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To illustrate both specialization and multi-level caching, here is the cache hierarchy of the K8 core in the AMD Athlon 64 CPU.
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The K8 has four specialized caches: an instruction cache, an instruction TLB, a data TLB, and a data cache. Each of these caches is specialized:
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The K8 also has multiple-level caches. There are second-level instruction and data TLBs, which store only PTEs mapping 4 KiB. Both instruction and data caches, and the various TLBs, can fill from the large unified L2 cache. This cache is exclusive to both the L1 instruction and data caches, which means that any 8-byte line can only be in one of the L1 instruction cache, the L1 data cache, or the L2 cache. It is, however, possible for a line in the data cache to have a PTE which is also in one of the TLBs—the operating system is responsible for keeping the TLBs coherent by flushing portions of them when the page tables in memory are updated.
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The K8 also caches information that is never stored in memory—prediction information. These caches are not shown in the above diagram. As is usual for this class of CPU, the K8 has fairly complex
branch prediction, with tables that help predict whether branches are taken and other tables which predict the targets of branches and jumps. Some of this information is associated with instructions, in both the level 1 instruction cache and the unified secondary cache.
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The K8 uses an interesting trick to store prediction information with instructions in the secondary cache. Lines in the secondary cache are protected from accidental data corruption by either ECC or parity, depending on whether those lines were evicted from the data or instruction primary caches. Since the parity code takes fewer bits than the ECC code, lines from the instruction cache have a few spare bits. These bits are used to cache branch prediction information associated with those instructions. The net result is that the branch predictor has a larger effective history table, and so has better accuracy.
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Other processors have other kinds of predictors , and various specialized predictors are likely to flourish in future processors.
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These predictors are caches in that they store information that is costly to compute. Some of the terminology used when discussing predictors is the same as that for caches , but predictors are not generally thought of as part of the cache hierarchy.
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The K8 keeps the instruction and data caches coherent in hardware, which means that a store into an instruction closely following the store instruction will change that following instruction. Other processors, like those in the Alpha and MIPS family, have relied on software to keep the instruction cache coherent. Stores are not guaranteed to show up in the instruction stream until a program calls an operating system facility to ensure coherency.
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In computer engineering, a tag RAM is used to specify which of the possible memory locations is currently stored in a CPU cache. For a simple, direct-mapped design fast SRAM can be used. Higher associative caches usually employ content-addressable memory.
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Cache reads are the most common CPU operation that takes more than a single cycle. Program execution time tends to be very sensitive to the latency of a level-1 data cache hit. A great deal of design effort, and often power and silicon area are expended making the caches as fast as possible.
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The simplest cache is a virtually indexed direct-mapped cache. The virtual address is calculated with an adder, the relevant portion of the address extracted and used to index an SRAM, which returns the loaded data. The data is byte aligned in a byte shifter, and from there is bypassed to the next operation. There is no need for any tag checking in the inner loop – in fact, the tags need not even be read. Later in the pipeline, but before the load instruction is retired, the tag for the loaded data must be read, and checked against the virtual address to make sure there was a cache hit. On a miss, the cache is updated with the requested cache line and the pipeline is restarted.
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An associative cache is more complicated, because some form of tag must be read to determine which entry of the cache to select. An N-way set-associative level-1 cache usually reads all N possible tags and N data in parallel, and then chooses the data associated with the matching tag. Level-2 caches sometimes save power by reading the tags first, so that only one data element is read from the data SRAM.
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The adjacent diagram is intended to clarify the manner in which the various fields of the address are used. Address bit 31 is most significant, bit 0 is least significant. The diagram shows the SRAMs, indexing, and multiplexing for a 4 KiB, 2-way set-associative, virtually indexed and virtually tagged cache with 64 byte lines, a 32-bit read width and 32-bit virtual address.
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Because the cache is 4 KiB and has 64 B lines, there are just 64 lines in the cache, and we read two at a time from a Tag SRAM which has 32 rows, each with a pair of 21 bit tags. Although any function of virtual address bits 31 through 6 could be used to index the tag and data SRAMs, it is simplest to use the least significant bits.
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Similarly, because the cache is 4 KiB and has a 4 B read path, and reads two ways for each access, the Data SRAM is 512 rows by 8 bytes wide.
