The overall power consumption of the system is referred to as System Power States. There are a total of six different power states ranging from S0 (the system is completely powered ON and fully operational) to S5 (the system is completely powered OFF) and the States (S1, S2, S3 and S4) are referred to as sleeping states, in which the system appears OFF because of low power consumption and retains enough of the hardware context to return to the working state without a system reboot.
Global states :
The ACPI specification defines the following four Global "Gx" states and six Sleep "Sx" states for an ACPI-compliant computer-system:
G0 (S0): Working. "Awaymode" is a subset of S0, where monitor is off but background tasks are running.
G1, Sleeping subdivides into the four states S1 through S4:
S1: All processor caches are flushed, and the CPU(s) stops executing instructions. Power to the CPU(s) and RAM is maintained; devices that do not indicate they must remain on may be powered down.
S2: CPU powered off. Dirty cache is flushed to RAM.
S3: Commonly referred to as Standby, Sleep, or Suspend to RAM (STR). RAM remains powered
S4: Hibernation or Suspend to Disk. All content of main memory is saved to non-volatile memory such as a hard drive, and is powered down.
G2 (S5), Soft Off: G2/S5 is almost the same as G3 Mechanical Off, except that the PSU still supplies power, at a minimum, to the power button to allow return to S0. A full reboot is required. No previous content is retained. Other components may remain powered so the computer can "wake" on input from the keyboard, clock, modem, LAN, or USB device.
G3, Mechanical Off: The computer's power has been totally removed via a mechanical switch (as on the rear of a PSU). The power cord can be removed and the system is safe for disassembly (typically, only the real-time clock continues to run - using its own small battery).
Furthermore, the specification defines a Legacy state: the state on an operating system which does not support ACPI. In this state, the hardware and power are not managed via ACPI, effectively disabling ACPI.
All about System Power States (S0-S5)
The key characteristics of the each state that we care about are:
The overall Power the system consumes in a given state - Power Consumption.
Retention of the system context (such as the volatile registers, memory caches, and RAM) - Context
Note on power state transition: System is Waking Up when the system is transitioning from the OFF State (S5) or any sleep state (S1-S4) to the ON State (S0) and the System is going to Sleep when the system is transitioning from ON state (S0) to OFF state (S5) or sleep state (S1-S4). Please note that the system cannot enter one sleep state directly from another, as it must enter the ON state before entering any other sleep state.
System Power State S0 the ON state: The system is completely operation, fully powered and completely retains the context.
System Power State S1 the Sleep state: The system consumes less power than S0 state. All Hardware & Processor context is maintained.
System Power State S2 the Sleep state: The system consumes less power than S1 state. Processor loses power and processor context and contents of the cache are lost.
System Power State S3 the Sleep state: The system consumes less power than S2 state. Processor & Hardware context, cache contents, and chipset context are lost. The system memory is retained.
System Power State S4 the Hibernate state: The system consumes the least power compared to all other sleep states. The system is almost at an OFF state, expect for a trickle power. The context data is written to hard drive (disk)and there is no context retained.
System Power State S5 the OFF state:The system is in a shutdown state and the system retains no context. Note that in power state S4 the system can restart from the context data stored on the disk, but in S5 the system requires a reboot.
Refer to the following documents for further information on System Power states