The Motorola 68000 is a 16/32-bit ^[1] CISC microprocessor core designed and marketed by Freescale Semiconductor (formerly Motorola Semiconductor Products Sector). A complex instruction set computer ( CISC, pronounced like " sisk " is a Microprocessor Instruction set architecture (ISA in which A microprocessor incorporates most or all of the functions of a Central processing unit (CPU on a single Integrated Freescale Semiconductor Inc is an American Semiconductor manufacturer Motorola Inc ( is an American, multinational Fortune 100, Telecommunications company based in Schaumburg Illinois. Introduced in 1979 as the first member of the successful 32-bit m68k family of microprocessors, it is generally software forward compatible with the rest of the line despite belonging to the 16-bit hardware technology generation. The range of Integer values that can be stored in 32 bits is 0 through 4294967295 or −2147483648 through 2147483647 using Two's complement encoding The Motorola 680x0 / m68k / 68k / 68K is a family of 32-bit CISC Microprocessor CPU chips and was the primary Forward compatibility (sometimes confused with Extensibility) is the ability of a system to gracefully accept Input intended for later versions of itself After twenty-nine years in production, the 68000 architecture is still in use. Semiconductor device fabrication is the process used to create chips the Integrated circuits that are present in everyday Electrical and electronic In Computer engineering, computer architecture is the conceptual design and fundamental operational structure of a Computer system

Pre-release XC68000 chip manufactured in 1979.

History

The 68000 grew out of the MACSS (Motorola Advanced Computer System on Silicon) project, begun in 1976 to develop an entirely new architecture without backward compatibility. Year 1976 ( MCMLXXVI) was a Leap year starting on Thursday (link will display full calendar of the Gregorian calendar. In Technology, especially Computing (irrespective of platform a product is said to be backward compatible when it is able to take the place of an older product It would be a higher-power sibling complementing the existing 8-bit 6800 line rather than a compatible successor. The 6800 is an 8-bit Microprocessor produced by Motorola and released shortly after the Intel 8080 in late 1974 In the end, the 68000 did retain a bus protocol compatibility mode for existing 6800 peripheral devices, and a version with an 8-bit data bus was produced. In computing, a protocol is a convention or standard that controls or enables the connection Communication, and Data transfer between two computing However, the designers mainly focused on the future, or forward compatibility, which gave the M68K platform a head start against later 32-bit instruction set architectures. Forward compatibility (sometimes confused with Extensibility) is the ability of a system to gracefully accept Input intended for later versions of itself In Computing, a platform describes some sort of Hardware architecture or Software framework (including Application frameworks, that allows An instruction set is a list of all the instructions and all their variations that a processor can execute For instance, the CPU registers are 32 bits wide, though few self-contained structures in the processor itself operate on 32 bits at a time. The 68000 may be considered a 16-bit microprocessor which is microcoded to accelerate 32-bit tasks. Microprogramming (ie writing microcode) is a method that can be employed to implement Machine instructions in a CPU relatively easily often using less The MACSS team drew heavily on the influence of minicomputer processor design, such as the PDP-11 and VAX systems, which were similarly microcoded. A minicomputer (colloquially mini) is a class of multi-user Computers that lies in the middle range of the computing spectrum in between the largest Multi-user The PDP-11 was a series of 16-bit Minicomputers sold by Digital Equipment Corp Name "VAX" was originally an Acronym for V irtual A ddress e' X' tension, both because the VAX was seen as a 32-bit

In the mid 1970s, the 8-bit processor manufacturers raced to introduce the 16-bit generation. This article is about the Decade 1970-1979 For the Year 1970 see 1970. National Semiconductor had been first with its IMP-16 and PACE processors in 1973-1975, but these had issues with speed. National Semiconductor ( is a Semiconductor manufacturer specializing in analog devices and subsystems headquartered in Santa Clara, California, The IMP-16, by National Semiconductor, was the first multi-chip 16-bit Microprocessor. The Intel 8086 in 1977 quickly gained popularity. The 8086 is a 16-bit Microprocessor chip designed by Intel and introduced on the market in 1978 which gave rise to the X86 architecture The decision to leapfrog the competition and introduce a hybrid 16/32-bit design was necessary, and Motorola turned it into a coherent mission. Leapfrog is a Children's game in which players vault over each other's stooped Backs The first participant rests hands on knees and bends over this is called Arriving late to the 16-bit arena afforded the new processor more transistors (roughly 40000 active versus 20000 active in the 8086), 32-bit macroinstructions, and acclaimed general ease of use.

