|General purpose||8 × 32-bit + 7 address registers also usable for most operations + stack pointer|
The Motorola 68000 ("'sixty-eight-thousand'"; also called the Motorola 68k, "sixty-eight-k") is a 16/32-bit CISC microprocessor core designed and marketed by Motorola Semiconductor Products Sector (now Freescale Semiconductor). Introduced in 1979 with HMOS technology 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 being limited to a 16-bit wide external bus. After 30 years in production, the 68000 architecture is still in use.
- 1 History
- 2 Applications
- 3 Architecture
- 4 Instruction set details
- 5 68EC000
- 6 Example code
- 7 See also
- 8 Notable users
- 9 Notes
- 10 References
- 11 External links
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. It would be a higher-power sibling complementing the existing 8-bit 6800 line rather than a compatible successor. 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. 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. 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 MACSS team drew heavily on the influence of minicomputer processor design, such as the PDP-11 and VAX systems, which were similarly microcoded.
In the mid 1970s, the 8-bit microprocessor manufacturers raced to introduce the 16-bit generation. National Semiconductor had been first with its IMP-16 and PACE processors in 1973–1975, but these had issues with speed. Intel had worked on their advanced 16/32-bit iAPX432 (alias 8800) since 1975 and their Intel 8086 since 1976 (it was introduced in 1978 but became really widespread in the form of the almost identical 8088 in the IBM PC a few years later). Arriving late to the 16-bit arena afforded the new processor more transistors (roughly 40 000 active versus 20 000 active in the 8086), 32-bit macroinstructions, and acclaimed general ease of use.
The original MC68000 was fabricated using an HMOS process with a 3.5 µm feature size. Formally introduced in September 1979, Initial samples were released in February 1980, with production chips available over the counter in November. Initial speed grades were 4, 6, and 8 MHz. 10 MHz chips became available during 1981, and 12.5 MHz chips by June 1982. The 16.67 MHz "12F" version of the MC68000, the fastest version of the original HMOS chip, was not produced until the late 1980s. Tom Gunter, retired Corporate Vice President at Motorola, is known as the "Father of the 68000".
The 68000 became the dominant CPU for Unix-based workstations including Sun workstations and Apollo/Domain workstations, and also was used for mass-market computers such as the Apple Lisa, Macintosh, Amiga, and Atari ST. The 68000 was used in Microsoft Xenix systems as well as a prototype NetWare Unix-based Server (after the MC6800 series was not found to be powerful enough and before the move to Intel x86 processors). The 68000 was used in the first generation of desktop laser printers including the original Apple Inc. LaserWriter and the HP LaserJet. In 1982, the 68000 received an update to its ISA allowing it to support virtual memory and to conform to the Popek and Goldberg virtualization requirements. The updated chip was called the 68010. A further extended version which exposed 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 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.
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, 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).
The 68HC000, the first CMOS version of the 68000, was designed by Hitachi and jointly introduced in 1985. Motorola's version was called the MC68HC000, while Hitachi's was the HD68HC000. The 68HC000 was eventually offered at speeds of 8-20 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 0.13 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.) Apple selected the 68HC000 for use in the Macintosh Portable.
Motorola replaced the MC68008 with the MC68HC001 in 1990. 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.
The later evolution of the 68000 focused on more modern embedded control applications and on-chip peripherals. The 68EC000 chip and SCM68000 core expanded the address bus to 32 bits, removed the M6800 peripheral bus, and excluded the MOVE from SR instruction from user mode programs. In 1996, Motorola updated the standalone core with fully static circuitry drawing only 2 µW in low-power mode, calling it the MC68SEC000.
Motorola ceased production of the HMOS MC68000 and MC68008 in 1996, but its spin-off company, Freescale Semiconductor, were 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 has been discontinued, leaving only the 68SEC000 in production. 
As a microcontroller core
At its introduction, the 68000 was first used in high-priced systems, including multiuser microcomputers like the WICAT 150, early Alpha Microsystems computers, Sage II / IV, 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. 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. The 68008, on the other hand, was only used in a few home computer systems. The Sinclair QL (though the QL was a sister machine to the ICL One Per Desk, which also used a 68008) was the most commercially important. Helix Systems (in Missouri, United States) designed an extension to the SWTPC SS-50 bus, the SS-64, and produced systems built around the 68008 processor.