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A more modern cache might be 16 KiB, 4-way set-associative, virtually indexed, virtually hinted, and physically tagged, with 32 B lines, 32-bit read width and 36-bit physical addresses. The read path recurrence for such a cache looks very similar to the path above. Instead of tags, vhints are read, and matched against a subset of the virtual address. Later on in the pipeline, the virtual address is translated into a physical address by the TLB, and the physical tag is read . Finally the physical address is compared to the physical tag to determine if a hit has occurred.
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Some SPARC designs have improved the speed of their L1 caches by a few gate delays by collapsing the virtual address adder into the SRAM decoders. See sum-addressed decoder.
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The early history of cache technology is closely tied to the invention and use of virtual memory. Because of scarcity and cost of semi-conductor memories, early mainframe computers in the 1960s used a complex hierarchy of physical memory, mapped onto a flat virtual memory space used by programs. The memory technologies would span semi-conductor, magnetic core, drum and disc. Virtual memory seen and used by programs would be flat and caching would be used to fetch data and instructions into the fastest memory ahead of processor access. Extensive studies were done to optimize the cache sizes. Optimal values were found to depend greatly on the programming language used with Algol needing the smallest and Fortran and Cobol needing the largest cache sizes.
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In the early days of microcomputer technology, memory access was only slightly slower than register access. But since the 1980s the performance gap between processor and memory has been growing. Microprocessors have advanced much faster than memory, especially in terms of their operating frequency, so memory became a performance bottleneck. While it was technically possible to have all the main memory as fast as the CPU, a more economically viable path has been taken: use plenty of low-speed memory, but also introduce a small high-speed cache memory to alleviate the performance gap. This provided an order of magnitude more capacity—for the same price—with only a slightly reduced combined performance.
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The first documented uses of a TLB were on the GE 645 and the IBM 360/67, both of which used an associative memory as a TLB.
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The first documented use of an instruction cache was on the CDC 6600.
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The first documented use of a data cache was on the IBM System/360 Model 85.
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The 68010, released in 1982, has a "loop mode" which can be considered a tiny and special-case instruction cache that accelerates loops that consist of only two instructions. The 68020, released in 1984, replaced that with a typical instruction cache of 256 bytes, being the first 68k series processor to feature true on-chip cache memory.
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The 68030, released in 1987, is basically a 68020 core with an additional 256-byte data cache, an on-chip memory management unit , a process shrink, and added burst mode for the caches. The 68040, released in 1990, has split instruction and data caches of four kilobytes each. The 68060, released in 1994, has the following: 8 KiB data cache , 8 KiB instruction cache , 96-byte FIFO instruction buffer, 256-entry branch cache, and 64-entry address translation cache MMU buffer .
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As the x86 microprocessors reached clock rates of 20 MHz and above in the 386, small amounts of fast cache memory began to be featured in systems to improve performance. This was because the DRAM used for main memory had significant latency, up to 120 ns, as well as refresh cycles. The cache was constructed from more expensive, but significantly faster, SRAM memory cells, which at the time had latencies around 10–25 ns. The early caches were external to the processor and typically located on the motherboard in the form of eight or nine DIP devices placed in sockets to enable the cache as an optional extra or upgrade feature.
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Some versions of the Intel 386 processor could support 16 to 256 KiB of external cache.
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With the 486 processor, an 8 KiB cache was integrated directly into the CPU die. This cache was termed Level 1 or L1 cache to differentiate it from the slower on-motherboard, or Level 2 cache. These on-motherboard caches were much larger, with the most common size being 256 KiB. There were some system boards that contained sockets for the Intel 485Turbocache daughtercard which had either 64 or 128 Kbyte of cache memory. The popularity of on-motherboard cache continued through the Pentium MMX era but was made obsolete by the introduction of SDRAM and the growing disparity between bus clock rates and CPU clock rates, which caused on-motherboard cache to be only slightly faster than main memory.
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The next development in cache implementation in the x86 microprocessors began with the Pentium Pro, which brought the secondary cache onto the same package as the microprocessor, clocked at the same frequency as the microprocessor.
