Sega's System 16, Capcom's CP System and CPS-2, and SNK's Neo Geo. By the late 1980s, the 68000 was inexpensive enough to power home game consoles, such as Sega's Sega Genesis console and also the Sega CD attachment for it (A Sega CD system has three CPUs, two of them 68000s). The multi-processor Atari Jaguar console from 1993 used a 68000 as a support chip, although some developers used it as the primary processor due to familiarity. The Sega Saturn console used the 68000 as a sound co-processor. In October 1995, the 68000 made it into a handheld game console, Sega's Genesis Nomad, as its CPU.[43]

Certain arcade games (such as Steel Gunner and others based on Namco System 2) use a dual 68000 CPU configuration,[44] and systems with a triple 68000 CPU configuration also exist (such as Galaxy Force and others based on the Sega Y Board),[45] along with a quad 68000 CPU configuration, which has been used by Jaleco (one 68000 for sound has a lower clock rate compared to the other 68000 CPUs)[46] for games such as Big Run and Cisco Heat; another, fifth 68000 (at a different clock rate than the other 68000 CPUs) was used in the Jaleco arcade game Wild Pilot for input/output (I/O) processing.[47]

The 68000 has a 24-bit external address bus and two byte-select signals "replaced" A0. These 24 lines can therefore address 16 MB of physical memory with byte resolution. Address storage and computation uses 32 bits internally; however, the 8 high-order address bits are ignored due to the physical lack of device pins. This allows it to run software written for a logically flat 32-bit address space, while accessing only a 24-bit physical address space. Motorola's intent with the internal 32-bit address space was forward compatibility, making it feasible to write 68000 software that would take full advantage of later 32-bit implementations of the 68000 instruction set.[4] 2b1af7f3a8