While the rapid pace of computer advancement quickly rendered the 68000 obsolete as desktop/workstation CPU, the processor found substantial use in embedded applications. By the early 1980s, quantities of 68000 CPUs could be purchased for less than $30 USD per part. 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 Mega Drive (Genesis) console. The 1993 multi-processor Atari Jaguar console used a 68000 as a support chip, although some developers used it as the primary processor due to familiarity. The 1994 multi-processor Sega Saturn console used the 68000 as a sound co-processor (much as the Mega Drive/Genesis use the Z80 as a sound co-processor).
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 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. 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.
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 uses the DragonBall family in later versions of its portable word processors. Texas Instruments uses the 68000 in its high-end graphing calculators, the TI-89 and TI-92 series and Voyage 200. Early versions of these used a specialized microcontroller with a static 68EC000 core; later versions use a standard MC68SEC000 processor.
A modified version of the 68000 formed the basis of the IBM XT/370 Hardware emulator of a System 370 processor.
|Motorola 68000 registers|
The 68000 has a 24-bit external address bus and two byte-select signals "replaced" A0. These 24 lines can therefore reach 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.[irrelevant citation]
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 68000 implementations. For example, early (pre-7.0) versions of Apple's Mac OS used the high byte of memory-block master pointers to hold flags such as locked and purgeable. Later versions of the OS moved the flags to a nearby location, and Apple began shipping computers which had "32-bit clean" ROMs beginning with the release of the 1989 Mac IIci.
The 68000 family stores multi-byte binary integers in memory in big-endian order.
The CPU has eight 32-bit general-purpose data registers (D0-D7), and eight address registers (A0-A7). The last address register is the stack pointer, and assemblers accept the label SP as equivalent to A7. This was a good number of registers in many ways. It was small enough to allow the 68000 to respond quickly to interrupts (even in the worst case where all 8 data registers D0–D7 and 7 address registers A0–A6 have to be saved, 15 registers in total), and yet large enough to make most calculations fast, because they can be done entirely within the processor without keeping any partial results in memory. (Note that an exception routine in supervisor mode can also save the user stack pointer A7, which would total 8 address registers. However, the dual stack pointer (A7 and A7') design of the 68000 makes this normally unnecessary, except when a task switch is performed in a multitasking system.)
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.
The 68000 comparison, arithmetic, and logic operations sets bit flags in a status register to record their results for use by later conditional jumps. The bit flags are "zero" (Z), "carry" (C), "overflow" (V), "extend" (X), and "negative" (N). The "extend" (X) flag deserves special mention, because it is separate from the carry flag. This permits the extra bit from arithmetic, logic, and shift operations to be separated from the carry for flow-of-control and linkage.
The designers attempted to make the assembly language orthogonal. That is, instructions are divided into operations and address modes, and almost all address modes are available for almost all instructions. There are 56 instructions and a minimum instruction size of 16 bits. Many instructions and addressing modes are longer to include additional address or mode bits.
The CPU, and later the whole family, implements two levels of privilege. User mode gives access to everything except the interrupt level control. Supervisor privilege gives access to everything. An interrupt always becomes supervisory. The supervisor bit is stored in the status register, and is visible to user programs.
An advantage of this system is that the supervisor level has a separate stack pointer. This permits a multitasking system to use very small stacks for tasks, because the designers do not have to allocate the memory required to hold the stack frames of a maximum stack-up of interrupts.
The CPU recognizes seven interrupt levels. Levels 1 through 7 are strictly prioritized. That is, a higher-numbered interrupt can always interrupt a lower-numbered interrupt. In the status register, a privileged instruction allows one to set the current minimum interrupt level, blocking lower or equal priority interrupts. For example, if the interrupt level in the status register is set to 3, higher levels from 4 to 7 can caused an exception. Level 7 is a level triggered Non-maskable interrupt (NMI). Level 1 can be interrupted by any higher level. Level 0 means no interrupt. The level is stored in the status register, and is visible to user-level programs.