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On-motherboard caches enjoyed prolonged popularity thanks to the AMD K6-2 and AMD K6-III processors that still used Socket 7, which was previously used by Intel with on-motherboard caches. K6-III included 256 KiB on-die L2 cache and took advantage of the on-board cache as a third level cache, named L3 . After the Socket 7 became obsolete, on-motherboard cache disappeared from the x86 systems.
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The three-level caches were used again first with the introduction of multiple processor cores, where the L3 cache was added to the CPU die. It became common for the total cache sizes to be increasingly larger in newer processor generations, and recently it is not uncommon to find Level 3 cache sizes of tens of megabytes.
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Intel introduced a Level 4 on-package cache with the Haswell microarchitecture. Crystalwell Haswell CPUs, equipped with the GT3e variant of Intel's integrated Iris Pro graphics, effectively feature 128 MiB of embedded DRAM on the same package. This L4 cache is shared dynamically between the on-die GPU and CPU, and serves as a victim cache to the CPU's L3 cache.
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Apple M1 CPU has 128 or 192 KiB instruction L1 cache for each core , depending on core type. This is an unusually large L1 cache for any CPU type ; the total cache memory size is not unusually large for a laptop, and much larger total sizes are available in IBM's mainframes.
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Early cache designs focused entirely on the direct cost of cache and RAM and average execution speed.
More recent cache designs also consider energy efficiency, fault tolerance, and other goals.
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There are several tools available to computer architects to help explore tradeoffs between the cache cycle time, energy, and area; the CACTI cache simulator and the SimpleScalar instruction set simulator are two open-source options.
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A multi-ported cache is a cache which can serve more than one request at a time. When accessing a traditional cache we normally use a single memory address, whereas in a multi-ported cache we may request N addresses at a time – where N is the number of ports that connected through the processor and the cache. The benefit of this is that a pipelined processor may access memory from different phases in its pipeline. Another benefit is that it allows the concept of super-scalar processors through different cache levels.
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Firmware is found in a wide-range of computing devices including personal computers, phones, home appliances, vehicles, computer peripherals and in many of the digital chips inside each of these larger systems.
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Firmware is stored in non-volatile memory – either read-only memory or programmable memory such as EPROM, EEPROM, or flash. Changing a device's firmware stored in ROM requires physically replacing the memory chip – although some chips are not designed to be removed after manufacture. Programmable firmware memory can be reprogrammed via a procedure sometimes called flashing.
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Common reasons for changing firmware include fixing bugs and adding features.
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Ascher Opler used the term firmware in a 1967 Datamation article, as an intermediary term between "hardware" and "software". In this article, Opler was referring to a new kind of computer program that had a different practical and psychological purpose from traditional programs from the user's perspective.
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As computers began to increase in complexity, it became clear that various programs needed to first be initiated and run to provide a consistent environment necessary for running more complex programs at the user's discretion. This required programming the computer to run those programs automatically. Furthermore, as companies, universities, and marketers wanted to sell computers to laypeople with little technical knowledge, greater automation became necessary to allow a lay-user to easily run programs for practical purposes. This gave rise to a kind of software that a user would not consciously run, and it led to software that a lay user wouldn't even know about.
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Originally, it meant the contents of a writable control store , containing microcode that defined and implemented the computer's instruction set, and that could be reloaded to specialize or modify the instructions that the central processing unit could execute. As originally used, firmware contrasted with hardware and software . It was not composed of CPU machine instructions, but of lower-level microcode involved in the implementation of machine instructions. It existed on the boundary between hardware and software; thus the name firmware. Over time, popular usage extended the word firmware to denote any computer program that is tightly linked to hardware, including BIOS on PCs, boot firmware on smartphones, computer peripherals, or the control systems on simple consumer electronic devices such as microwave ovens, remote controls.
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In some respects, the various firmware components are as important as the operating system in a working computer. However, unlike most modern operating systems, firmware rarely has a well-evolved automatic mechanism of updating itself to fix any functionality issues detected after shipping the unit.
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A computer's firmware may be manually updated by a user via a small utility program. In contrast, firmware in mass storage devices is less frequently updated, even when flash memory storage is used for the firmware.
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Most computer peripherals are themselves special-purpose computers. Devices such as printers, scanners, webcams, and USB flash drives have internally-stored firmware; some devices may also permit field upgrading of their firmware.
