??All Unix kernels are reentrant : this means that several processes may be executing in Kernel
Mode at the same time. Of course, on uniprocessor systems only one process can progress,
but many of them can be blocked in Kernel Mode waiting for the CPU or the completion of
some I/O operation. For instance, after issuing a read to a disk on behalf of some process, the
kernel will let the disk controller handle it and will resume executing other processes.
An interrupt notifies the kernel when the device has satisfied the read, so the former process
can resume the execution.
