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Other articles > X86 Glossary > SL Enhanced

Created by Intel for its 386, SL Technology consists of a number of microprocessor features that deliver power-management capabilities.
These features operate at two levels: at the system level, controlling the way power is used by the entire system (including peripherals); and at the microprocessor level.
Power management at these levels involves putting the cpu into low-power state during non-cpu intensive tasks (such as word processing), or into a very low-power state when the computer is not in use ("sleep" mode).

System-Level Power Management
At the system level, Intel's SL technology centers on System Management Mode (SMM), an operating mode first introduced in Intel's SL family of microprocessors (the Intel386SL and Intel486SL cpus), which has become a standard in the notebook market.
SMM provides intelligent system management that allows the microprocessor to slow down, suspend, or completely shut down various system components so as to maximize energy savings.
The CPU enters SMM upon reception of a System Management Interrupt (SMI), the highest priority non-maskable interrupt the cpu can receive. When an event generates an SMI (the standby button is pressed, or a timer indicates an energy-efficient desktop is idle), the cpu responds by saving the current state of the processor (i.e., the software currently running) to a dedicated area called SMRAM, to which operating systems and applications do not have access. The cpu then switches into SMM and executes the SMM power-management code, which is also stored in the SMRAM. Once the task for which normal operation was interrupted is complete, the SMI handler executes a RESUME instruction, which restores the former state of the processor by copying its "context" from SMRAM. In this way, SMM operates transparently to operating system and application software. This means the user does not have to worry whether new or existing software will conflict with the power-management operation. The system hardware can provide the same power-management capabilities whether DOS, UNIX or Windows is used. The user is also freed from acquiring different power-management software drivers whenever upgrading to a new revision of an operating system or installing a new application. SMM enables computer manufacturers to provide users with worry-free power management.

CPU-Level Power Management
The SL Enhanced Intel486 cpu family incorporates power-saving technology at the cpu level beginning with the manufacturing process; the chips are implemented with fully static CMOS process technology. This static design allows the cpu frequency to be reduced to 0 MHz, where the cpu uses very little power.
One of the SL Technology features used to manage the power consumption of the cpu is Stop Clock. Stop Clock is a microprocessor input that provides fine-tuned control over the cpu's clock frequency, enabling a variety of energy-conservation techniques. When Stop Clock is enabled, the internal frequency of the cpu can be lowered to 0 MHz, causing the cpu to consume only 20-50 milliamps or less than 0.25 watts when the stop clock is asserted; when the clock is taken away, the processor power consumption drops to the micro amp range. This is the cpu's low-power sleep mode, needed when the PC is put in its sleep mode to meet the Energy Star guidelines. Stop Clock also allows the cpu to return to "full-on" state within microseconds--a necessity for desktop systems that are, for example, connected to a network.
With Stop Clock enabled, the external clock frequency can also be removed altogether (for example, when the system is left unattended for a set period of time) lowering power consumption to the microamp range, resulting in even greater power savings. This additional level of power saving is especially critical in battery-operated notebook systems, since every extra minute of battery life is precious. The ability of Stop Clock to vary clock input and cpu speed results in dramatic power savings.
Another approach to cpu power management involves the Auto Halt feature, an enhancement of the existing HALT instruction of Intel486 cpus. In SL Enhanced Intel486 cpus, when a HALT is executed (stopping the cpu from executing further instructions) the cpu automatically enters its low-power (20-50 milliamp) sleep mode. This software approach to invoking sleep mode provides savings of up to four watts over the standard Intel486 microprocessor HALT instruction.
A new SL Technology feature for the SL Enhanced Intel486 DX2 microprocessor is Auto Idle, which reduces the internal clock frequency of clock-doubled cpus when the cpu is idle during a memory or I/O read or write. For example, when the cpu is writing information to a disk drive, a clock-doubled 66-MHz CPU is slowed to 33-MHz during idle cycles, resulting in immediate 50 percent power savings, during disk activity, without affecting the system's performance. On average, a SL Enhanced Intel486 DX2 CPU will use 10 percent less power due to the Auto Idle feature. This savings occurs automatically, without an impact to performance or a need to be controlled by the system or software.

Please note that all cpus after 486 have all SL functions implemented. Therefore, "SL Enhanced" feature is not mentionned for any of them.

Used for : Intel 386, Intel 486