Juniper Systems Allegro DOS Manual - Manual (Page 238)

Page 8-4 Software Developer's Guide for DOS
Power Management
The Allegro has several power management functions integrated within
the BIOS. Applications can use these functions to greatly enhance
performance and operating time between battery charges. There is an
internal automatic Power Manager, which can be set off or on, and five
levels of performance, from 0 (miser) to 4 (hyper).
There is a trade-off between battery operating time and performance. As
the processor speed is increased the Allegro uses more power, causing
the batteries to drain more quickly. For optimal power management, the
processor should be run at high speed only when there is actual
processing to be done, and run at low speed or stopped (sleep mode) the
rest of the time. It is analogous to driving a car. Think of the processor as
the car engine and the battery as the car fuel tank. When you press the
gas pedal, the engine runs faster and in turn uses fuel more quickly.
On most computers that run DOS, the processor is running at full
throttle all of the time, even when there is no work to be done. While the
processor is waiting for input, it is running at full speed in circles
(programmers call this a spin loop). This is a tremendous waste of
power, yet when DOS was designed desktop computers did not run on
batteries and so power saving was not a concern. It is a serious problem
in a battery-operated system. Properly controlling CPU power during
idle times has a dramatic effect on battery operating time. It can be
handled at either the BIOS/operating system level, or at the application
level.
Starting and stopping the processor as needed is a simple concept, but
can be difficult to implement effectively. The problem is determining
when the processor is not doing any real work and can be safely shut
down. When writing an application, the programmer usually knows
when the processor is idly waiting for input, and by using Allegro’s
power management API can design a very power efficient program. If
the program has not been designed to use power management, the
Allegro BIOS attempts to manage power automatically. However, the
BIOS does not know for sure what the processor is doing. It can only
guess based on the level of activity it can see. For this reason, it is better
to design power management into the program itself.