Reset
A reset ([ ɹiː'sεt ]; English reset , "reset") is the anglicism for a process by which an electronic system is brought into a defined initial state. This may be necessary if the system no longer functions properly and does not respond to the usual inputs .
General
In the German language, the reset in information technology is also known as the “AEG principle” (abbreviation for A ausschalt- E inschalt- G eht) - a principle for eliminating hardware errors . Regardless of this emergency situation , every electronic system that requires a defined initial status for its proper functionality automatically carries out a reset during normal commissioning . Systems monitored by a watchdog circuit are automatically reset in the event of a malfunction.
Computer technology and other electronic systems
When a PC is reset , the entire PC, including the processors , controllers and other functional units and assemblies contained therein, assumes its own initial state. This is why a reset is often imprecisely referred to as a restart in German .
In other electronic systems with widely distributed components, the reset can also be limited to individual components or, within the components, to individual functional units. Examples of this are controls of industrial plants, electronic systems in motor vehicles, building management systems, computer networks, etc.
Basic digital circuits such as flip-flops are reset with a signal on input "R" (for reset). In circuits consisting of flip-flops such as asynchronous counters and synchronous counters , the reset input is the resetting to zero equivalent to the button on the trip meter .
Reset types of PCs
There are three types of reset for a single PC:
- Warm start, reboot, restart
- Hardware reset
- Cold start
Warm start (reboot)
The software restart of a system is a shortened boot procedure, triggered by keyboard input or commands.
Hardware reset
The hardware reset is triggered by pressing the reset button on the computer housing. This also works when the operating system no longer reacts to inputs (usually from the keyboard or mouse). The reset button acts directly on the hardware and thus bypasses the failed operating system.
On some newer PCs, the BIOS can be used to set which event should be triggered by pressing the reset button. This allows z. B. the not crashed operating system can be informed that the reset button has been pressed. The operating system can then react accordingly and z. B. Shut down the computer cleanly, finally write cached data to the data carrier and the like. a. m.
The reset button is usually located near the power button. Many computers have this "emergency switch" with which the PC can be restarted if it has "crashed". It is not on the keyboard . With some PC cases it can only be reached via a small hole into which you have to insert a thin, pointed object (e.g. the tip of a ballpoint pen). With some laptops, you have to hold down the power button for several seconds to reset.
On many small computers (mobile devices) such as MP3 players , the reset button is a tiny, recessed button, usually on the back of the housing and can usually only be operated with a thin needle .
Older home computers like the C64 didn't have a reset button. It was only available as an accessory from third-party suppliers (see picture on the right).
Cold start
It can happen that a PC no longer reacts to the reset button. Then the power supply must be interrupted. This also interrupts the power supply to all devices that are supplied with power via the PC (e.g. hard disks and other drives with data carriers located in them). This can affect these devices. However, modern hard drives have a mechanism that allows the head to be parked anyway and thus to prevent a head crash .
Problem of the reset with PCs
Files and data that are not saved on the hard drive or other data carriers are as good as lost after a reset. Sometimes fragments of the old data can be found in the unused memory of the restarted system.
Since the operating system does not normally get to complete all its operations properly before a reset , a reset can result in an inconsistency in the file system . The operating system usually detects this the next time it boots up , then checks the file systems and tries to repair them. Nevertheless, it can happen that data on the hard disk is lost if it cannot be assigned to the correct files and folders (directories) when the file system is repaired. In the worst case, the entire file system of a partition can become unusable and all data on it is lost or can only be recovered through data recovery . If the operating system (or important parts of it) is located on such a damaged file system, it may or may not want to start (to prevent further damage), so that the computer becomes unusable.
Because the reset button on the front of the PC is easily accessible, this button was repeatedly pressed unintentionally in the past. As an antidote, the PC manufacturers have recessed the buttons, hidden them behind small access holes and built in time delays for triggering. On some newer PCs, the reset button is even omitted entirely. Their function is then taken over by the power button, which must be pressed for several seconds.
Power On Reset (POR)
A power on reset is a reset that ensures a defined start of the circuit in non-trivial digital circuits ( computers , embedded systems , ...) after the supply voltages have been applied, as soon as the system supply voltages have reached their nominal values. With the PC this is z. B. signaled by the power-good signal of the power supply unit . In the case of embedded systems, either the microcontrollers used have a built-in POR circuit, or an external component is used, e.g. B. a system basis chip or a fixed voltage regulator with reset output.
Further examples
On programmable pocket calculators from the 1970s and 1980s, the reset button had to be pressed after every battery change in order to neutralize incorrect charges. This regulated short circuit was z. For example, on the Sharp “PC”, it was reached by a button under the back of the computer that could be pushed through a small hole with a ballpoint pen or the tip of a pencil. This also brought the computer's memory from an undefined state to a defined initial state .
Program crashes can also occur with other electronically controlled devices. In this case, the last resort is often a cold start using the power switch or plug, removing the battery or pressing a specific “hard” reset combination. You may need to turn off power for up to a minute to allow the capacitors to discharge. If that doesn't work, there is usually a hardware fault. In the case of smaller devices, the main memory is often supplied by a backup battery, so that removing the battery for a short period of time does not work.