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51-576: The specification describes a system memory structure for computer hardware vendors to exchange data between host system memory and attached storage devices . AHCI gives software developers and hardware designers a standard method for detecting, configuring, and programming SATA/AHCI adapters. AHCI is separate from the SATA 3 Gbit/s standard, although it exposes SATA's advanced capabilities (such as hot swapping and native command queuing ) such that host systems can utilize them. For modern solid state drives ,

102-441: A mass storage cache and write buffer to improve both reading and writing performance. Operating systems borrow RAM capacity for caching so long as it is not needed by running software. If needed, contents of the computer memory can be transferred to storage; a common way of doing this is through a memory management technique called virtual memory . Modern computer memory is implemented as semiconductor memory , where data

153-634: A notification will tell you that you are in safe mode. In that mode, some apps may not launch. The way that safe mode in Android can be activated differs per vendor. Safe mode can be disabled by rebooting the device. When you reboot into safe mode in Android, downloaded apps and widgets are automatically disabled, but built-in apps remain available. A watermark in the bottom-left corner also appears if you're booted to either normal or safe mode. Some devices also turn their radios off automatically upon booting into safe mode. Application software sometimes offers

204-520: A safe mode as well. In the PHP interpreter , prior to version 5.4, safe mode offers stricter security measures. The Glasgow Haskell Compiler from version 7.2 offers "Safe Haskell" mode, restricting usage of functions such as unsafePerformIO . Mozilla Firefox 's safe mode allows the user to remove extensions which may be preventing the browser from loading. Internet Explorer can run in "No Add-Ons" mode and Protected Mode. Cydia 's MobileSubstrate also has

255-513: A safe mode with networking enabled, in the macOS Safe Boot always includes networking. On the Classic Mac OS versions 6, 7, 8, and 9, a similar mode to the Unix root is achieved by holding down the shift key while booting, which starts the system without extensions . iOS does not have a safe mode, however some jailbreaks add a safe mode where all tweaks are disabled, the wallpaper is black and

306-593: Is semi-volatile . The term is used to describe a memory that has some limited non-volatile duration after power is removed, but then data is ultimately lost. A typical goal when using a semi-volatile memory is to provide the high performance and durability associated with volatile memories while providing some benefits of non-volatile memory. For example, some non-volatile memory types experience wear when written. A worn cell has increased volatility but otherwise continues to work. Data locations which are written frequently can thus be directed to use worn circuits. As long as

357-505: Is a software backward-compatibility mechanism intended to allow the SATA controller to run in legacy operating systems which are not SATA-aware or where a driver does not exist to make the operating system SATA-aware. When a SATA controller is configured to operate in IDE Mode, the number of storage devices per controller is usually limited to four (two IDE channels, master device and slave device with up to two devices per channel), compared to

408-464: Is a system where each program is given an area of memory to use and is prevented from going outside that range. If the operating system detects that a program has tried to alter memory that does not belong to it, the program is terminated (or otherwise restricted or redirected). This way, only the offending program crashes, and other programs are not affected by the misbehavior (whether accidental or intentional). Use of protected memory greatly enhances both

459-545: Is accessed by pressing the F8 key as the operating system boots. Also, in a multi-boot environment with multiple versions of Windows installed side by side, the F8 key can be pressed at the OS selector prompt to get to safe mode. However, under Windows 8 (released in 2012), the traditional press-F8-for-safe-mode-options UI convention no longer works, and either Shift-F8 or a special GUI-based workaround

510-409: Is also used to describe semi-volatile behavior constructed from other memory types, such as nvSRAM , which combines SRAM and a non-volatile memory on the same chip , where an external signal copies data from the volatile memory to the non-volatile memory, but if power is removed before the copy occurs, the data is lost. Another example is battery-backed RAM , which uses an external battery to power