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Examples of computer firmware include:
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Consumer appliances like gaming consoles, digital cameras and portable music players support firmware upgrades. Some companies use firmware updates to add new playable file formats . Other features that may change with firmware updates include the GUI or even the battery life. Smartphones have a firmware over the air upgrade capability for adding new features and patching security issues.
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Since 1996, most automobiles have employed an on-board computer and various sensors to detect mechanical problems. As of 2010, modern vehicles also employ computer-controlled anti-lock braking systems and computer-operated transmission control units . The driver can also get in-dash information while driving in this manner, such as real-time fuel economy and tire pressure readings. Local dealers can update most vehicle firmware.
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Other firmware applications include:
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Flashing involves the overwriting of existing firmware or data, contained in EEPROM or flash memory module present in an electronic device, with new data. This can be done to upgrade a device or to change the provider of a service associated with the function of the device, such as changing from one mobile phone service provider to another or installing a new operating system. If firmware is upgradable, it is often done via a program from the provider, and will often allow the old firmware to be saved before upgrading so it can be reverted to if the process fails, or if the newer version performs worse. Free software replacements for vendor flashing tools have been developed, such as Flashrom.
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Sometimes, third parties develop an unofficial new or modified version of firmware to provide new features or to unlock hidden functionality; this is referred to as custom firmware. An example is Rockbox as a firmware replacement for portable media players. There are many homebrew projects for various devices, which often unlock general-purpose computing functionality in previously limited devices .
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Firmware hacks usually take advantage of the firmware update facility on many devices to install or run themselves. Some, however, must resort to exploits to run, because the manufacturer has attempted to lock the hardware to stop it from running unlicensed code.
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Most firmware hacks are free software.
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The Moscow-based Kaspersky Lab discovered that a group of developers it refers to as the "Equation Group" has developed hard disk drive firmware modifications for various drive models, containing a trojan horse that allows data to be stored on the drive in locations that will not be erased even if the drive is formatted or wiped. Although the Kaspersky Lab report did not explicitly claim that this group is part of the United States National Security Agency , evidence obtained from the code of various Equation Group software suggests that they are part of the NSA.
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Researchers from the Kaspersky Lab categorized the undertakings by Equation Group as the most advanced hacking operation ever uncovered, also documenting around 500 infections caused by the Equation Group in at least 42 countries.
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Mark Shuttleworth, the founder of the company Canonical, which created the Ubuntu Linux distribution, has described proprietary firmware as a security risk, saying that "firmware on your device is the NSA's best friend" and calling firmware "a trojan horse of monumental proportions". He has asserted that low-quality, closed source firmware is a major threat to system security: "Your biggest mistake is to assume that the NSA is the only institution abusing this position of trust – in fact, it's reasonable to assume that all firmware is a cesspool of insecurity, courtesy of incompetence of the highest degree from manufacturers, and competence of the highest degree from a very wide range of such agencies". As a potential solution to this problem, he has called for declarative firmware, which would describe "hardware linkage and dependencies" and "should not include executable code". Firmware should be open-source so that the code can be checked and verified.
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Custom firmware hacks have also focused on injecting malware into devices such as smartphones or USB devices. One such smartphone injection was demonstrated on the Symbian OS at MalCon, a hacker convention. A USB device firmware hack called BadUSB was presented at the Black Hat USA 2014 conference, demonstrating how a USB flash drive microcontroller can be reprogrammed to spoof various other device types to take control of a computer, exfiltrate data, or spy on the user. Other security researchers have worked further on how to exploit the principles behind BadUSB, releasing at the same time the source code of hacking tools that can be used to modify the behavior of different USB devices.
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An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is embedded as part of a complete device often including electrical or electronic hardware and mechanical parts.
Because an embedded system typically controls physical operations of the machine that it is embedded within, it often has real-time computing constraints. Embedded systems control many devices in common use. In 2009, it was estimated that ninety-eight percent of all microprocessors manufactured were used in embedded systems.
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Modern embedded systems are often based on microcontrollers , but ordinary microprocessors are also common, especially in more complex systems. In either case, the processor used may be types ranging from general purpose to those specialized in a certain class of computations, or even custom designed for the application at hand. A common standard class of dedicated processors is the digital signal processor .
