Expects: AX    5305H
Returns: CY    clear (always succeeds)
Info: This function informs the BIOS that the system is currently idle
and that processing should be suspended until the next system
event (typically an interrupt) occurs.  The BIOS may then take
action such as issuing a HLT opcode or slowing or stopping the
CPU clock.

You may access this service via INT 15H or by using the APM entry
address obtained via INT 15H 5302H or INT 15H 5303H (protected
mode connect).

See Also: APM API
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INT 15H 5305H: CPU Idle