Chapter 34 · Power Management

Knowledge Check

8 min read

Try to answer before you open the answers. Guessing is fine; checking your guess is how you learn.

QUICK REVIEW (5 minutes)

  1. In ACPI, who decides when to sleep, and who tells how?
  2. What are the DSDT and SSDT, and what language do they contain?
  3. Which S-state is "Sleep" and which is "Hibernate"?
  4. What file holds the saved RAM when you hibernate?
  5. What does S0 Low Power Idle mean, and what is its everyday name?
  6. What is the difference between Restart and Shut down when Fast Startup is on?
  7. Which command shows the sleep states your PC supports?
  8. What is the difference between a C-state and a P-state?
  9. What do D0 and D3 mean for a device?
  10. How do you calculate battery health from the battery report?

ANSWER KEY — DO NOT READ UNTIL YOU'VE ANSWERED

Click to reveal answers

1. Policy and mechanism

Windows (the Power Manager in the kernel) decides when. The firmware's ACPI tables say how for this exact motherboard.

2. DSDT and SSDT

Firmware tables that describe the machine's devices and hold code. The code is AML (ACPI Machine Language), run by an interpreter inside acpi.sys.

3. S3 and S4

S3 is Sleep (suspend to RAM). S4 is Hibernate (suspend to disk).

4. hiberfil.sys

The hidden hiberfil.sys file in the root of the system drive.

5. S0 Low Power Idle

The state used by Modern Standby: the PC is still in S0, looks asleep, but stays network-connected at very low power for instant wake and background tasks.

6. Restart vs Shut down

Restart fully ends the kernel and boots fresh. Shut down with Fast Startup ends your session but saves the kernel and drivers into hiberfil.sys for a faster next boot.

7. powercfg /a

powercfg /a lists the available sleep states and why others are not.

8. C-state vs P-state

A C-state is how deeply an idle core naps (C0 is running; deeper numbers save more). A P-state is the speed and voltage level of a running core (P0 is fastest).

9. D0 and D3

D0 is fully on. D3 is off (the lowest level of power).

10. Battery health

Full charge capacity ÷ design capacity, as a percentage.

UNDERSTANDING QUESTIONS

Q1. Why does Windows keep policy in the OS and put the "how" in firmware tables?

Answer Different boards need different steps to power parts on and off. By reading tables with code (AML), one Windows works on many boards. The OS also sees the whole picture (what you are doing, battery level), so it can make better decisions than firmware alone, as the old APM system did.

Q2. Why is waking from S3 much faster than starting from S5?

Answer In S3 your work is still in powered RAM, so nothing is loaded from disk, and the firmware only does a short restart of the CPU. From S5 the PC does the full chain: firmware tests, boot loader, kernel, drivers, services and logon.

Q3. Why is hibernate safe against a power cut, but sleep is not?

Answer Sleep needs power to keep RAM alive (self-refresh). If power is lost, RAM goes blank. Hibernate writes RAM to the drive, which keeps its data without power.

Q4. Why can Fast Startup keep a driver problem alive after "Shut down"?

Answer The kernel and driver state are saved in `hiberfil.sys` and restored on the next boot, instead of being rebuilt. A glitch in a driver's state is restored with it. A Restart discards that state and starts clean.

Q5. Why does a deeper C-state save more power but cost more time?

Answer Deeper states turn off more of the core (clocks, then voltage, and sometimes flush caches). That saves more power, but turning everything back on and refilling caches takes longer. The CPU picks a depth that fits how long it expects to stay idle.

SCENARIO QUESTIONS (Troubleshooting)

S1. A laptop in Modern Standby is warm and half-empty after a night in a bag. What do you do?

Answer Something kept the PC active. Run `powercfg /sleepstudy` (hedge: Modern Standby PCs) and look at the sessions with high drain to find the app or driver. Update or remove that item. For long periods, hibernate or shut down.

S2. A desktop wakes itself at 3 a.m. every night. How do you find the cause?

Answer Run `powercfg /lastwake` and `powercfg /waketimers`. A timer from Task Scheduler ("wake the computer to run this task") or Windows maintenance is the usual cause. Check `powercfg /devicequery wake_armed` for devices that can wake it. Then turn off the task's wake option, set Allow wake timers to Disable, or untick the device's wake option.

S3. The PC never goes to sleep, even when idle. What is the first command?

