It was a hardware change. In those older CPU designs, the external clock signal was directly driving a state machine, so for as long as the clock was applied,

It was a hardware change. In those older CPU designs, the external clock signal was directly driving a state machine, so for as long as the clock was applied, the state machine would go. It's important to realise that there was no good reason to have the ability to stop the CPU in those days - power consumption by the CPU itself was truly trivial compared to the memory and peripherals it was attached to, and those CPUs weren't really damaged or worn out by running continuously. Having the CPU spin in software when there was nothing else to do was perfectly fine.