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 system level (SL) 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. In November 1992, the 386SL was followed by the 486SL processor, 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.