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Since the embedded system is dedicated to specific tasks, design engineers can optimize it to reduce the size and cost of the product and increase its reliability and performance. Some embedded systems are mass-produced, benefiting from economies of scale.
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Embedded systems range in size from portable personal devices such as digital watches and MP3 players to bigger machines like home appliances, industrial assembly lines, robots, transport vehicles, traffic light controllers, and medical imaging systems. Often they constitute subsystems of other machines like avionics in aircraft and astrionics in spacecraft. Large installations like factories, pipelines, and electrical grids rely on multiple embedded systems networked together. Generalized through software customization, embedded systems such as programmable logic controllers frequently comprise their functional units.
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Embedded systems range from those low in complexity, with a single microcontroller chip, to very high with multiple units, peripherals and networks, which may reside in equipment racks or across large geographical areas connected via long-distance communications lines.
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The origins of the microprocessor and the microcontroller can be traced back to the MOS integrated circuit, which is an integrated circuit chip fabricated from MOSFETs and was developed in the early 1960s. By 1964, MOS chips had reached higher transistor density and lower manufacturing costs than bipolar chips. MOS chips further increased in complexity at a rate predicted by Moore's law, leading to large-scale integration with hundreds of transistors on a single MOS chip by the late 1960s. The application of MOS LSI chips to computing was the basis for the first microprocessors, as engineers began recognizing that a complete computer processor system could be contained on several MOS LSI chips.
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The first multi-chip microprocessors, the Four-Phase Systems AL1 in 1969 and the Garrett AiResearch MP944 in 1970, were developed with multiple MOS LSI chips. The first single-chip microprocessor was the Intel 4004, released in 1971. It was developed by Federico Faggin, using his silicon-gate MOS technology, along with Intel engineers Marcian Hoff and Stan Mazor, and Busicom engineer Masatoshi Shima.
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One of the first recognizably modern embedded systems was the Apollo Guidance Computer, developed ca. 1965 by Charles Stark Draper at the MIT Instrumentation Laboratory. At the project's inception, the Apollo guidance computer was considered the riskiest item in the Apollo project as it employed the then newly developed monolithic integrated circuits to reduce the computer's size and weight.
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An early mass-produced embedded system was the Autonetics D-17 guidance computer for the Minuteman missile, released in 1961. When the Minuteman II went into production in 1966, the D-17 was replaced with a new computer that represented the first high-volume use of integrated circuits.
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Since these early applications in the 1960s, embedded systems have come down in price and there has been a dramatic rise in processing power and functionality. An early microprocessor, the Intel 4004 , was designed for calculators and other small systems but still required external memory and support chips. By the early 1980s, memory, input and output system components had been integrated into the same chip as the processor forming a microcontroller. Microcontrollers find applications where a general-purpose computer would be too costly. As the cost of microprocessors and microcontrollers fell, the prevalence of embedded systems increased.
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A comparatively low-cost microcontroller may be programmed to fulfill the same role as a large number of separate components. With microcontrollers, it became feasible to replace, even in consumer products, expensive knob-based analog components such as potentiometers and variable capacitors with up/down buttons or knobs read out by a microprocessor. Although in this context an embedded system is usually more complex than a traditional solution, most of the complexity is contained within the microcontroller itself. Very few additional components may be needed and most of the design effort is in the software. Software prototype and test can be quicker compared with the design and construction of a new circuit not using an embedded processor.
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Embedded systems are commonly found in consumer, industrial, automotive, home appliances, medical, telecommunication, commercial, aerospace and military applications.
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Telecommunications systems employ numerous embedded systems from telephone switches for the network to cell phones at the end user. Computer networking uses dedicated routers and network bridges to route data.
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Consumer electronics include MP3 players, television sets, mobile phones, video game consoles, digital cameras, GPS receivers, and printers. Household appliances, such as microwave ovens, washing machines and dishwashers, include embedded systems to provide flexibility, efficiency and features. Advanced heating, ventilation, and air conditioning systems use networked thermostats to more accurately and efficiently control temperature that can change by time of day and season. Home automation uses wired- and wireless-networking that can be used to control lights, climate, security, audio/visual, surveillance, etc., all of which use embedded devices for sensing and controlling.