The original MC68000 was fabricated using an HMOS process with a 3. Semiconductor device fabrication is the process used to create chips the Integrated circuits that are present in everyday Electrical and electronic HMOS, high-performance n-channel MOS, also called depletion-load NMOS, is a Digital circuit style which uses n-type MOSFETs to implement 5-micron (3500 nanometre) feature size. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre Initial engineering samples were released in late 1979. Year 1979 ( MCMLXXIX) was a Common year starting on Monday (link displays the 1979 Gregorian calendar) Production chips were available in 1980, with initial speed grades of 4, 6, and 8 MHz. Year 1980 ( MCMLXXX) was a Leap year starting on Tuesday (link displays the 1980 Gregorian calendar) The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. 10 MHz chips became available during 1981, and 12. Year 1981 ( MCMLXXXI) was a Common year starting on Thursday (link displays the 1981 5 MHz chips during 1982. Year 1982 ( MCMLXXXII) was a Common year starting on Friday (link displays the 1982 Gregorian calendar) The 16. 67 MHz "12F" version of the MC68000, the fastest version of the original HMOS chip, was not produced until the late 1980s.

The 68000 had many high-end design wins early on. It became the dominant CPU for Unix based workstations including Sun workstations and Apollo/Domain worksations, found its way into heralded computers such as the Amiga, Atari ST, Apple Lisa and Macintosh, and was used in the first generation of desktop laser printers including the original Apple LaserWriter. Sun Microsystems Inc ( is a multinational vendor of Computers computer components Computer software, and Information technology services The Amiga is a family of Personal computers originally developed by Amiga Corporation. The Atari ST is a home / Personal computer that was commercially available from 1985 to the early 1990s For the MOS 6502 assembler for Apple II computers see Lisa assembler. The Macintosh is the original Apple Macintosh Personal computer. A laser printer is a common type of Computer printer that rapidly produces high quality text and graphics on plain paper The apple is the pomaceous Fruit of the apple tree Species Malus domestica in the Rose family Rosaceae. The Apple LaserWriter was one of the first Laser printers available to the mass market In 1982, the 68000 received an update to its ISA allowing it to support virtual memory by conforming to the Popek and Goldberg virtualization requirements. An instruction set is a list of all the instructions and all their variations that a processor can execute Virtual memory is a Computer system technique which gives an application program the impression that it has contiguous working memory while in fact it may be physically The Popek and Goldberg virtualization requirements are a set of sufficient conditions for a computer architecture to efficiently support system Virtualization. The updated chip was called the 68010. The Motorola MC68010 processor is a 16/32-bit Microprocessor from Motorola, released in 1982. A further extended version which exposed 31 bits of the address bus was also produced, in small quantities, as the 68012. The Motorola MC68012 processor is a 16/32-bit Microprocessor from the early 1980s

To support lower-cost systems and control applications with smaller memory sizes, Motorola introduced the 8-bit compatible MC68008, also in 1982. The Motorola 68008 is an 8/16/32-bit Microprocessor made by Motorola. This was 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. The Motorola 68020 is a 32-bit Microprocessor from Motorola, released in 1984. The 88000 ( m88k for short is a Microprocessor design produced by Motorola.

