The idle loop of an OS usually halts the core and waits for an actual hardware interrupt, too. And if your user space program is e.g. waiting on a disk read, th

The idle loop of an OS usually halts the core and waits for an actual hardware interrupt, too. And if your user space program is e.g. waiting on a disk read, there is a pretty good chance that the OS has nothing else to do and hits that loop, halting the core, until the disk delivers, or maybe a timer interrupt kicks in. So there's not much difference between a Game Boy and a modern computer in that particular regard, other than the many thick abstraction layers between the system call invocation and the halt instruction.