Global System States (G-States)

Global system states apply to the entire system and it is user-visible.

G0 Working State

• This is the power state where computer is able to run applications.
• The computer system as a whole is working, but the peripheral devices and CPU can change their power states dynamically. For example, a monitor can be turned off when we are just listening to music.
• When a laptop is running in the maximum battery saving mode, the CPU can be suspended after idling for some time.
• Power consumption is the highest among all G states.
• Example : When doing light work like surfing and chatting, CPU may run at its lowest clock speed and CD-ROM drive can be turned off to save power.

G1 Sleeping State

• Applications cannot run in the G1 state. The computer appears "turned off" to the user.
• The operating system can switch to the normal (G0) state without rebooting.
• Most of the system context will be saved in memory - either RAM or the hard drive.
• The wake up latency (transition from G1 to G0) varies, depending on the S-state selected within G1 state.
• Power consumption is small, and may drop to only a few watts (depending on the S-state, of course).
• Example : Switching the computer into "Standby" or "Hibernate" mode in Windows XP.

G2 Soft Off

• No application and operating system context are retained in the G2 state.
• Basically, the whole is system is turned off, with the exception of the main switch of the power supply unit.
• At this point, some debug and machine check registers still retain their error codes, if there are any.
• This information is not really useful for end users, except for motherboards that have debug LEDs.
• Power consumption is almost zero.
• The operating system needs a reboot, and the wake up latency is long.
• It is still not safe to disassemble any device from the computer system, because some of them are still powered.
• Example : Choose "Shut Down Computer" in Windows XP, but the main power switch is not turned off.

G3 Mechanical Off

• The computer is completely turned off by cutting the main power into power supply unit.
• Only the real time clock is still active, using the built-in battery.
• Power consumption is zero, if we don't take the battery into consideration.
• It takes the longest time to go back into the working state (G0).
• It is safe to disassemble devices from the computer.

Global States Summary