Nitpicking: arbitrary pauses can occur even without syscalls, when the OS preempts the program.
More nitpicking: on x86-64, SMI interrupts can cause arbitrary pauses even without any software control involved. Hard realtime on x86-64 is not possible.
More nitpicking: on x86-64, SMI interrupts can cause arbitrary pauses even without any software control involved. Hard realtime on x86-64 is not possible.