Hardware interrupts are signaled to the CPU using three inputs that encode the highest pending interrupt priority. A separate Encoder 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 (used in the Atari ST range of computers), which also provided a UART, timer, and parallel I/O.
The "exception table" (interrupt vector table interrupt vector addresses) is fixed at addresses 0 through 1023, permitting 256 32-bit vectors. The first vector (RESET) consists of 2 vectors, namely the starting stack address, and the starting code address. Vectors 3 through 15 are used to report various errors: bus error, address error, illegal instruction, zero division, CHK and CHK2 vector, privilege violation (to block privilege escalation), and some reserved vectors that became line 1010 emulator, line 1111 emulator, and hardware breakpoint. Vector 24 starts the real interrupts: spurious interrupt (no hardware acknowledgement), and level 1 through level 7 autovectors, then the 16 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. 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 does not meet the Popek and Goldberg virtualization requirements for full processor virtualization because it has a single unprivileged instruction "MOVE from SR", which allows user-mode software read-only access to a small amount of privileged state.
The 68000 is also unable to easily support virtual memory, which requires the ability to trap and recover from a failed memory access. 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 push 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 is provided for use in its place by user mode software; an operating system can trap and emulate user-mode MOVE from SR instructions if desired.
Instruction set details
The standard addressing modes are:
- Register direct
- data register, e.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)
- Register indirect with index register & 8-bit signed offset e.g. 8(A0, D0) or 8(A0, A1)
- Note that with (A0)+ and -(A0), the actual increment or decrement value is dependent on the operand size: a byte access increments the address register by 1, a word by 2, and a long by 4.
- PC (program counter) relative with displacement
- Relative 16-bit signed offset, e.g. 16(PC). This mode was very useful for position-independent code.
- Relative with 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" or a trailing H.
- There were 16 and 32-bit version of this addressing mode
- Immediate mode
- Data stored in the instruction, e.g. "#400"
- Quick Immediate mode
- 3 bit unsigned (or 8 bit signed with moveq) with value stored in Opcode
- In addq and subq, 0 is the equivalent to 8
- e.g. moveq #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.
Most instructions have 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 comparison done by subtracting the arguments and setting the status bits, but did not store the result)
- Binary Coded Decimal Arithmetic: ABCD, and SBCD
- Logic: EOR (exclusive or), AND, NOT (logical not), OR (inclusive or)
- Shifting: (logical, i.e. right shifts put zero in the most significant bit) LSL, LSR, (arithmetic shifts, i.e. sign-extend the most significant bit) ASR, ASL, (Rotates through eXtend and not:) ROXL, ROXR, ROL, ROR
- Bit test and manipulation in memory: BSET (to 1), BCLR (to 0), BCHG (invert Bit) and BTST (set the Zero bit if tested bit is 0)
- 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.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. This also facilitated nesting of DBcc.
The processors are available in a variety of speeds including 8 and 16 MHz configurations, producing 2,100 and 4,376 Dhrystones each. These processors have no floating point unit and it is difficult to implement an FPU coprocessor (MC68881/2) with one because the EC series lacks necessary coprocessor instructions.
The 68EC000 was used as a controller in many audio applications, including Ensoniq musical instruments and sound cards where it was part of the MIDI synthesizer. On Ensoniq sound boards, the controller provided several advantages compared to competitors without a CPU on board. The processor allowed the board to be configured to perform various audio tasks, such as MPU-401 MIDI synthesis or MT-32 emulation, without the use of a TSR program. This improved software compatibility, lowered CPU usage, and eliminated host system memory usage.
The 68000 assembler code below is for a subroutine named
strtolower which copies a source null-terminated ASCIZ character string to another destination string, converting all alphabetic characters to lower case.
