Section 2: System Power States

The Big Picture: Awake, Napping, Deep Sleep, Gone

A person can be wide awake, dozing, in deep sleep, or away from home. The PC has the same kinds of levels. ACPI gives each one a name, so Windows, the firmware and the drivers all mean the same thing when they say "sleep".

In Chapter 4, Section 5.5, you met the short table of S0, S3, S4 and S5. Now we look at the whole ladder, and at how to read what your PC supports.

2.1 The Two Families: G-States and S-States

What it is: ACPI describes the whole machine with two sets of names that fit together.

• G-states ("Global" states) are the big picture: working, sleeping, off.
• S-states ("Sleeping" states) are the detail inside them.

 G0 Working        G1 Sleeping                      G2 Soft off     G3 Mech. off
 +--------+   +-----------------------------+      +-----------+   +-----------+
 |   S0   |   | S1   S2   S3 (RAM)  S4 (disk)|     |    S5     |   | no power  |
 +--------+   +-----------------------------+      +-----------+   +-----------+
 most power  ------------------- less and less power ------------>  none at all

(Hibernate, S4, is counted by ACPI inside G1 even though the PC is fully powered down, because the saved state lets it resume where it left off.)

2.2 S0 (Fully On)

What it is: S0 is the working state. The CPU runs code, RAM is in use, the screen is on, and you are using the PC.

Two things to know:

• S0 is not always "full power". Inside S0 the CPU can nap between tasks (C-states) and slow down (P-states). You will meet them in Section 8.
• Modern Standby (Section 5) is also an S0 state, a very quiet one called S0 Low Power Idle. That is why "S0" no longer always means "the user is working".

2.3 S3 (Sleep, Suspend to RAM)

What it is: In S3, the PC keeps only RAM alive. Everything else is switched off. Your open apps and windows stay in RAM, so waking is quick.

Section 3 covers sleep in detail.

2.4 S4 (Hibernate, Suspend to Disk)

What it is: In S4, Windows copies the contents of RAM to a file on the system drive named hiberfil.sys, then switches the PC fully off. On wake, the firmware starts the PC, the loader finds hiberfil.sys, and Windows reads it back into RAM, so your apps appear again.

Section 4 covers hibernate in detail.

2.5 S5 (Soft Off, Shutdown)

What it is: S5 is the normal "Shut down". Nothing about your running session is saved. Only the standby power stays on, so the power button, Wake-on-LAN and a few other parts can still work ("soft" off, because the PSU is still partly alive).

Starting from S5 needs a full boot (Chapter 6).

Fast Startup twist: On Windows 10 and 11, "Shut down" can leave the PC in S5 while having saved the kernel in hiberfil.sys first. The power is off, but the next boot is faster. Section 6 explains this.

2.6 S1 and S2 (Rare Today)

What they are: S1 and S2 are light sleep states from older PCs. In S1 the CPU stops but power stays on to the CPU, RAM and most devices; wake is very fast but the power saving is small. S2 is similar and was almost never used.

Today most PCs support S3 and S4, or Modern Standby instead of S3. S1 and S2 are rare, so powercfg /a almost never lists them as available.

2.7 The Full Table

Power used      S0 --- S0 LPI --- S1 --- S3 ---- S4 ---- S5
(highest->0)    high    very low   med   tiny    ~0      ~0 (standby only)
Time to wake    --      instant    fast  1-3 s   longer  full boot

2.8 Which States Does My PC Support? (powercfg /a)

What it is: powercfg is a Windows command-line tool for power. The switch /a ("available") lists the sleep states this machine supports, and why others are missing. It only reads information.

Open Command Prompt or PowerShell and type:

powercfg /a

Typical results (your text may differ):

Why a state might be missing: the firmware does not offer it, a driver does not support it, hibernation is switched off, or (on virtual machines) the virtual hardware does not provide it. The command prints the reason next to each state.

Summary