Sleeping States (S-States)
Sleeping states define the computer's ‘sleeping methods’ in the G1 (sleeping) state. In all the sleeping states (except S0 and S5),
- The CPU executes no instructions. It is having a good sleep!
- User applications will not run (duh… the CPU is sleeping!)
- Some devices sleep partially because they need to generate wake up events.
- When the system is ‘awakened’, it will continue working from the point before it slept.
S1 State
- The hardware maintains all its system context.
- The CPU input clock will be stopped, and its caches will be invalidated.
- The system memory goes into its self-refreshing mode.
- All system clocks are turned off, except the real time clock.
- Power consumption is much lower than G0 working state.
- Wake up latency is low. It takes about 2 seconds to go back to the G0 working state. The hardware will be responsible for restarting the system clocks.
- Example : “Stand-by” mode in Windows XP, if the S3 state isn’t supported.
S2 State
- Similar to S1 state - the only difference is the CPU power state.
- In S2, the CPU and its caches are powered down, instead of just gating the clock input and invalidating the caches.
- The S2 wake up latency is slightly longer than S1, but it saves slightly more power.
S3 State
- The S3 state powers down the CPU, cache, chipset and peripherals, except RAM.
- Some devices necessary to maintain memory context will still run.
- RAM goes into a low-power, self-refreshing mode.
- The power consumption is as low as the power requirement of the RAM (at idle power), plus some necessary onboard devices only.
- The wake-up latency is about 5-6 seconds.
- Example : "Stand-by" mode in Windows XP if the S3 state is supported by hardware.
S4 State
- All devices including system RAM are powered down.
- Only platform settings are maintained, while other settings are stored in a special partition in the hard drive.
- When successfully switched into the S4 state, the system appears to be turned off to the user.
- The power consumption is very low (< 3W), as almost everything has turned off.
- We need to go through the BIOS boot sequence again when the computer is awakened.
- The OS reboot is not required. It will automatically return to where you last left it.
- Example : "Hibernate" mode in Windows XP
Sleeping States Summary
Sleep State |
Wake Up Latency |
Power Consumption |
BIOS Reboot |
OS Reboot |
CPU |
Cache |
Chipset |
RAM |
S0 (G0) |
none |
large |
no |
no |
on |
on |
on |
on |
S1 (G1) |
2-3s |
medium |
no |
no |
no clock |
invalidated |
no clock |
self refresh |
S2 (G1) |
3-4s |
mid-low |
no |
no |
off |
off |
no clock |
self refresh |
S3 (G1) |
5-6s |
low |
no |
no |
off |
off |
off |
low power self refresh |
S4 (G1) |
20-30s |
very low |
yes |
no |
off |
off |
off |
off |
S5 (G2) |
> 30s |
near zero |
yes |
yes |
off |
off |
off |
off |
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