Answer `powercfg /requests` (may need an administrator window). It lists the programs and drivers that hold a power request, such as a video player or a device driver. Close or fix the one named.

S4. A laptop is plugged in all the time, and its battery now lasts 40 minutes. What do the numbers mean?

Answer Run `powercfg /batteryreport` and compare full charge capacity with design capacity. A big gap means the battery is worn (heat and months at 100% speed up wear). Use a charge limit if the maker offers one and replace the battery if it is very worn. If the case is swollen, stop using it at once.

S5. After a driver problem, a user "shuts down" and powers on, but the fault is still there. Why?

Answer Fast Startup restored the saved kernel and driver state. Use **Restart**, or hold Shift while clicking Shut down, to get a full clean boot. (Do not use the Power Button hold unless you must.)

INTERVIEW QUESTIONS

I1. Explain G0, G1, G2 and G3 and where S0 to S5 fit.

Answer G0 is working (S0). G1 is sleeping (S1 to S4). G2 is soft off (S5). G3 is mechanical off. The G-states are the big picture; the S-states are the sleeping detail.

I2. What is the role of acpi.sys and IRP_MJ_POWER?

Answer `acpi.sys` reads the firmware's ACPI tables, runs their AML methods and acts as a bus-level driver for devices described by ACPI. The Power Manager sends `IRP_MJ_POWER` requests down each device's driver stack to change the device's power state; `acpi.sys` performs the board-specific part.

I3. How does Modern Standby differ from S3?

Answer S3 turns off the CPU and keeps only RAM alive; nothing runs. Modern Standby stays in S0 at very low power, keeps the network connected, runs short background tasks and wakes instantly, but can use more battery than S3, especially if something stays active.

I4. Describe what Fast Startup does and why it is called a hybrid shutdown.

Answer It logs off all users and closes their programs as in a normal shutdown, then hibernates only the kernel session into `hiberfil.sys` and powers off. The next boot restores that image instead of reloading the kernel and drivers from scratch. It mixes shutdown (user session) and hibernate (kernel).

I5. Walk through the chain from clicking Shut down to the PSU switching off.

Answer Apps are asked to close, the session ends, services stop in dependency order, the kernel flushes caches to disk, drivers get power IRPs and put devices into D3, `acpi.sys` runs the firmware's S5 method (or the kernel is first saved for Fast Startup), the PS_ON signal drops and the PSU shuts its main rails, leaving only standby power.

PRACTICAL TASK

Write a one-page "power profile" of your PC. Using your results from the Practical Lab, record: the supported sleep states (S3 or Modern Standby, Hibernate, Fast Startup), the active power plan and the power mode, what is allowed to wake it, what was keeping it awake (if anything), and the battery health from the battery report (laptops). Then write three suggestions for saving power or battery life.

Why This Matters

When a PC will not sleep, wakes up on its own, or has a battery that dies too quickly, you will know exactly which command to use and what the answer means.

CHAPTER SUMMARY

  • ACPI: Windows (the Power Manager) sets policy; firmware tables (DSDT, SSDT with AML) and acpi.sys provide the mechanism; power changes travel as IRP_MJ_POWER.
  • System states: G0/S0 working, S3 sleep, S4 hibernate, S5 soft off, with S0 Low Power Idle for Modern Standby. powercfg /a shows what your PC supports.
  • Sleep keeps RAM alive and wakes in seconds; hibernate writes RAM to hiberfil.sys and survives power loss; Modern Standby is a phone-like low-power S0.
  • Shutdown and Restart: graceful shutdown ends apps, services, flushes caches, powers devices down and enters S5; Fast Startup saves the kernel; Restart is a full fresh boot.
  • Power plans bundle settings; powercfg /list, /getactivescheme and /query read them.
  • C-states (idle depth), P-states (speed), D-states (device power), ASPM, USB selective suspend and disk spin-down all save power.
  • Batteries: Li-ion packs with protection and gauge boards; health is full charge ÷ design capacity; heat is the enemy.
  • Wake events: power button, keyboard, lid, timers, Wake-on-LAN; use /lastwake, /devicequery wake_armed, /waketimers and /requests to investigate.
  • Complete chain: shutdown goes from the click to S5 and standby power; startup goes from PS_ON/PWR_OK through firmware, loader, kernel, services and logon.