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86-406: Hal was the editor of the newsletter and wrote almost all of its content. The ostensible purpose of the newsletter was to promote Hal's company's line of 68000-based hardware and software. However, Hal never let that get in the way of telling a good story or explaining how to design or build a fast computer. He was a hardware engineer and a businessman who also knew how to write software. His newsletter

172-423: A 32-bit instruction set , with 32-bit registers and a 16-bit internal data bus . The address bus is 24 bits and does not use memory segmentation , which made it easier to program for. Internally, it uses a 16-bit data arithmetic logic unit (ALU) and two more 16-bit ALUs used mostly for addresses, and has a 16-bit external data bus . For this reason, Motorola termed it a 16/32-bit processor. As one of

258-525: A 16-bit status register. The upper 8 bits is the system byte, and modification of it is privileged. The lower 8 bits is the user byte, also known as the condition code register (CCR), and modification of it is not privileged. The 68000 comparison, arithmetic, and logic operations modify condition codes to record their results for use by later conditional jumps. The condition code bits are "carry" (C), "overflow" (V), "zero" (Z), "negative" (N) and "extend" (X). The "extend" (X) flag deserves special mention, because it

344-413: A 64-pin package. This became known as the "Texas Cockroach". By the mid-1970s, Motorola's MOS design techniques had become less advanced than their competition, and their fabrication lines at times struggled with low yields . By the late-1970s, the company had entered a technology exchange program with Hitachi , dramatically improving their production capabilities. As part of this, a new fab named MOS-8

430-452: A design with "DTACK grounded" (i.e., DTACK always asserted). When many different peripherals with different maximum speed capabilities must be used in a small system, another solution to keep DTACK grounded is to slow down the clock rate of the CPU so that all memory and peripheral devices can transfer data at the (reduced) maximum speed of the CPU. In turn, using a lower CPU clock frequency enables

516-467: A dual 68000 CPU configuration, and systems with a triple 68000 CPU configuration also exist (such as Galaxy Force and others based on the Sega Y Board), 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) for games such as Big Run and Cisco Heat ; another, fifth 68000 (at a different clock rate than

602-491: A higher-numbered interrupt can always interrupt a lower-numbered interrupt. In the status register, a privileged instruction allows setting 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 cause 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

688-437: 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. However, this did not prevent programmers from writing forward incompatible software. "24-bit" software that discarded

774-413: A minimum instruction size of 16 bits. Many instructions and addressing modes are longer to include more address or mode bits. The CPU, and later the whole family, implements two levels of privilege. User mode gives access to everything except privileged instructions such as interrupt level controls. Supervisor privilege gives access to everything. An interrupt always becomes supervisory. The supervisor bit

860-513: A result, it was not very popular. In the 1990s, Hal revived DTACK Grounded as an occasional column in Dr. Dobb's Journal called "DTACK Revisited". Motorola 68000 The Motorola 68000 (sometimes shortened to Motorola 68k or m68k and usually pronounced "sixty-eight-thousand") is a 16/32-bit complex instruction set computer (CISC) microprocessor , introduced in 1979 by Motorola Semiconductor Products Sector. The design implements

946-560: 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. The 68HC000, the first CMOS version of the 68000, was designed by Hitachi and jointly introduced in 1985. Motorola's version

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1032-607: A similar function to DTACK, but it is usually named READY or RDY. (On the Z80, it is named WAIT.) Therefore, a reference to DTACK distinctively implies the 68000 family of CPUs. In order to discourage photocopying of the newsletter, a portion of the DTACK Grounded newsletter was printed on dark red paper. This made it harder to photocopy those pages. This section of the newsletter was called "the Redlands". It typically contained

1118-526: A simpler, less expensive circuit board design, with fewer layers, to be used successfully. This, too, like limiting the system to a few peripherals of low diversity, is a design strategy that is typically applied to small or embedded systems but would not be appropriate for high-performance workstations. Many other microprocessors, including the Intel 8086 , 8088 , 80286 , 80386 , 8080 , 8085 , Zilog Z80 , and 6502 family , each have an input pin that serves

