Mobile Processor Features


As with most portable system components , the main concern with mobile processors is reducing their size, power usage, and heat generation. This allows them to function in the tight confines of a laptop system without overheating , while allowing the longest possible battery life. Mobile processors usually differ from desktop processors in packaging and power consumption, and they can have special features not found in desktop processors. Some of the special features first debuted in mobile processors are subsequently implemented in desktop processors as well. Features unique to mobile processors are discussed in the following sections.

SL Technology

SL technology and SL architecture are terms that Intel used to describe the first power-management improvements that were specially designed for mobile processors and later incorporated into all desktop processors. This technology was first introduced in the 386SL processor in October 1990 and was the first mobile-specific PC processor on the market. The 386SL was based on the 386SX core (16-bit data bus), with added power-management features that Intel then called SL technology. The 386SL was followed by the 486SL processor in November 1992, which was essentially a 486DX with the same SL technology included in the 386SL. At first, the 486SL was a unique model. However, starting in June 1993, SL technology was available in all desktop 486 processors and all Pentium processors from 75MHz and faster. Every Intel x86 processor introduced since then, from the Pentium II through the Pentium 4 and beyond, has incorporated SL technology.

SL technology consists of a number of processor features that operate at the system hardware level, independent of the operating system or application software. SL technology includes the following features:

  • System Management Mode . This dedicated special-purpose interrupt and memory address space implements power management transparent to operating system and applications software.

  • I/O Restart . An I/O instruction interrupted by a System Management Interrupt (SMI) can automatically be restarted following the execution of the Resume (RSM) instruction.

  • Stop Clock . This control mechanism provides a fastwake-up Stop Grant state and a slowwake-up Stop Clock state, with the CPU operating at 0MHz.

  • AutoHALT powerdown . After executing a HALT instruction, the processor issues a normal HALT bus cycle, and the clock input to the processor core is automatically stopped .

  • Auto Idle powerdown . This allows the processor to reduce the core frequency to the bus frequency when both the core and the bus are idle.

The most important part of SL technology is System Management Mode (SMM), which can control and power up/down components without interfering with other system resources. SMM software executes in a dedicated memory space called System Management RAM (SMRAM), which is invisible to operating system and applications software. The CPU enters SMM upon receiving a System Management Interrupt (SMI), the highest-priority nonmaskable interrupt that the CPU can receive. When an event generates an SMI (for example, accessing a device that is currently powered down), the CPU responds by saving the state of the processor to SMRAM. The CPU then switches into SMM and executes the SMM code (also stored in the SMRAM). When the SMM task is complete (for example, powering on the device that was being accessed), the SMI handler executes a Resume (RSM) instruction, which restores the former state of the processor from the SMRAM.

I/O Restart is one of the SL technology functions used with System Management Mode. For example, if an application executes an I/O instruction that tries to access a disk drive that is powered down for battery savings, a System Management Interrupt occurs, powering up the drive and re-executing the I/O instruction automatically. This is transparent to the operating system and application program, allowing the software to run seamlessly.

SL technology also added special clock controls, including Stop Clock, AutoHALT, and Auto Idle. Stop Clock is an instruction that allows control over the CPU clock frequency. When Stop Clock is enabled, the internal frequency of the CPU can be throttled down as low as 0MHz, causing the CPU to consume only a few milliamps of power. This is also called sleep mode. For further power reductions, the external clock signal can be removed altogether, lowering power consumption to the microamp range. This is also called suspend mode.

AutoHALT is an enhancement to the existing HALT instruction and is related to Stop Clock. When a HALT instruction is executed (which stops the CPU from executing further instructions), the CPU automatically executes the Stop Clock instruction and enters sleep mode.

Auto Idle reduces the clock speed of the CPU from normal (clock multiplied) speed down to the CPU bus speed whenever the processor is idle during memory or I/O operations. For example, when the processor is executing an I/O instruction and waiting for the device to respond, the processor speed is automatically reduced to match the CPU bus speed, resulting in power savings without affecting overall performance.

Processor Performance Technology (SpeedStep/PowerNow!/LongRun)

Overall power consumption is one of the biggest issues faced when designing mobile processors and systems. Most laptops are designed to run off battery power when disconnected from an AC power source. The less power is required, the longer the system can run between recharges. In this case, battery life is not so much how many times you can discharge and recharge ( expressed as the total number of cycles), but how long each cycle lasts. The less power the system requires, the greater the time you can run off an existing charge. Conserving power when connected to AC can also be useful for reducing component temperatures and heat generation in a laptop.

The Mobile Pentium III/Celeron, Pentium 4, Pentium M, AMD K6-2P, AMD K6-IIIP, AMD K6-III+, AMD Mobile Athlon 4, AMD Mobile Duron, as well as the Transmeta processors all feature processor performance control technology to allow for longer battery life in mobile operation, as well as reduced thermal generation when under AC power. Intel calls this technology SpeedStep (originally code-named Geyserville), AMD calls it PowerNow!, and Transmeta calls it LongRun. This technology enables these processors to optionally reduce both speed and voltage when running on batteries. Earlier processors could reduce speed using SL technology, but by also reducing voltage the overall power consumption (and heat production) is significantly reduced as well. More recent versions of this technology allow modes to be dynamically switched based on processor demand, not just by whether the system is running on battery or AC power.

