What happens in the CPU when there is no user code to run?

Question

It sounds reasonable that the os/rtos would schedule an "Idle task". In that case, wouldn't it be power consuming? (it sounds reasonable that the idle task will execute: while (true) {} )

Answer 1

This depends on the OS and the CPU architecture. On x86 (Intel compatible) the operating system might execute HLT instructions, making the CPU wait until something interesting happens, such as a hardware interrupt. This supposedly consumes very little power. Operating systems report the time spent doing this as "idle" and may even assign it to a fictional "idle" process.

So, when in Windows task manager you see that the System Idle Process is consuming 90% CPU what it really means is that the CPU does not have an actual a program to run 90% of the time.

Here's a good article on the subject: What does an idle CPU do?