When you restart a PC, does it completely “shut down”? If it does, what tells it to power up again? If it doesn’t, why does it behave like it has been shut down?
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The basic operations necessary to operate an operating system are stored in bios(the chip for Binary Input Output System). The operating system when tells that it wants to restart, the bios changes it’s order of operations from clean up the cache, shutting down, ànd the restart, and reload the OS etc.
No, restart and shutdown are different and there are separate power level commands implemented by each. When restarting your computer doesn’t shut down first.
When the computer is shutdown, the OS will send a shutdown command to the BIOS using the appropriate driver, the BIOS will then send a command to attached devices to place them in a safe working mode, hard drives will park their read write heads to avoid damage.
Once the devices report ready and everything is made safe the BIOS will send a signal to the power supply to terminate power to most attached devices and enter standby power mode.
Even when shutdown ATX and above will maintain a standby power to the motherboard when plugged in. This allows external devices to power on the machine.
Flicking the switch or removing the power cord will remove all power but the power supply itself remains charged with residual current so never open it up.
During a restart devices such as the hard drive will not park and will continue to be active, certain POST operations may be skipped.
There is a special ATX power signal used to move from standby to full power. The cases switch is wired into a molex connector onto the motherboard, when you press the switch it operates the pins that generate this power signal.
Every CPU has a start up routine. Similar to a chef setting up his kitchen before he starts cooking (cleaning, placing the pots & pans & ingredients in the right location etc.) then he starts cooking.
The reset button just tells the CPU to start that routine. So to compare, let’s say the chef was in the middle of making pasta, but got a reset. He would clean his kitchen and start cooking from scratch again.
Well, what is the difference between an intitial boot and a reset? It is nothing. The list of activities that a CPU needs to do before it can start executing other programs is defined at a certain address. Let’s say $0000. When the computer is turned on, it is hardwired to start following instructions from address $0000 onwards.
During the execution of code, the processor keeps track where he is (same as following steps in a receipe) with a counter called a ‘program counter’. What the reset button is pressed, it writes $0000 to the program counter making the CPU think it was just turned on.
A computer powersupply is actually two powersupply in one box.
You have the main powersupply, which power your PC. It provide normally the -12, 3.3, 5 and 12V at 400-1000W or sometime even more if the guy that built the pc was crazy! This one is switched on and off by the motherboard.
You also have a secondary powersupply. This one is a way smaller one that deliver 5V at 1-3A only. Some phone chargers can actually provide more power than most of them! That is 5-15W only. Most are in the 1-2A range, so 5-10W. This one is always on. If the powersupply is plugged in the wall and the switch on the back of the powersupply is on (if it even exists) then that one is on. Ever saw that always on green led on the motherboard? Well, it is there to say “Hey! Don’t forget to disconnect the power before going any maintenance because there is still power here!!!”
Now… That 5V always on, called 5V Standby, shortened to 5VSTB, also power a microcontroller and some other part of the motherboard. When you select restart in windows, what actually happen is that windows send a signal to that microcontroller to reset everything. Some computer will not shutdown, some like mine does. So, what actually happen is that the microcontroller get the “reset” signal, turn off the powersupply, wait a second or two, then turn it back on.
The same microcontroller is often the one that manage the power button on the front of your case. When you press on it, that microcontroller get a signal, and can take a few actions:
* if the computer is off, turn it on.
* if the computer is on, and the bios setting is set to delay, then send a signal to the OS to shut down NOW. Else turn off the power.
* If the computer is on and the button is held for more than 4 seconds, then turn off the power. Take note that this can cause data corruption so don’t do that unless the computer is frozen. It is also the equivalent of pulling the plug off the wall.
Now, remember I said something about some that just reset? Well, instead of shutting off and on the power, another way is to actually issue a reset to most controllers in the computers. There is basically a wire that connect those chips together, and when you send the signal (usually that wire get grounded to reset), the chip should return to it’s powered on state and lose all the data and state it was currently at. Magic word: should. See, sometime the electronics crash in some weird way, and the chip can’t reset it for some reason to a default state. This can be by design (aka this shouln’t happen so let’s not put the circuit to reset it), by design flaw (the circuit is there, but a ‘bug’ exists that make it not work proprelly), or sometime it is because the chip itself is damaged (but can still work flawlessly in most cases) or some part can have been skipped on purpose (the programmer just need to send some command to reset the values). For the last one, surprise, some programmers don’t do what they must do, and skip over the “Those default values are not reset” part of the datasheet thru created a software bug that cause it to then misbehave.