561-706: Is compiled as a kernel module rather than built into the kernel image , as it may not be included in the initrd (initial RAM disk) created when the controller is configured to run in Legacy Mode. The solution is either to build a new initrd containing the AHCI module, or to build the AHCI driver into the kernel image. Power management is handled by the Aggressive Link Power Management (ALPM) protocol. Computer memory Computer memory stores information, such as data and programs, for immediate use in

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612-470: Is computer memory that requires power to maintain the stored information. Most modern semiconductor volatile memory is either static RAM (SRAM) or dynamic RAM (DRAM). DRAM dominates for desktop system memory. SRAM is used for CPU cache . SRAM is also found in small embedded systems requiring little memory. SRAM retains its contents as long as the power is connected and may use a simpler interface, but commonly uses six transistors per bit . Dynamic RAM

663-429: Is intended to help fix most, if not all, problems within an operating system. It is also widely used for removing rogue security software. Microsoft Windows , macOS , Android and Linux distributions such as Ubuntu and Linux Mint are examples of contemporary operating systems that implement a safe mode as well as other complex electronic devices . In safe mode, an operating system has reduced functionality, but

714-918: Is more complicated for interfacing and control, needing regular refresh cycles to prevent losing its contents, but uses only one transistor and one capacitor per bit, allowing it to reach much higher densities and much cheaper per-bit costs. Non-volatile memory can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory , flash memory , most types of magnetic computer storage devices (e.g. hard disk drives , floppy disks and magnetic tape ), optical discs , and early computer storage methods such as magnetic drum , paper tape and punched cards . Non-volatile memory technologies under development include ferroelectric RAM , programmable metallization cell , Spin-transfer torque magnetic RAM , SONOS , resistive random-access memory , racetrack memory , Nano-RAM , 3D XPoint , and millipede memory . A third category of memory

765-512: Is necessary. An equivalently minimal setting in Unix-like operating systems is single-user mode , in which daemons and the X Window System are not started, and only the root user can log in . It can do emergency repairs or maintenance, including resetting users' passwords on the machine without the need to know the old one. In macOS holding the shift key after powering up activates Safe Boot that has background maintenance features (Besides

816-411: Is organized into memory cells each storing one bit (0 or 1). Flash memory organization includes both one bit per memory cell and a multi-level cell capable of storing multiple bits per cell. The memory cells are grouped into words of fixed word length , for example, 1, 2, 4, 8, 16, 32, 64 or 128 bits. Each word can be accessed by a binary address of N bits, making it possible to store 2 words in

867-479: Is physically stored or whether the user's computer will have enough memory. The operating system will place actively used data in RAM, which is much faster than hard disks. When the amount of RAM is not sufficient to run all the current programs, it can result in a situation where the computer spends more time moving data from RAM to disk and back than it does accomplishing tasks; this is known as thrashing . Protected memory

918-488: Is stored within memory cells built from MOS transistors and other components on an integrated circuit . There are two main kinds of semiconductor memory: volatile and non-volatile . Examples of non-volatile memory are flash memory and ROM , PROM , EPROM , and EEPROM memory. Examples of volatile memory are dynamic random-access memory (DRAM) used for primary storage and static random-access memory (SRAM) used mainly for CPU cache . Most semiconductor memory

969-497: Is that the system drive typically fails to boot, with an ensuing error message, if the SATA controller (in BIOS) is inadvertently switched to AHCI mode after OS installation. In Microsoft Windows the symptom is a boot loop which begins with a Blue Screen error, if not rectified. Technically speaking, this is an implementation bug with AHCI that can be avoided, but it has not been fixed yet. As an interim resolution, Intel recommends changing

1020-498: The BIOS will allow the user to boot into Windows, and thereby the required registry change can be performed. Consequently, the user then has the option of continuing to use the system in Combined mode or switching to AHCI mode. Inter alia with Windows 10 and 8, this can be fixed by forcing the correct drivers to reload during Safe Mode . In Windows 8, Windows 8.1 and Windows Server 2012 ,

