<ul><li>Operating systems have limited direct execution to maintain control over resource permissions and process operations.</li><li>User programs run in user mode by default and issue system calls to perform restricted operations.</li><li>The operating system has a trap table that maps system calls to instructions, and writing to it is a restricted operation.</li><li>Context switching involves saving the registers of the currently executing process and restoring those of the soon-to-be-executing process.</li></ul>

Operating Systems: Limited Direct Execution

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