1204-585: A very simple 8-bit ALU: Mathematician John von Neumann proposed the ALU concept in 1945 in a report on the foundations for a new computer called the EDVAC . The cost, size, and power consumption of electronic circuitry was relatively high throughout the infancy of the Information Age . Consequently, all early computers had a serial ALU that operated on one data bit at a time although they often presented

1290-601: A wider word size to programmers. The first computer to have multiple parallel discrete single-bit ALU circuits was the 1951 Whirlwind I , which employed sixteen such "math units" to enable it to operate on 16-bit words. In 1967, Fairchild introduced the first ALU-like device implemented as an integrated circuit, the Fairchild 3800, consisting of an eight-bit arithmetic unit with accumulator. It only supported adds and subtracts but no logic functions. Full integrated-circuit ALUs soon emerged, including four-bit ALUs such as

1376-427: Is a combinational digital circuit that performs arithmetic and bitwise operations on integer binary numbers . This is in contrast to a floating-point unit (FPU), which operates on floating point numbers. It is a fundamental building block of many types of computing circuits, including the central processing unit (CPU) of computers, FPUs, and graphics processing units (GPUs). The inputs to an ALU are

1462-405: Is a group of signals that conveys one binary integer number. Typically, the A, B and Y bus widths (the number of signals comprising each bus) are identical and match the native word size of the external circuitry (e.g., the encapsulating CPU or other processor). The opcode input is a parallel bus that conveys to the ALU an operation selection code, which is an enumerated value that specifies

1548-474: Is also powered by the 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, speeding up 32-bit operations and allowing 32-bit addressing, rather than the 24-bit addressing of the 68000 and 68010 or the 31-bit addressing of the Motorola 68012 . The original 68k is generally software forward-compatible with

1634-567: Is also the CPU of the Sega Pico , a young childrens' educational game console. The multi-processor Atari Jaguar console from 1993 used a 68000 as a support chip, although, due to familiarity, some developers used it as the primary processor. The 1994 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. Certain arcade games (such as Steel Gunner and others based on Namco System 2 ) use

1720-436: Is an algorithm that operates on integers which are larger than the ALU word size. To do this, the algorithm treats each integer as an ordered collection of ALU-size fragments, arranged from most-significant (MS) to least-significant (LS) or vice versa. For example, in the case of an 8-bit ALU, the 24-bit integer 0x123456 would be treated as a collection of three 8-bit fragments: 0x12 (MS), 0x34 , and 0x56 (LS). Since

1806-771: Is called the MC68HC000, while Hitachi's is the HD68HC000. The 68HC000 offers 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

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1892-410: Is referred to as the "status register" or "condition code register". Depending on the ALU operation being performed, some status register bits may be changed and others may be left unmodified. For example, in bitwise logical operations such as AND and OR, the carry status bit is typically not modified as it is not relevant to such operations. In CPUs, the stored carry-out signal is usually connected to

1978-407: Is repeated for all operand fragments so as to generate a complete collection of partials, which is the result of the multiple-precision operation. In arithmetic operations (e.g., addition, subtraction), the algorithm starts by invoking an ALU operation on the operands' LS fragments, thereby producing both a LS partial and a carry out bit. The algorithm writes the partial to designated storage, whereas

2064-410: Is separate from the carry flag . This permits the extra bit from arithmetic, logic, and shift operations to be separated from the carry multiprecision arithmetic . 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

2150-458: 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 5 are strictly prioritized. That is,

2236-412: Is stored in the status register, and is visible to user-level programs. Hardware interrupts are signalled 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

2322-450: 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

2408-540: 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. Since being succeeded by "true" 32-bit microprocessors, the 68000 is used as the core of many microcontrollers . In 1989, Motorola introduced the MC68302 communications processor. IBM considered

2494-466: The Am2901 and 74181 . These devices were typically " bit slice " capable, meaning they had "carry look ahead" signals that facilitated the use of multiple interconnected ALU chips to create an ALU with a wider word size. These devices quickly became popular and were widely used in bit-slice minicomputers. Microprocessors began to appear in the early 1970s. Even though transistors had become smaller, there