Other manufacturers

The 68HC000, the CMOS version of the 68000, was designed by Hitachi and jointly introduced in 1985. Complementary metal–oxide–semiconductor ( CMOS) (pronounced "see-moss" siːmɔːs ˈsiːmɒs is a major class of Integrated circuits CMOS technology ^[2] Motorola's version was called the MC68HC000, while Hitachi's was the HD68HC000. The 68HC000 was eventually offered at speeds from 8 MHz to 20 MHz. Except for using CMOS circuitry, it behaved identically to the HMOS MC68000, but the change to CMOS greatly reduced its power consumption. The original HMOS MC68000 consumed around 1. 35 watts at an ambient temperature of 25 °C, regardless of clock speed, while the MC68HC000 consumed only 0. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. The Celsius Temperature scale was previously known as the centigrade scale. 13 watts at 8 MHz and 0. 38 watts at 20 MHz. (Unlike CMOS circuits, HMOS still draws power when idle, so power consumption varies little with clock rate. ) Motorola replaced the MC68008 with the MC68HC001 in 1990. ^[3] This chip resembled the 68HC000 in most respects, but its data bus could operate in either 16-bit or 8-bit mode, depending on the value of an input pin at reset. Thus, like the 68008, it could be used in systems with cheaper 8-bit memories.

Several other companies were second-source manufacturers of the HMOS 68000. In the Electronics industry a second source is a company that is licensed to manufacture and sell components originally designed by another company (the first source These included Hitachi (HD68000), Mostek (MK68000), Rockwell (R68000), Signetics (SCN68000), Thomson/SGS-Thomson (originally EF68000 and later TS68000), and Toshiba (TMP68000). Mostek Podlaskie Voivodeship Mostek was an Integrated circuit manufacturer founded in 1969 by ex-employees of Texas Instruments. Rockwell International was the ultimate incarnation of a series of companies under the sphere of influence of Willard Rockwell, who had made his fortune after the invention and Signetics, once a major player in semiconductor manufacturing made a variety of devices which included Integrated circuits bipolar and MOS, the Dolby Thomson-CSF was a major electronics and Defense contractor. In December 2000 it was renamed Thales Group. STMicroelectronics (,)is an franco-italian Electronics and Semiconductor manufacturer headquartered in in Geneva, Switzerland. ( is a multinational conglomerate manufacturing company headquartered in Tokyo, Japan. Toshiba was also a second-source maker of the CMOS 68HC000 (TMP68HC000).

As a microcontroller core

After being succeeded by "true" 32-bit microprocessors, the 68000 was used as the core of many microcontrollers. A microcontroller (also MCU or µC is a functional Computer system-on-a- chip. In 1989, Motorola introduced the MC68302 communications processor,^[4]. In 1991 Motorola introduced a separate processor chip based on this core, the MC68EC000. ^[5] In 1996 Motorola introduced this static core as a separate processor, the MC68SEC000. ^[6]

Motorola ceased production of the HMOS MC68000 and MC68008 in 1996,^[7] but its spin-off company, Freescale Semiconductor, is still producing the MC68HC000, MC68HC001, MC68EC000, and MC68SEC000, as well as the MC68302 and MC68306 microcontrollers and later versions of the DragonBall family. Year 1996 ( MCMXCVI) was a Leap year starting on Monday (link will display full 1996 Gregorian calendar) Freescale Semiconductor Inc is an American Semiconductor manufacturer The 68000's architectural descendants, the 680x0, CPU32, and Coldfire families, are also still in production.

Applications

The 68000 was first used during the early 1980s in high-priced systems, including multiuser microcomputers like the WICAT 150 [1], Tandy 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. microcomputer is a Computer with a Microprocessor as its Central processing unit. Tandy Corporation was a family-owned Leather goods company based in Fort Worth Texas, which is best known for purchasing and giving its name to the Fort Worth TRS-80 was Tandy Corporation 's desktop Microcomputer model line sold through Tandy's Radio Shack stores in the late 1970s and early A workstation, such as a Unix workstation, RISC workstation or Engineering workstation, is a high-end Microcomputer HP 9000 is the name for a line of Workstation and server Computer systems produced by the Hewlett-Packard (HP company Sun Microsystems Inc ( is a multinational vendor of Computers computer components Computer software, and Information technology services Sun-1 was the first generation of UNIX Computer workstations and servers produced by Sun Microsystems, launched in May 1982 Corvus Systems was a technology company founded by Michael D'Addio and Mark Hahn in 1979 and located in San Jose, Silicon Valley in the U A computer terminal is an electronic or electromechanical hardware device that is used for entering data into and displaying data from a Computer or a Computing Digital Equipment Corporation was a pioneering American company in the Computer industry Name "VAX" was originally an Acronym for V irtual A ddress e' X' tension, both because the VAX was seen as a 32-bit Silicon Graphics Inc (commonly initialised to SGI, historically sometimes referred to as Silicon Graphics Computer Systems or SGCS) is a company Unix systems rapidly moved to the more capable later generations of the 68k line, which remained popular in that market throughout the 1980s. Unix (officially trademarked as UNIX, sometimes also written as Unix with Small caps) is a computer