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Transportation systems from flight to automobiles increasingly use embedded systems. New airplanes contain advanced avionics such as inertial guidance systems and GPS receivers that also have considerable safety requirements. Spacecraft rely on astrionics systems for trajectory correction. Various electric motors — brushless DC motors, induction motors and DC motors — use electronic motor controllers. Automobiles, electric vehicles, and hybrid vehicles increasingly use embedded systems to maximize efficiency and reduce pollution. Other automotive safety systems using embedded systems include anti-lock braking system , electronic stability control , traction control and automatic four-wheel drive.
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Medical equipment uses embedded systems for monitoring, and various medical imaging , single-photon emission computed tomography , computed tomography , and magnetic resonance imaging for non-invasive internal inspections. Embedded systems within medical equipment are often powered by industrial computers.
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Embedded systems are used for safety-critical systems in aerospace and defense industries. Unless connected to wired or wireless networks via on-chip 3G cellular or other methods for IoT monitoring and control purposes, these systems can be isolated from hacking and thus be more secure. For fire safety, the systems can be designed to have a greater ability to handle higher temperatures and continue to operate. In dealing with security, the embedded systems can be self-sufficient and be able to deal with cut electrical and communication systems.
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Miniature wireless devices called motes are networked wireless sensors. Wireless sensor networking makes use of miniaturization made possible by advanced integrated circuit design to couple full wireless subsystems to sophisticated sensors, enabling people and companies to measure a myriad of things in the physical world and act on this information through monitoring and control systems. These motes are completely self-contained and will typically run off a battery source for years before the batteries need to be changed or charged.
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Embedded systems are designed to perform a specific task, in contrast with general-purpose computers designed for multiple tasks. Some have real-time performance constraints that must be met, for reasons such as safety and usability; others may have low or no performance requirements, allowing the system hardware to be simplified to reduce costs.
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Embedded systems are not always standalone devices. Many embedded systems are a small part within a larger device that serves a more general purpose. For example, the Gibson Robot Guitar features an embedded system for tuning the strings, but the overall purpose of the Robot Guitar is to play music. Similarly, an embedded system in an automobile provides a specific function as a subsystem of the car itself.
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The program instructions written for embedded systems are referred to as firmware, and are stored in read-only memory or flash memory chips. They run with limited computer hardware resources: little memory, small or non-existent keyboard or screen.
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Embedded systems range from no user interface at all, in systems dedicated to one task, to complex graphical user interfaces that resemble modern computer desktop operating systems. Simple embedded devices use buttons, light-emitting diodes , graphic or character liquid-crystal displays with a simple menu system. More sophisticated devices that use a graphical screen with touch sensing or screen-edge soft keys provide flexibility while minimizing space used: the meaning of the buttons can change with the screen, and selection involves the natural behavior of pointing at what is desired.
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Some systems provide user interface remotely with the help of a serial or network connection. This approach extends the capabilities of the embedded system, avoids the cost of a display, simplifies the board support package and allows designers to build a rich user interface on the PC. A good example of this is the combination of an embedded HTTP server running on an embedded device . The user interface is displayed in a web browser on a PC connected to the device.
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Examples of properties of typical embedded computers when compared with general-purpose counterparts, are low power consumption, small size, rugged operating ranges, and low per-unit cost. This comes at the expense of limited processing resources.
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Numerous microcontrollers have been developed for embedded systems use. General-purpose microprocessors are also used in embedded systems, but generally, require more support circuitry than microcontrollers.
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PC/104 and PC/104+ are examples of standards for ready-made computer boards intended for small, low-volume embedded and ruggedized systems. These are mostly x86-based and often physically small compared to a standard PC, although still quite large compared to most simple embedded systems. They may use DOS, FreeBSD, Linux, NetBSD, OpenHarmony or an embedded real-time operating system such as MicroC/OS-II, QNX or VxWorks.