2580-670: The Data General Nova or PDP-8 . Based on the semiconductor manufacturing processes of the era, these were often multi-chip solutions like the National Semiconductor IMP-16 , or the single-chip PACE that had issues with speed. With the sales prospects for the 6800 dimming, but still cash-flush from the engine control sales, in late 1976 Colin Crook, Operations Manager, began considering how to successfully win future sales. They were aware that Intel

2666-542: The Macintosh moved from the 6809 to the 68k. The average price eventually reached $ 14.76. 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 . It also adds a new "loop mode" which speeds up small loops, and increases overall performance by about 10% at

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2752-510: 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. At its introduction, the 68000 was first used in high-priced systems, including multiuser microcomputers like

2838-858: 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 . A modified version of the 68000 formed the basis of the IBM XT/370 hardware emulator of the System 370 processor. Video game manufacturers used

2924-572: The WICAT 150, 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

3010-406: The electrical conductors used to convey digital signals between the ALU and external circuitry. When an ALU is operating, external circuits apply signals to the ALU inputs and, in response, the ALU produces and conveys signals to external circuitry via its outputs. A basic ALU has three parallel data buses consisting of two input operands ( A and B ) and a result output ( Y ). Each data bus

3096-439: 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

3182-550: The 6800, as they felt the 8-bit designs were too limited to be the basis for new designs. The new system was influenced by the PDP-11 , the most popular minicomputer design of the era. At the time, a key concept in minis was the concept of an orthogonal instruction set , in which every operation was allowed to work on any sort of data. To feed the correct data into the internal units, MACSS made extensive use of microcode , essentially small programs in read only memory that gathered up

3268-450: 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

3354-528: 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 Genesis console, and also the Sega CD attachment for it (a Sega CD system has three CPUs, two of them 68000s.) The 68000

3440-460: The 68000 for the IBM PC but chose the Intel 8088 ; however, IBM Instruments briefly sold the 68000-based IBM System 9000 laboratory computer systems. The 68k instruction set is particularly well suited to implement Unix, 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

3526-503: The 68000 itself had to succeed despite initially adopting a metal-gate design. Though the point about playing catch-up is clear, this could not have been an entirely accurate summary because Motorola's 1976 datasheets, predating the inception of the MACCS project, denote the majority of its 6800 family in silicon-gate. Indeed, Gunter's own 1979 article introducing the 68000 highlighted it as a silicon-gate depletion-mode HMOS design. Whatever

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3612-472: 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 needed to be saved, 15 registers in total), and yet large enough to make most calculations fast, because they could 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,

3698-685: The 68HC000 for use in the Macintosh Portable . Motorola replaced the MC68008 with the MC68HC001 in 1990. This chip resembles the 68HC000 in most respects, but its data bus can operate in either 16-bit or 8-bit mode, depending on the value of an input pin at reset. Thus, like the 68008, it can 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 remove

3784-451: The ALU circuitry before sampling the ALU outputs. In general, external circuitry controls an ALU by applying signals to the ALU inputs. Typically, the external circuitry employs sequential logic to generate the signals that control ALU operation. The external sequential logic is paced by a clock signal of sufficiently low frequency to ensure enough time for the ALU outputs to settle under worst-case conditions (i.e., conditions resulting in

3870-403: The ALU inputs and, when enough time (known as the " propagation delay ") has passed for the signals to propagate through the ALU circuitry, the result of the ALU operation appears at the ALU outputs. The external circuitry connected to the ALU is responsible for ensuring the stability of ALU input signals throughout the operation, and for allowing sufficient time for the signals to propagate through

3956-400: The ALU's carry-in net. This facilitates efficient propagation of carries (which may represent addition carries, subtraction borrows, or shift overflows) when performing multiple-precision operations, as it eliminates the need for software-management of carry propagation (via conditional branching, based on the carry status bit). In integer arithmetic computations, multiple-precision arithmetic