During the mid 1980s, the 68000 was first used in personal and home computers, starting with the Apple Lisa and Macintosh, and followed by the Commodore Amiga, Atari ST, and Sharp X68000. A personal computer ( PC) is any Computer whose original sales price size and capabilities make it useful for individuals and which is intended to be operated A home computer was a class of Personal computer entering the market in 1977 and becoming common during the 1980s Apple Inc, ( formerly Apple Computer Inc, is an American Multinational corporation with a focus on designing and manufacturing Consumer electronics For the MOS 6502 assembler for Apple II computers see Lisa assembler. Macintosh, commonly nicknamed Mac is a Brand name which covers several lines of Personal computers designed developed and marketed by Apple Inc Commodore, the commonly used name for Commodore International, was a US-American Electronics company based in West Chester Pennsylvania The Amiga is a family of Personal computers originally developed by Amiga Corporation. Atari is a corporate and brand name owned by several entities since its inception in 1972. The Atari ST is a home / Personal computer that was commercially available from 1985 to the early 1990s () is a Japanese Electronics manufacturer founded in 1912 It takes its name from one of its founder's first inventions the Ever-Sharp Mechanical pencil, which The Sharp X68000, often referred to as the X68k, is a home computer released only in Japan by the Sharp Corporation. The 68008, on the other hand, was only used in one home computer system, the Sinclair QL (though the QL was a sister machine to the ICL One Per Desk, which also used a 68008). The Sinclair QL (for Quantum Leap) was a Personal computer launched by Sinclair Research in 1984 as the successor to the ZX Spectrum

The 68000 eventually saw its greatest success as a controller. As early as 1981, laser printers such as the Imagen Imprint-10 were driven by external controllers using the 68000 as the CPU. A laser printer is a common type of Computer printer that rapidly produces high quality text and graphics on plain paper Imagén is a Puerto Rican fashion Magazine. It is printed monthly by Casiano Communications. The first HP LaserJet, introduced in 1984, used an 8 MHz 68000 in its built-in controller. LaserJet is the brand name used by the American computer company Hewlett-Packard (HP for their line of dry electrophotographic (DEP Laser printers Similar 68000-based integrated controllers were subsequently used in many other laser printers, including the Apple LaserWriter, the first PostScript laser printer, introduced in 1985. The Apple LaserWriter was one of the first Laser printers available to the mass market PostScript ( PS) is a dynamically typed concatenative Programming language created by John Warnock and Charles Geschke in 1982 The 68000 continued to be widely used in laser printers throughout the rest of the 1980s, persisting well into the 1990s in low-end printers.

Outside traditional commercial or domestic computing applications, the 68000 also saw success in the field of industrial control systems. Among the systems which 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. A programmable logic controller ( PLC) or programmable controller is a Digital computer used for Automation of industrial processes such as Texas Instruments ( better known in the electronics industry (and popularly as TI, is an American company based in Dallas, Texas, USA 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.

As technological advances obsoleted the 68000 from use in the standalone computing market, its use grew in consumer and embedded applications. Video game manufacturers used the 68000 as the backbone of many arcade games and home game consoles. A video game is a Game that involves interaction with a User interface to generate visual feedback on a video device. An arcade game is a coin-operated entertainment machine typically installed in businesses such as Restaurants Pubs Video arcades and Family Entertainment Atari's Food Fight, from 1983, was one of the first 68000-based arcade games. Food Fight is a 1983 Arcade game by Atari. The player guides an ambitious youth trying to eat an Ice cream cone before it melts The 68000 was the main CPU of many arcade systems during the late 1980s and early 1990s, such as Sega's System 16, Capcom's CPS-1 and CPS-2, and SNK's Neo Geo. An arcade system board is a dedicated Computer system created for the purpose of running Arcade games. is a multinational Video game Software and Hardware development company and a former Home computer The following is a list of Arcade system boards released by Sega. For the original NASA meaning see capsule communicator. is a leading Japanese International developer The or Capcom Play System is an Arcade system board by Capcom that debuted in 1988 with Forgotten Worlds. The Neo Geo is a cartridge -based arcade and home Video game system released in 1990 by Japanese game company SNK. A number of arcade systems used two 68000s (such as OutRun; some even used three. (also spelled OutRun and Outrun) is a 1986 Arcade game designed by Yu Suzuki and Sega-AM2. During the 1990s, as arcade systems switched to more powerful processors for the main CPU, they often continued to use the 68000 as a sound controller.

