What Does an Idle CPU Do?
In the days of OS/360 and OS/370 (IBM's mainframe OS in the 60's and '70s) no instructions at all executed in the wait state.  There was no OS level idle process.  Instead, the internal CPU microcode itself ran a little loop without dispatching any machine instructions until the tentacles that reached out from that loop were tickled by an external interrupting event.  That loop had to be very short for latency reasons and very prickly to test for all the conditions that could take the processor out of it to resume dispatching instructions or do critical internal things.<p>I know this because I wrote that 6 (initially 8) micro-instruction loop for the IBM System/370 Model 155-157 in the late '60s.  I was told back then that since that model became the most widely sold machine ever I had written the code most executed in the history of computing.  :-)<p>The status latch in the machine that indicated that loop was running was fed to a light on the console and, yep, it was nearly always lit other than when booting the OS.