; strtolower: ; Copy a null-terminated ASCII string, converting ; all alphabetic characters to lower case. ; ; Entry parameters: ; (SP+0): Source string address ; (SP+4): Target string address org $00100000 ;Start at 00100000 00100000 strtolower public 00100000 CE56 0000 link a6,#0 ;Set up stack frame 00100004 206E 0004 movea 4(a6),a0 ;A0 = src, from stack 00100008 226E 0008 movea 8(a6),a1 ;A1 = dst, from stack 0010000C 1018 loop move.b (a0)+,d0 ;Load D0 from (src) 0010000E 0C00 0041 cmpi #'A',d0 ;If D0 < 'A', 00100012 650A blo copy ;skip 00100014 0C00 0059 cmpi #'Z',d0 ;If D0 > 'Z', 00100018 6204 bhi copy ;skip 0010001A 0600 0020 addi #'a'-'A',d0 ;D0 = lowercase(D0) 0010001E 12E0 copy move.b d0,(a1)+ ;Store D0 to (dst) 00100020 66E8 bne loop ;Repeat while D0 <> NUL 00100022 4E5E unlk a6 ;Restore stack frame 00100024 4E75 rts ;Return 00100026 end
The subroutine establishes a call frame using register A6 as the frame pointer. This kind of calling convention supports reentrant and recursive code, and is typically used by languages like C and C++. The subroutine then retrieves the parameters passed to it (
dst) from the stack. It then loops, reading an ASCII character (a single byte) from the
src string, checking if it is an alphabetic character, and if so converting it into a lower-case character, then writing the character into the
dst string. Finally, it checks if the character was a null character; if not, it repeats the loop, otherwise it restores the previous stack frame (and A6 register) and returns. Note that the string pointers (registers A0 and A1) are auto-incremented in each iteration of the loop.
- Motorola 68000 family
- Motorola 6800, 8-bit predecessor
- Motorola 6809, 8-bit
- Freescale 68HC11, 8-bit embedded
- MacsBug, low-level assembly/machine-level debugger
- x86, the Intel competitor
- Zilog Z8000, 16-bit
- Sega Genesis/Mega Drive uses the 68000 processor as its CPU
- Neo Geo
- Some Amiga systems use the 68000 processor as their CPU.
- CDTV the world's first compact disc based multimedia platform uses the 68000 processor as its CPU.
- Motorola Literature Distribution, Phonenix, AZ (1992). M68000 Family Programmer's Reference Manual. [motorola]. pp. 1–1. ISBN 0-13-723289-6.
- Ken Polsson. "Chronology of Microprocessors". Processortimeline.info. Retrieved 2013-09-27.
- DTACK GROUNDED, The Journal of Simple 68000/16081 Systems (29), March 1984, p. 9
- "Company Briefs", The New York Times, September 21, 1985, available from TimesSelect (subscription).
- "68HC001 obsoletes 68008".[dead link], Microprocessor Report, June 20, 1990; available from HighBeam Research (subscription).
- "Motorola streamlines 68000 family; "EC" versions of 68000, '020, '030, and '040, plus low-end 68300 chip".[dead link], Microprocessor Report, April 17, 1991; available from HighBeam Research (subscription).
- "Motorola reveals MC68SEC000 processor for low power embedded applications", Motorola press release, November 18, 1996; archived by Internet Archive on March 28, 1997.
- comp.sys.m68k Usenet posting, May 16, 1995; also see other posts in thread. The end-of-life announcement was in late 1994; according to standard Motorola end-of-life practice, final orders would have been in 1995, with final shipments in 1996.
- "Multiprotocol processor marries 68000 and RISC"., ESD: The Electronic System Design Magazine, November 1, 1989; available from AccessMyLibrary.
- "musem ~ WICAT 150". Old-computers.com. Retrieved 2013-09-27.
- Boys, Robert. M68k Frequently Asked Questions (FAQ), comp.sys.m68k, October 19, 1994.
- Soundscape Elite Specs. from Fax Sheet, Google Groups, April 25, 1995.
- "Commodore Amiga 1000 computer". Oldcomputers.net. Retrieved 2013-09-27.
- Gareth Knight (2002-08-13). "Amiga CDTV". Amigahistory.co.uk. Retrieved 2013-09-27.
- M68K FAQ
- Descriptions of assembler instructions
- 68000 images and descriptions at cpu-collection.de
- 'Chips : Of Diagnostics & Debugging' Article
- The Vintage Mac Museum: 9inch/mono Display 68000 ONLY
- EASy68K, an open-source 68k assembler for Windows.
- Feralcore, an open-source 68k emulator, disassembler, and debugger for Java.
- "Kiwi - a 68k Homebrew Computer"