Mechanism: Limited Direct Execution

6 Mechanism: Limited Direct ExecutionIn order to virtualize the CPU, the operating system needs to somehowshare the physical CPU among many jobs running seemingly at thesametime. The basic idea is simple: run one process for a little while, thenrun another one, and so forth. Bytime sharingthe CPU in this manner,virtualization is are a few challenges, however, in building such virtualizationmachinery. The first isperformance: how can we implement virtualiza-tion without adding excessive overhead to the system ? The secondiscontrol: how can we run processes efficiently while retaining control overthe CPU? Control is particularly important to the OS, as it is in charge ofresources; without control, a process could simply run forever and takeover the machine, or access information that it should not be allowedtoaccess.

You may wonder why a call to a system call, such as open()or read(), looks exactly like a typical procedure call in C; that is, if it looks just like a procedure call, how does the system know it’s a system call, and do all the right stuff? The simple reason: it is a procedure call, but hidden in-side that procedure call is the famous trap ...

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