Operating System: Three Easy Pieces --- Mechanism: Limited Direct Execution (Note)

In order to virtualize the CPU, the operating system needs to somehow share the physical

CPU among many jobs running seemingly at the same time. The basic idea is simple: run

one process for a little while, then run another, and so forth. By time sharing the CPU in this

manner, virtualization is achieved. There are a few challenges, however, in building such

virtualization machinery. The first is performance: how can we implement virtualization without

adding excessive overhead to the system? The second is control: how can we processes efficiently

while retaining control over the CPU? Control is particularly important to the OS, as it is in

charge of resources; without control, a process could simpley run forever and take over the

machine, or access information that it should not be allowed to access. Attaining performance

while maintaining control is thus one of the central challenges in building an Operating System.