4042-550: 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

4128-518: The LS bit of each partial—which is conveyed via the stored carry bit—must be obtained from the MS bit of the previously left-shifted, less-significant operand. Conversely, operands are processed MS first in right-shift operations because the MS bit of each partial must be obtained from the LS bit of the previously right-shifted, more-significant operand. In bitwise logical operations (e.g., logical AND, logical OR),

4214-632: 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. In 1996, Motorola updated the standalone core with fully static circuitry, drawing only 2  μW in low-power mode, calling it

4300-736: The MC68000, the fastest version of the original HMOS chip, was not produced until the late 1980s. By the start of 1981, the 68k was winning orders in the high end, and Gunter began to approach Apple to win their business. At that time, the 68k sold for about $ 125 in quantity. In meetings with Steve Jobs , Jobs talked about using the 68k in the Apple Lisa , but stated "the real future is in this product that I'm personally doing. If you want this business, you got to commit that you'll sell it for $ 15." Motorola countered by offering to sell it at $ 55 at first, then step down to $ 35, and so on. Jobs agreed, and

4386-518: The MC68SEC000. Motorola ceased production of the HMOS MC68000, as well as the MC68008, MC68010, MC68330, and MC68340 in on June 1, 1996, 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,

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4472-671: The addition operation) upon operation completion. The ALU's input signals, which are held stable until the next clock, are allowed to propagate through the ALU and to the destination register while the CPU waits for the next clock. When the next clock arrives, the destination register stores the ALU result and, since the ALU operation has completed, the ALU inputs may be set up for the next ALU operation. A number of basic arithmetic and bitwise logic functions are commonly supported by ALUs. Basic, general purpose ALUs typically include these operations in their repertoires: ALU shift operations cause operand A (or B) to shift left or right (depending on

4558-649: The assembly source code for high performance math subroutines. A major topic in later years of the newsletter was the Halgol interpreter, later converted to the DBASIC interpreter. Hal produced a fast Basic interpreter for the Atari ST computer. It was written in hand-tuned assembly language, and ran very fast compared to other Basics. But it was non-standard: it couldn't run Microsoft BASIC programs, and it couldn't read or write MS-DOS format floppy disks. Unfortunately, as

4644-573: The cost of more software complexity. The interrupt controller can be as simple as a 74LS148 priority encoder, or may be part of a very large-scale integration (VLSI) peripheral chip such as the MC68901 Multi-Function Peripheral (used in the Atari ST range of computers and X68000 ), which also provides a UART , timer, and parallel I/O. Arithmetic logic unit In computing , an arithmetic logic unit ( ALU )

4730-429: The data to be operated on, called operands , and a code indicating the operation to be performed; the ALU's output is the result of the performed operation. In many designs, the ALU also has status inputs or outputs, or both, which convey information about a previous operation or the current operation, respectively, between the ALU and external status registers . An ALU has a variety of input and output nets , which are

4816-694: The degree of Motorola's process and manufacturing deficits in the early days, the team was undeterred and would not compromise in its pursuit of a microprocessor with industry-leading performance. 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

4902-520: The desired arithmetic or logic operation to be performed by the ALU. The opcode size (its bus width) determines the maximum number of distinct operations the ALU can perform; for example, a four-bit opcode can specify up to sixteen different ALU operations. Generally, an ALU opcode is not the same as a machine language instruction , though in some cases it may be directly encoded as a bit field within such instructions. The status outputs are various individual signals that convey supplemental information about

4988-416: The dual stack pointer (A7 and supervisor-mode A7') design of the 68000 makes this normally unnecessary, except when a task switch is performed in a multitasking system.) Having the two types of registers allows one 32-bit address and one 16-bit data calculation to take place at the same time. This results in reduced instruction execution time as addresses and data can be processed in parallel. The 68000 has