The last one is why they are sometime doing the power reset instead: it do not soft reset, but hard reset. This ensure that everything is always resetted, all the time, no matter what crashed and how.
As a side note, when the computer goes into standby, the main powersupply is turned off, and the standby one is what power the ram and some other stuff with important volatile data. The rest is saved to disk or just reinitialised when it get powered on again. This is why it wake up so fast when in standby: everything is still loaded in ram. All it need to do is reinitialise the video card and the like. But all of your opened programs are still in memory. The computer still use a tiny bit of power, a few watts. Lose the power and you lose everything that was opened. The downside is that it take time to dump the ram content to disk, which may be significant if you have lots of ram and a mechanical hard drive. Hard drives are SLOW!
For completeness, hybernation is simmilar to standby, except that it dump the content of the ram on the disk then shut off the computer. At power on it reload the content into ram and reinit the hardware.The computer basically take no power.
Also, microsoft also made an hybrid mode. It hybernate, but do not shut down the computer (it put it in standby). If you turn it back on, it behave like in standby, but if it lose power then it behave like in hybernation. Kinda the best of both world. The downside is: it take some time to dump the ram content to the disk at standby time, same as in hybernation mode.
The motherboard of a pc is always on if it is plugged in, it listens for commands on various interfaces in order to know when to power on or power off
On Windows 10 you have to hold shift when you click shut down to do a full shutdown. Otherwise it “turns off” but caches everything so it can boot up faster the next time.
I’ve always wondered this, especially when I set my phone to turn itself off at a certain time and it knows what the time is to turn itself back on. It must obviously not be completely off to keep track of time. Thanks for asking this!
Change computer to analog, for a 5 year old.
**Turn your computer on:** show up to school, get your books and supplies out. Student is a computer, the teacher is giving it directions.
**Put your computer to sleep:** go to recess or lunch. You leave your books and supplies out, the teacher turns off the lights in the classroom, but you haven’t put anything away yet. You can pick up where you left off without losing your place or any work that’s unfinished.
**Hibernating your computer:** you go home for the night and safely organize your work at your desk, bookmark your book, and put it away. The books are not out of the desk, but they are bookmarked so you can find your place quickly.
**Shutting down your computer:** You close all of your books and put everything away safely and neatly. You have to find and open everything manually to find where you left off the previous day. It takes the longest, but you start fresh and don’t have a lot of clutter right away.
**Restarting your computer:** the teacher tells you to pack everything up neatly, wash your desk, and then get everything back out. You never leave the room.
**Power outage:** the teacher walks around and throws everything on your desk in a box. Some of it might fall on the floor, but most of it is in the box. most of your work is still there, but it might take you longer to organize it all and start up again.
A little more in-depth: Windows 10 for most users combines sleep, hibernation, and shutdown. There’s still different sleep states, but those system buttons behave similarly. If you ever look at *system uptime* in Windows 10, it might show its been running for months. That’s because when you shut it down, it’s actually hibernating.
Restarting closes all of the program that’s running, tells the motherboard to stay powered on, and then naturally begins running those programs again. The computer never powers off, but it does terminate every piece of code that is being processed.
Between those two piece of information, and with my experience in IT, ever since like 2005 era computing, restarting is VERY different than shutdown and manually turning it back on.
There is a component in the computer called ROM (Read only Memory) which is always on (even when the computer is shutdown and the power is disconnected) – it is possible because of a special battery attached to it. This ROM recieves a signal when you switch on the computer and this communicates the first ever instruction required for the computer to start doing everything else.
There are many subsystems inside your PC. Some have smarts to do things on their own. Using the shutdown gracefully prepares the most of PC for a loss of power. Then the motherboard signals the power supply to cut power. The power supply goes into a low power mode and waits for the power button in the case it be pushed.
A restart is all of that, except instead of the motherboard signaling the power supply to cut power, the motherboard just starts a normal start sequence.
It’s virtually the same thing. But sometimes you do have to shutdown, flip the physical switch on the power supply and unplug when you need a true full power loss to clear some low level caches or some cranky hardware. By this point you are usually following some advice on the web.
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