The 68000 was also the central processor in several home game consoles of the late 1980s/early 1990s, including the Sega Mega Drive (Sega Genesis), the Sega Mega-CD (Sega CD), and the console version of the Neo Geo. The is a 16-bit Video game console released by Sega in Japan in 1988 North America in 1989 and the PAL region in 1990 The is an add-on device for the Sega Mega Drive that was released in Europe, Australia, New Zealand, Japan. The Neo Geo is a cartridge -based arcade and home Video game system released in 1990 by Japanese game company SNK. Some later game consoles still included the 68000: the Sega Saturn used it as a dedicated sound controller, and in the Atari Jaguar it co-ordinated the activities of the other specialized graphics and sound chips. The is a 32-bit Video game console that was first released on November 22 1994 in Japan, May 11 1995 in North America The Atari Jaguar is a Video game console, released by Atari Corporation in.

The 68000-based 683XX microcontrollers have been utilized in a wide variety of applications, including networking and telephone equipment, television set-top boxes, and laboratory and medical instruments, among others. The MC68302 and its derivatives have been used in many communication products from Cisco, 3com, Ascend, Marconi and others. The DragonBall family was used in Palm Computing's popular Palm PDAs and in the Handspring Visor series, until Palm gradually phased out the architecture in favor of ARM processors. Palm Inc is a Personal digital assistant and Smartphone manufacturer headquartered in Sunnyvale, California that is responsible for popular Palm handhelds are Personal Digital Assistants ( PDAs which run the Palm OS. The ARM architecture (previously the Advanced RISC Machine, and prior to that Acorn RISC Machine) is a 32-bit RISC processor architecture AlphaSmart uses the DragonBall family in later versions of its portable word processors. The AlphaSmart is a brand of portable battery powered word-processing keyboards manufactured by AlphaSmart Inc

Texas Instruments uses the 68000 in its high-end graphing calculators, the TI-89 and TI-92 series and Voyage 200. Texas Instruments ( better known in the electronics industry (and popularly as TI, is an American company based in Dallas, Texas, USA The TI-89 and the TI-89 Titanium are Graphing calculators developed by Texas Instruments. The Texas Instruments TI-92 Calculator, originally released in 1995, was a large calculator with a QWERTY keyboard Early versions of these used a specialized microcontroller with a static 68EC000 core; later versions use a standard MC68SEC000 processor.

Architecture

Address bus

The 68000 implements a 24-bit address bus, allowing it to address up to 16 MB of physical memory. Address storage and computation used 32 bits, however, with the high-order byte ignored due to the physical lack of pins. This allowed it to run software written for a flat 32-bit address space. In Computing, an address space defines a range of discrete addresses each of which may correspond to a physical or virtual Memory register, a network host By modern definition this meant that the 68000 was a 32-bit microprocessor. Motorola's intent with the internal 32-bit address space was forwards compatibility, making it feasible to write 68000 software that would take full advantage of later 32-bit implementations of the 68000 instruction set.

However, this did not prevent programmers from writing forward incompatible software. "24-bit" software that discarded the upper address byte, or used it for purposes other than addressing, could fail on 32-bit 68K implementations.