5074-672: The end of 1976. Crook formed the Motorola Advanced Computer System on Silicon (MACSS) project to build the design and hired Tom Gunter to be its principal architect. Gunter began forming his team in January 1977. The performance goal was set at 1 million instructions per second (MIPS). They wanted the design to not only win back microcomputer vendors like Apple Computer and Tandy , but also minicomputer companies like NCR and AT&T . The team decided to abandon an attempt at backward compatibility with

5160-409: 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 Sega Genesis/Mega Drive console, released in 1988,

5246-692: The ground plane (or "grounded") to produce the fastest-possible memory read/write time. However, any complex system would almost certainly have several different types of devices (RAM, ROM, various peripherals) that would support different speeds of access, which in a large system would normally be accommodated by using the DTACK line to insert "wait states"—delays—into bus cycles. (The 68000 has no separate I/O address space, so all I/O devices must be memory-mapped.) The logic circuitry required to pulse DTACK high to add these delays could be fairly complex. Therefore, typically only relatively simple systems can use

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5332-421: The market. In order to compete, they set themselves the goal of being two times as powerful at the same cost, or one-half the cost with the same performance. Crook decided that they would attack the high-end of the market with the most powerful processor on the market. Another 16-bit would not do, their design would have to be bigger, and that meant having some 32-bit features. Crook had decided on this approach by

5418-420: The maximum possible propagation delay). For example, a CPU starts an addition operation by routing the operands from their sources (typically processor registers ) to the ALU's operand inputs, while simultaneously applying a value to the ALU's opcode input that configures it to perform an addition operation. At the same time, the CPU enables the destination register to store the ALU output (the resulting sum from

5504-543: 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 computers starting with the Apple Lisa and Macintosh , and followed by the Amiga , Atari ST , and X68000 . The Sinclair QL microcomputer, along with its derivatives, such as the ICL One Per Desk business terminal,

5590-406: The opcode) and the shifted operand appears at Y. Simple ALUs typically can shift the operand by only one bit position, whereas more complex ALUs employ barrel shifters that allow them to shift the operand by an arbitrary number of bits in one operation. In all single-bit shift operations, the bit shifted out of the operand appears on carry-out; the value of the bit shifted into the operand depends on

5676-588: The operand fragments may be processed in any arbitrary order because each partial depends only on the corresponding operand fragments (the stored carry bit from the previous ALU operation is ignored). Although it is possible to design ALUs that can perform complex functions, this is usually impractical due to the resulting increases in circuit complexity, power consumption, propagation delay, cost and size. Consequently, ALUs are typically limited to simple functions that can be executed at very high speeds (i.e., very short propagation delays), with more complex functions being

5762-527: The other 68000 CPUs) was used in the Jaleco arcade game Wild Pilot for input/output (I/O) processing. 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

5848-410: The partial is written to designated storage. This process repeats until all operand fragments have been processed, resulting in a complete collection of partials in storage, which comprise the multi-precision arithmetic result. In multiple-precision shift operations, the order of operand fragment processing depends on the shift direction. In left-shift operations, fragments are processed LS first because

5934-415: The processor's state machine typically stores the carry out bit to an ALU status register. The algorithm then advances to the next fragment of each operand's collection and invokes an ALU operation on these fragments along with the stored carry bit from the previous ALU operation, thus producing another (more significant) partial and a carry out bit. As before, the carry bit is stored to the status register and

6020-417: The release of the 1989 Mac IIci. The 68000 family stores multi-byte 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 at the time in many ways. It was small enough to allow

6106-508: The required data, performed the operations and wrote out the results. MACSS was among the first to use this technique in a microprocessor. There was a large amount of support hardware for the 6800 that would remain useful, things like UARTs and similar interfacing systems. For this reason, the new design retained a bus protocol compatibility mode for existing 6800 peripheral devices. A chip with 32 data and 32 addressing pins would require 64 pins, plus more for power and other features. At

6192-455: The responsibility of external circuitry. For example: An ALU is usually implemented either as a stand-alone integrated circuit (IC), such as the 74181 , or as part of a more complex IC. In the latter case, an ALU is typically instantiated by synthesizing it from a description written in VHDL , Verilog or some other hardware description language . For example, the following VHDL code describes