1071-490: The Electrotechnical Laboratory in 1972. Flash memory was invented by Fujio Masuoka at Toshiba in the early 1980s. Masuoka and colleagues presented the invention of NOR flash in 1984, and then NAND flash in 1987. Toshiba commercialized NAND flash memory in 1987. Developments in technology and economies of scale have made possible so-called very large memory (VLM) computers. Volatile memory

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1122-488: The Royal Radar Establishment proposed digital storage systems that use CMOS (complementary MOS) memory cells, in addition to MOSFET power devices for the power supply , switched cross-coupling, switches and delay-line storage . The development of silicon-gate MOS integrated circuit (MOS IC) technology by Federico Faggin at Fairchild in 1968 enabled the production of MOS memory chips . NMOS memory

1173-544: The System/360 Model 95 . Toshiba introduced bipolar DRAM memory cells for its Toscal BC-1411 electronic calculator in 1965. While it offered improved performance, bipolar DRAM could not compete with the lower price of the then dominant magnetic-core memory. MOS technology is the basis for modern DRAM. In 1966, Robert H. Dennard at the IBM Thomas J. Watson Research Center was working on MOS memory. While examining

1224-535: The Whirlwind I computer in 1953. Magnetic-core memory was the dominant form of memory until the development of MOS semiconductor memory in the 1960s. The first semiconductor memory was implemented as a flip-flop circuit in the early 1960s using bipolar transistors . Semiconductor memory made from discrete devices was first shipped by Texas Instruments to the United States Air Force in 1961. In

1275-434: The computer . The term memory is often synonymous with the terms RAM , main memory , or primary storage . Archaic synonyms for main memory include core (for magnetic core memory) and store . Main memory operates at a high speed compared to mass storage which is slower but less expensive per bit and higher in capacity. Besides storing opened programs and data being actively processed, computer memory serves as

1326-541: The Arma Division of the American Bosch Arma Corporation. In 1967, Dawon Kahng and Simon Sze of Bell Labs proposed that the floating gate of a MOS semiconductor device could be used for the cell of a reprogrammable ROM, which led to Dov Frohman of Intel inventing EPROM (erasable PROM) in 1971. EEPROM (electrically erasable PROM) was developed by Yasuo Tarui, Yutaka Hayashi and Kiyoko Naga at

1377-595: The box. Some operating systems, notably Windows Vista , Windows 7 , Windows 8 , Windows 8.1 and Windows 10 , do not configure themselves to load the AHCI driver upon boot if the SATA controller was not in AHCI mode at the time the operating system was installed. Although this is an easily rectifiable condition, it remains an ongoing issue with the AHCI standard. The most prevalent symptom for an operating system (or systems) that are installed in IDE mode (in some BIOS firmware implementations otherwise called 'Combined IDE mode'),

1428-487: The characteristics of MOS technology, he found it was possible to build capacitors , and that storing a charge or no charge on the MOS capacitor could represent the 1 and 0 of a bit, while the MOS transistor could control writing the charge to the capacitor. This led to his development of a single-transistor DRAM memory cell. In 1967, Dennard filed a patent for a single-transistor DRAM memory cell based on MOS technology. This led to

1479-419: The controller driver has changed from msahci to storahci , and the procedures to upgrade to the AHCI controller is similar to that of Windows 7. On Windows 8, 8.1 and Windows Server 2012, changing from IDE mode to AHCI mode without first updating the registry will make the boot drive inaccessible (i.e. resulting in a recurring boot loop, which begins with a Blue Screen error). In Windows 10, after changing

1530-501: The controller to AHCI mode, if the OS is allowed to reboot a couple of times after the start of the boot loop, which starts with an INACCESSIBLE_BOOT_DEVICE BSOD, Windows presents recovery options. Out of the Advanced options, if Startup Repair option is selected, Windows attempts to fix the issue and the PC begins to function normally. A similar problem can occur on Linux systems if the AHCI driver

1581-582: The delay line, the Williams tube and Selectron tube , originated in 1946, both using electron beams in glass tubes as means of storage. Using cathode-ray tubes , Fred Williams invented the Williams tube, which was the first random-access computer memory . The Williams tube was able to store more information than the Selectron tube (the Selectron was limited to 256 bits, while the Williams tube could store thousands) and