Internal registers

The CPU had eight 32-bit general-purpose data registers (D0-D7), and eight address registers (A0-A7). In Computer architecture, a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage The last address register was also the standard stack pointer, and could be called either A7 or SP. In Computer science, a stack is an Abstract data type and Data structure based on the principle of Last In First Out (LIFO This was a good number of registers in many ways. It was small enough to allow the 68000 to respond quickly to interrupts (because only 15 or 16 had to be saved), and yet large enough to make most calculations fast. In Computing, an interrupt is an asynchronous signal from hardware indicating the need for attention or a synchronous event in software indicating the need for a change

Having two types of registers was mildly annoying at times, but not hard to use in practice. Reportedly, it allowed the CPU designers to achieve a higher degree of parallelism, by using an auxiliary execution unit for the address registers. In Computer engineering, an execution unit is a part of a CPU that performs the operations and calculations called for by the program.

Integer representation in the 68000 family is big-endian.

Status register

The 68000 comparison, arithmetic and logic operations set bit flags in a status register to record their results for use by later conditional jumps. The bit flags were "zero" (Z), "carry" (C), "overflow" (V), "extend" (X), and "negative" (N). The "extend" (X) flag deserves special mention, because it was separated from the carry flag. In Computer processors the carry flag (usually indicated as the C flag is a single Bit in a system status (flag register used to indicate when This permitted the extra bit from arithmetic, logic, and shift operations to be separated from the carry for flow-of-control and linkage.

The instruction set

The designers attempted to make the assembly language orthogonal. Orthogonal instruction set is a term used in Computer engineering. That is, instructions were divided into operations and address modes, and almost all address modes were available for almost all instructions. Addressing modes are an aspect of the Instruction set architecture in most Central processing unit (CPU designs Many programmers disliked the "near" orthogonality, while others were grateful for the attempt.

At the bit level, the person writing the assembler would clearly see that these "instructions" could become any of several different op-codes. See the terminology section below for information regarding inconsistent use of the terms assembly and assembler It was quite a good compromise because it gave almost the same convenience as a truly orthogonal machine, and yet also gave the CPU designers freedom to fill in the op-code table.

With only 56 instructions the minimal instruction size was huge for its day at 16 bits. Furthermore, many instructions and addressing modes added extra words on the back for addresses, more address-mode bits, etc.

Many designers believed that the MC68000 architecture had compact code for its cost, especially when produced by compilers. This belief in more compact code led to many of its design wins, and much of its longevity as an architecture.

This belief (or feature, depending on the designer) continued to make design wins for the instruction set (with updated CPUs) up until the ARM architecture introduced the Thumb instruction set that was similarly compact. The ARM architecture (previously the Advanced RISC Machine, and prior to that Acorn RISC Machine) is a 32-bit RISC processor architecture

Privilege levels

The CPU, and later the whole family, implemented exactly two levels of privilege. User mode gave access to everything except the interrupt level control. Supervisor privilege gave access to everything. An interrupt always became supervisory. The supervisor bit was stored in the status register, and visible to user programs.

A real advantage of this system was that the supervisor level had a separate stack pointer. This permitted a multitasking system to use very small stacks for tasks, because the designers did not have to allocate the memory required to hold the stack frames of a maximum stack-up of interrupts. In computing Multitasking is a method by which multiple tasks also known as processes, share common processing resources such as a CPU.

Interrupts

The CPU recognized 7 interrupt levels. In Computing, an interrupt is an asynchronous signal from hardware indicating the need for attention or a synchronous event in software indicating the need for a change Levels 1 through 7 were strictly prioritized. That is, a higher-numbered interrupt could always interrupt a lower-numbered interrupt. In the status register, a privileged instruction allowed one to set the current minimum interrupt level, blocking lower priority interrupts. Level 7 was not maskable - in other words, an NMI. A non-maskable interrupt ( NMI) is a computer processor Interrupt that can not be ignored by standard interrupt masking techniques in the system Level 1 could be interrupted by any higher level. Level 0 means no interrupt. The level was stored in the status register, and was visible to user-level programs.