Although processor performance-control technology is mainly designed to work when the laptop is running on battery power, it can also be used dynamically when under AC power to help reduce CPU temperature and energy consumption. When the laptop computer is connected to the AC outlet, CPU speed and voltage are normally at or near their maximum. When powered by a battery, the processor automatically drops to a lower frequency (by changing ratios, the bus frequency remains constant) and voltage, thus conserving battery life while still maintaining a relatively high level of performance. In most cases, the actual power consumption drops by half, which means about double the battery life as compared to full power, while reducing speed by only a small amount. For example, a 3.06GHz Mobile Pentium 4 consumes up to 101.4W at full power (3.06GHz and 1.55V), whereas in SpeedStep mode, the power consumption drops to only 40.9W (1.6GHz and 1.2V). This means that although power consumption drops by nearly 60%, the speed drops by only about 48%.

When the system is first powered up, the processor starts in the lower of its two speedsthat is, it starts in Battery Optimized mode. From there, BIOS, driver, or operating system instructions can rapidly switch the processor from mode to mode.

The requirements for this technology to work are as follows :

  • A processor that supports SpeedStep/PowerNow!/LongRun technology

  • A supporting motherboard (chipset, BIOS, and voltage regulator )

  • A supporting operating system (Win9 x /Me, WinNT/2000/XP)

  • SpeedStep/PowerNow!/LongRun driver (included with XP)

In general, all laptops that came with processors supporting performance-control technology included all of the other required support as well. Note that although it is possible to upgrade processors in many laptops, you generally cannot install a processor supporting SpeedStep/PowerNow!/LongRun technology into an older system that was originally equipped with a processor that did not support that technology.

Systems running Windows 9 x /Me or NT/2000 require a special configuration utility or driver to control the processor performance settings. Because the driver must be configured to the specific laptop motherboard, it is available only from the manufacturer or vendor of a given system. Typically, the driver automatically switches processor performance modes when the power source changes from AC to battery power. Normally, the driver displays an indicator (usually a small flag) in the notification area or system tray of the Windows taskbar indicating in which mode the CPU is currently running. The driver also typically adds a processor performance-control tab to the Power Management tool in the Control Panel. By clicking on the indicator in the system tray, you can switch among Maximum Performance, Automatic (dynamically switchable), or Battery Optimized modes on demand. By using the control tab added to the Power Management tool, you can select options allowing you to disable the processor performance-control technology, add or remove the icon from the taskbar, and enable or disable audio notification when performance changes.

Windows XP includes native support for processor performance-control technologies such as SpeedStep, PowerNow!, and LongRun, which means that manufacturer supplied drivers are no longer necessary. This native support also includes an algorithm that dynamically balances system performance and power consumption, based on the current CPU workload and remaining battery life.

Windows XP uses four specific processor policies (modes of operation) to control processor performance. The processor policies used by Windows XP are shown in Table 4.1 in order of power consumption from highest to lowest .

Table 4.1. Windows XP Processor Policies

Processor Policy

Description

None

The CPU always runs at the highest performance level.

Adaptive

The CPU performance level varies based on CPU demand.

Constant

The CPU always runs at the lowest performance level.

Degrade

The CPU starts at the lowest performance level and uses stop clock throttling to reduce power even further as the battery discharges.

These processor policies are directly tied into and selected via the various power schemes selected in the Power Options application in the Windows XP Control Panel. By setting a specific power scheme, you are also selecting the processor policies that will be used when under AC and battery power. Windows XP includes several standard power schemes with predefined processor policies. Your laptop might include a power-management accessory provided by the manufacturer, which allows you to select additional schemes provided or even to create your own custom schemes. Table 4.2 shows the standard power schemes and related processor policies that are provided with Windows XP, listed from highest to lowest power.

Table 4.2. Windows XP Standard Power Schemes and Related Processor Policies

Power Scheme

Processor Policy ( AC Power)

Processor Policy (Battery Power)

Always On

None

None

Home/Office Desk

None

Adaptive

Minimal Power Management

Adaptive

Adaptive

Portable/Laptop

Adaptive

Adaptive

Presentation

Adaptive

Degrade

Max Battery

Adaptive

Degrade

Reducing processor speed and voltage conserves power and dramatically increases battery life, but it also significantly reduces processor performance. This means that applications that require extremely high performance could suffer when in a battery-optimized mode. Additionally, when the processor speed changes, access to memory is temporarily blocked. This can cause problems with applications that require streaming access to memory (such as video playback), resulting in glitches or dropouts in the display. If you want maximum performance when running under battery power, you can manually override or disable the performance control technology.

To disable processor performance-control technology and force the processor to run at maximum performance, if you are running Windows 9 x /Me or NT/2000, you should use the application supplied by your laptop manufacturer to disable the technology. If you are using Windows XP, you should select the Always On power scheme, which can be accomplished using the following steps:

  1. Select Start, Control Panel (make sure you're using the Control Panel classic view).

  2. Double-click the Power Options tool and select the Power Schemes tab.

  3. Under Power Schemes, select the Always On scheme.

As you can see from the previous tables, the Always On power scheme in Windows XP automatically selects processor policies of None for both AC and battery power, which means that the processor will be forced to run its highest performance level at all times.

Caution

If you use the Always On power scheme on a laptop, battery life will be greatly reduced and the system might be prone to run extremely hot or even to overheat. If you find that the system is running too hot when using AC power, you can try selecting a power scheme such as Minimal Power Management or Portable/Laptop, which uses the Adaptive processor policy to reduce power under periods of lower demand. For minimum heat production and maximum battery life, you can try the Presentation or Max Battery schemes.




Upgrading and Repairing Laptops
Scott Muellers Upgrading and Repairing Laptops, Second Edition
ISBN: 0789733765
EAN: 2147483647
Year: 2003
Pages: 182
Authors: Scott Mueller

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