INT 15 - Advanced Power Management Specification - CPU BUSY
AX = 5306h
Return: CF clear
Notes:  called to ensure that the system runs at full speed even on systems
where the BIOS is unable to recognize increased activity (especially
if interrupts are hooked by other programs and not chained to the
BIOS)
this call may be made even when the system is already running at full
speed, but it will create unnecessary overhead
should not be called from within a hardware interrupt handler to avoid
reentrance problems

See also: 155305

155306 - Advanced Power Management Specification - CPU BUSY