Hardware interrupts are signalled to the CPU using three inputs that encode the highest pending interrupt priority. A separate interrupt controller is usually required to encode the interrupts, though for systems that do not require more than three hardware interrupts it is possible to connect the interrupt signals directly to the encoded inputs at the cost of additional software complexity. The interrupt controller can be as simple as a 74LS148 priority encoder, or may be part of a VLSI peripheral chip such as the MC68901 Multi-Function Peripheral, which also provided a UART, timer, and parallel I/O. The 7400 series of Transistor-transistor logic Integrated circuits are historically important as the first widespread family of TTL integrated circuit A universal asynchronous receiver/transmitter (usually abbreviated UART and ˈjuːɑrt is a type of "asynchronous receiver/transmitter" a piece of Computer

The "exception table" (interrupt vector addresses) was fixed at addresses 0 through 1023, permitting 256 32-bit vectors. The first vector was the starting stack address, and the second was the starting code address. Vectors 3 through 15 were used to report various errors: bus error, address error, illegal instruction, zero division, CHK & CHK2 vector, privilege violation, and some reserved vectors that became line 1010 emulator, line 1111 emulator, and hardware breakpoint. Vector 24 started the real interrupts: spurious interrupt (no hardware acknowledgement), and level 1 through level 7 autovectors, then the 15 TRAP vectors, then some more reserved vectors, then the user defined vectors.

Since at a minimum the starting code address vector must always be valid on reset, systems commonly included some nonvolatile memory (e. g. ROM) starting at address zero to contain the vectors and bootstrap code. In Computing, booting ( booting up) is a bootstrapping process that starts Operating systems when the user turns on a Computer system However, for a general purpose system it is desirable for the operating system to be able to change the vectors at runtime. This was often accomplished by either pointing the vectors in ROM to a jump table in RAM, or through use of bank-switching to allow the ROM to be replaced by RAM at runtime.

The 68000 did not meet the Popek and Goldberg virtualization requirements for full processor virtualization because it had a single unprivileged instruction "MOVE from SR", which allowed user-mode software read-only access to a small amount of privileged state. The Popek and Goldberg virtualization requirements are a set of sufficient conditions for a computer architecture to efficiently support system Virtualization.

The 68000 was also unable to easily support virtual memory, which requires the ability to trap and recover from a failed memory access. Virtual memory is a Computer system technique which gives an application program the impression that it has contiguous working memory while in fact it may be physically The 68000 does provide a bus error exception which can be used to trap, but it does not save enough processor state to resume the faulted instruction once the operating system has handled the exception. Several companies did succeed in making 68000 based Unix workstations with virtual memory that worked, by using two 68000 chips running in parallel on different phased clocks. When the "leading" 68000 encountered a bad memory access, extra hardware would interrupt the "main" 68000 to prevent it from also encountering the bad memory access. This interrupt routine would handle the virtual memory functions and restart the "leading" 68000 in the correct state to continue properly synchronized operation when the "main" 68000 returned from the interrupt.

These problems were fixed in the next major revision of the 68K architecture, with the release of the MC68010. The Bus Error and Address Error exceptions pushed a large amount of internal state onto the supervisor stack in order to facilitate recovery, and the MOVE from SR instruction was made privileged. A new unprivileged "MOVE from CCR" instruction was provided for use in its place by user mode software; an operating system could trap and emulate user-mode MOVE from SR instructions if desired.

Instruction set details

The standard addressing modes are:

• Register direct
  • data register, e. Addressing modes are an aspect of the Instruction set architecture in most Central processing unit (CPU designs g. "D0"
  • address register, e. g. "A6"

• Register indirect
  • Simple address, e. g. (A0)
  • Address with post-increment, e. g. (A0)+
  • Address with pre-decrement, e. g. -(A0)
  • Address with a 16-bit signed offset, e. g. 16(A0)
  • Note that the actual increment or decrement size was dependent on the operand request: a byte read instruction incremented the address register by 1, a word read by 2, and a long read by 4.

• Register indirect with an Index
  • 8-bit signed offset, e. g. 8(A0, D0) or 8(A0, A1)

• PC (program counter) relative with displacement
  • 16-bit signed offset, e. g. 16(PC). This mode was very useful.
  • 8-bit signed offset with index, e. g. 8(PC, D2)

• Absolute memory location
  • Either a number, e. g. "$4000", or a symbolic name translated by the assembler
  • Most 68000 assemblers used the "$" symbol for hexadecimal, instead of "0x". In Mathematics and Computer science, hexadecimal (also base -, hexa, or hex) is a Numeral system with a

• Immediate mode
  • Stored in the instruction, e. g. "#400".

