Transcription of Mechanism: Limited Direct Execution
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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. Obtaining high performance while maintaining controlis thusone of the central challenges in building an operating :HOWTOEFFICIENTLYVIRTUALIZETHECPU WITHCONTROLThe OS must virtualize the CPU in an efficient manner while retainingcontrol over the system.
Direct execution has the obvious advantage of being fast; the program runs natively on the hardware CPU and thus executes as quickly as one would expect. But running on the CPU introduces a problem: what if ... follow convention in order to process arguments and return values cor-rectly, as well as execute the hardware-specific trap ...
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