Section 4: Hibernate

The Big Picture: Saving a Game and Switching Off the Console

When you save a video game and switch the console completely off, nothing uses power. Next day you load the save and carry on exactly where you were. Hibernate does this for the whole PC. It is the S4 state from Section 2.

Sleep (Section 3) keeps your work in RAM, which needs power. Hibernate writes your work to the drive, which needs none.

4.1 RAM Contents Written to hiberfil.sys

What it is: hiberfil.sys is a large hidden system file in the root of the system drive (usually C:\hiberfil.sys). When you hibernate, Windows writes the contents of RAM into it, in a compressed form.

BEFORE                         DURING HIBERNATE               AFTER
RAM  [ apps, kernel, data ]    RAM  [ apps, kernel, data ]    RAM  [ empty ]
Disk [ hiberfil.sys: old ]  -> Disk [ hiberfil.sys: filling ] Disk [ hiberfil.sys: full ]
                               then power goes completely off

How big is it? Windows reserves the file in advance so there is always room to save. By default it is a fraction of your RAM size, roughly 40% to 75% (hedge: this depends on the Windows version and settings, because the data is compressed). A PC with 16 GB of RAM may therefore have a hiberfil.sys of several gigabytes to about 12 GB.

Why it is hidden: it is a protected operating-system file. To see it in File Explorer you must show hidden items and protected operating system files (a View option). It cannot be opened or edited.

Who writes and reads it: the kernel's power code does the writing and the Windows boot loader (Chapter 6) does the reading on resume. The loader reads the file before the full OS is running, so the system comes back as one block.

4.2 Full Power-Off, Restore on Wake

Going to hibernate (S4):

1. Trigger        Start -> Power -> Hibernate (if shown), or a timer or low battery
2. Notify         Apps and services are told; drivers get power IRPs (Chapter 13)
3. Freeze         Devices go to D3; only one CPU core keeps working
4. Write          RAM contents are compressed and written to hiberfil.sys
5. Power off      acpi.sys puts the machine into S4; all power is removed
                  (only standby power for the power button, as with S5)

Coming back:

1. Power button   Same as a normal start
2. Firmware       Does its normal startup checks (Chapter 5)
3. Boot manager   Notices a valid hiberfil.sys (the "resume" case)
4. Resume loader  Reads hiberfil.sys back into RAM
5. Windows        Continues where it stopped; drivers restore devices
6. You            See your apps exactly as you left them

An SSD (Chapter 18) makes hibernate and resume much faster than a hard drive.

4.3 Hibernate Is Not Always in the Menu

On many PCs, Hibernate is not shown in the Start menu Power list. That does not mean it is missing; the menu entry can be hidden by a setting. In Control Panel → Power Options → "Choose what the power buttons do", the Hibernate option can be shown or hidden (it can need "Change settings that are currently unavailable" first).

Use the read-only command from Section 2 to see the truth:

powercfg /a

If Hibernate is listed as available, hiberfil.sys exists and S4 works.

4.4 Turning Hibernation On or Off (Do Not Try in the Lab)

The command powercfg /h (or /hibernate) changes the feature. It needs an administrator window.

Do not run these in the practical lab. They change the system. They are safe and reversible, but this chapter's lab only reads information.

Why someone might turn it off: to free several gigabytes on a small drive. The cost: no hibernate, and no Fast Startup (Section 6). On PCs with Modern Standby, Windows may still use hibernation as a backup, so check powercfg /a before deciding (hedge: this depends on the PC).

4.5 Hybrid Sleep (Mostly on Desktops)

What it is: Hybrid sleep is sleep plus a hibernate save. Windows writes RAM to hiberfil.sys and then goes into S3. If the power stays on, you wake in a second or two from RAM. If power is lost, the PC starts from the saved file instead of losing your work.

It is usually switched on by default on desktop PCs and off on laptops, where the battery gives protection (hedge: it varies by PC and version). The setting lives in the advanced part of the power plan (Section 7).

4.6 Hibernate After Sleep

Windows can also hibernate after a long sleep. The power plan has a setting called Hibernate after (a number of minutes). The PC sleeps first; if it is still asleep when the time ends, Windows wakes it briefly, saves RAM to hiberfil.sys and powers off. This protects a laptop whose battery would otherwise slowly drain in S3.

Summary