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1632-402: The drive controller to AHCI or RAID before installing an operating system. (It may also be necessary to load chipset-specific AHCI or RAID drivers at installation time, for example from a USB flash drive). On Windows Vista and Windows 7, this can be fixed by configuring the msahci device driver to start at boot time (rather than on-demand). Setting non-AHCI mode (i.e. IDE or Combined mode) in

1683-446: The dysfunctional OS but with features, such as video drivers, audio, and networking, disabled. Safe mode typically provides access to utility and diagnostic programs so a user can troubleshoot what is preventing the operating system from working normally. Safe mode is intended for maintenance, not functionality, and it provides minimal access to features. Microsoft Windows' safe mode (for 7 / Vista / XP / 2000 / ME / 98 / 95 )

1734-401: The early 1940s. Through the construction of a glass tube filled with mercury and plugged at each end with a quartz crystal, delay lines could store bits of information in the form of sound waves propagating through the mercury, with the quartz crystals acting as transducers to read and write bits. Delay-line memory was limited to a capacity of up to a few thousand bits. Two alternatives to

1785-577: The first commercial DRAM IC chip, the Intel 1103 in October 1970. Synchronous dynamic random-access memory (SDRAM) later debuted with the Samsung KM48SL2000 chip in 1992. The term memory is also often used to refer to non-volatile memory including read-only memory (ROM) through modern flash memory . Programmable read-only memory (PROM) was invented by Wen Tsing Chow in 1956, while working for

1836-518: The following types: Virtual memory is a system where physical memory is managed by the operating system typically with assistance from a memory management unit , which is part of many modern CPUs . It allows multiple types of memory to be used. For example, some data can be stored in RAM while other data is stored on a hard drive (e.g. in a swapfile ), functioning as an extension of the cache hierarchy . This offers several advantages. Computer programmers no longer need to worry about where their data

1887-466: The form of a parallel "miniature" operating system that has no configuration information shared with the normal operating system. For example, on Microsoft Windows, the user can also choose to boot to the Recovery Console , a small text-based troubleshooting mode kept separate from the main operating system (which can also be accessed by booting the install CD) or to various "safe mode" options that run

1938-526: The interface has been superseded by NVMe . The current version of the specification is 1.3.1. Many SATA controllers offer selectable modes of operation: legacy Parallel ATA emulation (more commonly called IDE Mode), standard AHCI mode (also known as Native Mode), or vendor-specific RAID (which generally enables AHCI in order to take advantage of its capabilities). Intel recommends choosing RAID mode on their motherboards (which also enables AHCI) rather than AHCI/SATA mode for maximum flexibility. Legacy mode

1989-449: The late 1960s. The invention of the metal–oxide–semiconductor field-effect transistor ( MOSFET ) enabled the practical use of metal–oxide–semiconductor (MOS) transistors as memory cell storage elements. MOS memory was developed by John Schmidt at Fairchild Semiconductor in 1964. In addition to higher performance, MOS semiconductor memory was cheaper and consumed less power than magnetic core memory. In 1965, J. Wood and R. Ball of

2040-447: The location is updated within some known retention time, the data stays valid. After a period of time without update, the value is copied to a less-worn circuit with longer retention. Writing first to the worn area allows a high write rate while avoiding wear on the not-worn circuits. As a second example, an STT-RAM can be made non-volatile by building large cells, but doing so raises the cost per bit and power requirements and reduces

2091-774: The maximum of 32 devices/ports when configured in AHCI mode. But the chipset SATA interfaces may emulate more than one "IDE controller" when configured in IDE Mode. AHCI is supported out of the box on Windows Vista and later, Linux -based operating systems (since version 2.6.19 of the kernel ), OpenBSD (since version 4.1), NetBSD (since version 4.0), FreeBSD (since version 8.0), macOS , GNU Mach , ArcaOS , eComStation (since version 2.1), and Solaris 10 (since version 8/07). DragonFlyBSD based its AHCI implementation on OpenBSD's and added extended features such as port multiplier support. Older versions of operating systems require hardware-specific drivers in order to support AHCI. Windows XP and older do not provide AHCI support out of