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In certain applications, where small size or power efficiency are not primary concerns, the components used may be compatible with those used in general-purpose x86 personal computers. Boards such as the VIA EPIA range help to bridge the gap by being PC-compatible but highly integrated, physically smaller or have other attributes making them attractive to embedded engineers. The advantage of this approach is that low-cost commodity components may be used along with the same software development tools used for general software development. Systems built in this way are still regarded as embedded since they are integrated into larger devices and fulfill a single role. Examples of devices that may adopt this approach are automated teller machines and arcade machines, which contain code specific to the application.
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However, most ready-made embedded systems boards are not PC-centered and do not use the ISA or PCI busses. When a system-on-a-chip processor is involved, there may be little benefit to having a standardized bus connecting discrete components, and the environment for both hardware and software tools may be very different.
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One common design style uses a small system module, perhaps the size of a business card, holding high density BGA chips such as an ARM-based system-on-a-chip processor and peripherals, external flash memory for storage, and DRAM for runtime memory. The module vendor will usually provide boot software and make sure there is a selection of operating systems, usually including Linux and some real-time choices. These modules can be manufactured in high volume, by organizations familiar with their specialized testing issues, and combined with much lower volume custom mainboards with application-specific external peripherals. Prominent examples of this approach include Arduino and Raspberry Pi.
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A system on a chip contains a complete system - consisting of multiple processors, multipliers, caches, even different types of memory and commonly various peripherals like interfaces for wired or wireless communication on a single chip. Often graphics processing units and DSPs are included such chips. SoCs can be implemented as an application-specific integrated circuit or using a field-programmable gate array which typically can be reconfigured.
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ASIC implementations are common for very-high-volume embedded systems like mobile phones and smartphones. ASIC or FPGA implementations may be used for not-so-high-volume embedded systems with special needs in kind of signal processing performance, interfaces and reliability, like in avionics.
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Embedded systems talk with the outside world via peripherals, such as:
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As with other software, embedded system designers use compilers, assemblers, and debuggers to develop embedded system software. However, they may also use more specific tools:
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Software tools can come from several sources:
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As the complexity of embedded systems grows, higher-level tools and operating systems are migrating into machinery where it makes sense. For example, cellphones, personal digital assistants and other consumer computers often need significant software that is purchased or provided by a person other than the manufacturer of the electronics. In these systems, an open programming environment such as Linux, NetBSD, FreeBSD, OSGi or Embedded Java is required so that the third-party software provider can sell to a large market.
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Embedded debugging may be performed at different levels, depending on the facilities available. Considerations include: does it slow down the main application, how close is the debugged system or application to the actual system or application, how expressive are the triggers that can be set for debugging , and what can be inspected in the debugging process .
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3,693
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From simplest to most sophisticated debugging techniques and systems be roughly grouped into the following areas:
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3,694
|
Unless restricted to external debugging, the programmer can typically load and run software through the tools, view the code running in the processor, and start or stop its operation. The view of the code may be as high-level programming language, assembly code or mixture of both.
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3,695
|
Real-time operating systems often support tracing of operating system events. A graphical view is presented by a host PC tool, based on a recording of the system behavior. The trace recording can be performed in software, by the RTOS, or by special tracing hardware. RTOS tracing allows developers to understand timing and performance issues of the software system and gives a good understanding of the high-level system behaviors. Trace recording in embedded systems can be achieved using hardware or software solutions. Software-based trace recording does not require specialized debugging hardware and can be used to record traces in deployed devices, but it can have an impact on CPU and RAM usage. One example of a software-based tracing method used in RTOS environments is the use of empty macros which are invoked by the operating system at strategic places in the code, and can be implemented to serve as hooks.
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3,696
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Embedded systems often reside in machines that are expected to run continuously for years without error, and in some cases recover by themselves if an error occurs. Therefore, the software is usually developed and tested more carefully than that for personal computers, and unreliable mechanical moving parts such as disk drives, switches or buttons are avoided.
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3,697
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Specific reliability issues may include:
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3,698
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A variety of techniques are used, sometimes in combination, to recover from errors—both software bugs such as memory leaks, and also soft errors in the hardware:
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3,699
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For high-volume systems such as mobile phones, minimizing cost is usually the primary design consideration. Engineers typically select hardware that is just good enough to implement the necessary functions.
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3,700
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For low-volume or prototype embedded systems, general-purpose computers may be adapted by limiting the programs or by replacing the operating system with an RTOS.
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