6278-415: The rest of the line despite being limited to a 16-bit wide external bus. After 45 years in production , the 68000 architecture is still in use. Motorola's first widely produced microprocessor was the 6800 , introduced in early 1974 and available in quantity late that year. The company set itself the goal of selling 25,000 units by September 1976, a goal they did meet. Although a capable design, it

6364-498: The result of the current ALU operation. General-purpose ALUs commonly have status signals such as: The status inputs allow additional information to be made available to the ALU when performing an operation. Typically, this is a single "carry-in" bit that is the stored carry-out from a previous ALU operation. An ALU is a combinational logic circuit, meaning that its outputs will change asynchronously in response to input changes. In normal operation, stable signals are applied to all of

6450-423: 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

6536-611: 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, the 68000 has been used as a waveform display processor; some models including the LeCroy 9400/9400A 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

6622-432: The size of a fragment exactly matches the ALU word size, the ALU can directly operate on this "piece" of operand. The algorithm uses the ALU to directly operate on particular operand fragments and thus generate a corresponding fragment (a "partial") of the multi-precision result. Each partial, when generated, is written to an associated region of storage that has been designated for the multiple-precision result. This process

6708-422: The time, 64-pin dual inline package (DIP)s were "large, heavy-cost" systems and "just terrible", making that the largest they could consider. To make it fit, Crook selected a hybrid design, with a 32-bit instruction set architecture (ISA) but 16-bit components implementing it, like the arithmetic logic unit (ALU). The external interface was reduced to 16 data pins and 24 for addresses, allowing it all to fit in

6794-404: The type of shift. Upon completion of each ALU operation, the ALU's status output signals are usually stored in external registers to make them available for future ALU operations (e.g., to implement multiple-precision arithmetic ) and for controlling conditional branching . The bit registers that store the status output signals are often collectively treated as a single, multi-bit register, which

6880-464: 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

6966-466: Was built using the latest 5-inch wafer sizes and Intel's HMOS process with a 3.5  μm feature size. This was an investment aimed at catching the competition: even upstart semiconductor companies such as Zilog and MOS Technology had introduced CPUs fabricated on depletion-mode NMOS logic before Motorola did. In fact, Motorola may have substantially lagged contemporaries in phasing out enhancement mode and metal gate, with Gunter recollecting that

7052-580: Was eclipsed by more powerful designs, such as the Zilog Z80 , and less expensive designs, such as the MOS Technology 6502 . By late 1976, the sales book was flat and the division was only saved by a project for General Motors that turned into a huge product line for engine control and other tasks. By the time the 6800 was introduced, a small number of 16-bit designs had come to market. These were generally modeled on minicomputer platforms like

7138-437: Was full of amusing and educational stories about the way the personal computer hardware business worked, and strong opinions about the best way to design personal computer hardware and software. "DTACK" is the name of a pin on the Motorola 68000 CPU that informs the CPU that data is ready to be read from memory. It stands for "Data Transfer Acknowledge." If a system has fast enough memory, this pin can be connected directly to

7224-530: Was sometimes insufficient die space for a full-word-width ALU and, as a result, some early microprocessors employed a narrow ALU that required multiple cycles per machine language instruction. Examples of this includes the popular Zilog Z80 , which performed eight-bit additions with a four-bit ALU. Over time, transistor geometries shrank further, following Moore's law , and it became feasible to build wider ALUs on microprocessors. Modern integrated circuit (IC) transistors are orders of magnitude smaller than those of

7310-751: Was the most commercially important utilisation of the 68008. 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 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. The 68000 also saw great success as an embedded controller. As early as 1981, laser printers such as

7396-457: Was working on a 16-bit extension of their 8080 series, which would emerge as the Intel 8086 , and had heard rumors of a 16-bit Zilog Z80 , which became the Z8000 . These would use new design techniques that would eliminate the problems seen in earlier 16-bit systems. Motorola knew that if they launched a product similar to the 8086, within 10% of its capabilities, Intel would outperform them in

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