• Quick Immediate mode
  • 2? bit Sign extended with value stored in Opcode
  • eg. moveq. l #0,d0 was quicker than clr. l d0 (though both made d0 equal 0)

Plus: access to the status register, and, in later models, other special registers. A status register (also flag register or condition code register (CCR) is a collection of flag Bits for a processor.

Most instructions had dot-letter suffixes, permitting operations to occur on 8-bit bytes (". b"), 16-bit words (". w"), and 32-bit longs (". l").

Most instructions are dyadic, that is, the operation has a source, and a destination, and the destination is changed. Notable instructions were:

• Arithmetic: ADD, SUB, MULU (unsigned multiply), MULS (signed multiply), DIVU, DIVS, NEG (additive negation), and CMP (a sort of subtract that set the status bits, but did not store the result)

• Binary Coded Decimal Arithmetic: ABCD, and SBCD

• Logic: EOR (exclusive or), AND, NOT (logical not)

• Shifting: (logical, i. In Computing and electronic systems binary-coded decimal ( BCD) is an encoding for decimal numbers in which each digit is represented by its own binary e. right shifts put zero in the most significant bit) LSL, LSR, (arithmetic shifts, i. History and Details The formal definition of an arithmetic shift from Federal Standard 1037C is that it is e. sign-extend the most significant bit) ASR, ASL, (Rotates through eXtend and not:) ROXL, ROXR, ROL, ROR

• Bit manipulation in memory: BSET (to 1), BCLR (to 0), and BTST (set the Zero bit)

• Multiprocessing control: TAS, test-and-set, performed an indivisible bus operation, permitting semaphores to be used to synchronize several processors sharing a single memory

• Flow of control: JMP (jump), JSR (jump to subroutine), BSR (relative address jump to subroutine), RTS (return from subroutine), RTE (return from exception, i. Bit manipulation is the act of algorithmically manipulating Bits or other pieces of data shorter than a Byte. Multiprocessing is the use of two or more central processing units (CPUs within a single computer system In Computer science, the test-and-set instruction is an instruction used to both test and (conditionally write to a memory location as part of a single atomic (i For other uses see Semaphore. A semaphore, in computer science is a protected Variable (an entity storing a value or Abstract In Computer science, a subroutine ( function, method, procedure, or subprogram) is a portion of code within a larger e. an interrupt), TRAP (trigger a software exception similar to software interrupt), CHK (a conditional software exception)

• Branch: Bcc (a branch where the "cc" specified one of 16 tests of the condition codes in the status register: equal, greater than, less-than, carry, and most combinations and logical inversions, available from the status register).

• Decrement-and-branch: DBcc (where "cc" was as for the branch instructions) which decremented a D-register and branched to a destination provided the condition was still true and the register had not been decremented to -1. This use of -1 instead of 0 as the terminating value allowed the easy coding of loops which had to do nothing if the count was 0 to begin with, without the need for an additional check before entering the loop.

Notes

See also

• 68k
• x86
• Macsbug
• Z80

References

• Motorola MC68000 Family Programmer's Reference Manual
• comp.sys.m68k FAQ

External links

• Descriptions of assembler instructions
• 68000 images and descriptions at cpu-collection.de
• 'Chips : Of Diagnostics & Debugging' Article
• A microprocessor simulation framework, useful for working with Motorola 68000
• Integrated Development Environment for the 68000 microcomputer
• The Vintage Mac Museum: 9inch/mono Display 68000 ONLY
• EASy68K, an open-source 68k assembler for Windows. The Motorola 680x0 / m68k / 68k / 68K is a family of 32-bit CISC Microprocessor CPU chips and was the primary See also X86 assembly language The generic term x86 refers to the most commercially successful Instruction set architecture in the history of Personal MacsBug is a low-level ( Assembly language /machine-level Debugger for pre- Mac OS X Apple Macintosh computers The Zilog Z80 is an 8-bit Microprocessor designed and sold by Zilog from July 1976 onwards