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2142-457: The memory device in case of external power loss. If power is off for an extended period of time, the battery may run out, resulting in data loss. Proper management of memory is vital for a computer system to operate properly. Modern operating systems have complex systems to properly manage memory. Failure to do so can lead to bugs or slow performance. Improper management of memory is a common cause of bugs and security vulnerabilities, including

2193-422: The memory used by other programs. This is done by viruses and malware to take over computers. It may also be used benignly by desirable programs which are intended to modify other programs, debuggers , for example, to insert breakpoints or hooks. Safe Mode Safe mode is a diagnostic mode of a computer operating system (OS). It can also refer to a mode of operation by application software . Safe mode

2244-474: The memory. In the early 1940s, memory technology often permitted a capacity of a few bytes. The first electronic programmable digital computer , the ENIAC , using thousands of vacuum tubes , could perform simple calculations involving 20 numbers of ten decimal digits stored in the vacuum tubes. The next significant advance in computer memory came with acoustic delay-line memory , developed by J. Presper Eckert in

2295-613: The mode selection, it runs a file system repair, and in Mac OS 10.4, it disables all fonts other than those in /System/Library/Fonts, moves to the Trash all font caches normally stored in /Library/Caches/com.apple.ATS/(uid)/, where (uid) is a user ID number such as 501, and disables all startup items and any Login Items). Unlike in Windows where safe mode with networking is disabled by default and requires using

2346-409: The reliability and security of a computer system. Without protected memory, it is possible that a bug in one program will alter the memory used by another program. This will cause that other program to run off of corrupted memory with unpredictable results. If the operating system's memory is corrupted, the entire computer system may crash and need to be rebooted . At times programs intentionally alter

2397-475: The same year, the concept of solid-state memory on an integrated circuit (IC) chip was proposed by applications engineer Bob Norman at Fairchild Semiconductor . The first bipolar semiconductor memory IC chip was the SP95 introduced by IBM in 1965. While semiconductor memory offered improved performance over magnetic-core memory, it remained larger and more expensive and did not displace magnetic-core memory until

2448-559: The task of isolating problems is easier since many non-core components are disabled, such as sound. An installation that will only boot into safe mode typically has a major problem, such as disk corruption or the installation of poorly-configured software that prevents the operating system from successfully booting into its normal operating mode. Though it varies by operating system, safe mode typically loads only essential executable modules and disables devices except for those necessary to display information and accept input. It can also take

2499-434: The write speed. Using small cells improves cost, power, and speed, but leads to semi-volatile behavior. In some applications, the increased volatility can be managed to provide many benefits of a non-volatile memory, for example by removing power but forcing a wake-up before data is lost; or by caching read-only data and discarding the cached data if the power-off time exceeds the non-volatile threshold. The term semi-volatile

2550-638: Was commercialized by IBM in the early 1970s. MOS memory overtook magnetic core memory as the dominant memory technology in the early 1970s. The two main types of volatile random-access memory (RAM) are static random-access memory (SRAM) and dynamic random-access memory (DRAM). Bipolar SRAM was invented by Robert Norman at Fairchild Semiconductor in 1963, followed by the development of MOS SRAM by John Schmidt at Fairchild in 1964. SRAM became an alternative to magnetic-core memory, but requires six transistors for each bit of data. Commercial use of SRAM began in 1965, when IBM introduced their SP95 SRAM chip for

2601-421: Was less expensive. The Williams tube was nevertheless frustratingly sensitive to environmental disturbances. Efforts began in the late 1940s to find non-volatile memory . Magnetic-core memory allowed for memory recall after power loss. It was developed by Frederick W. Viehe and An Wang in the late 1940s, and improved by Jay Forrester and Jan A. Rajchman in the early 1950s, before being commercialized with

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