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For a fully PREEMPTIVE scheduler to function correctly, the OS must support
Aa CPU timer that triggers periodic interrupts
Ba single non-interruptible user-mode loop
Ca hard requirement that all jobs voluntarily yield
Dno special hardware — software polling suffices
Answer & Solution
Correct answer: A. a CPU timer that triggers periodic interrupts
1. PREEMPTIVE scheduling requires the OS to take the CPU AWAY from a running job, even if the job doesn't voluntarily yield.
2. The mechanism: a HARDWARE TIMER interrupt fires periodically (e.g. every 10 ms in Linux). The interrupt traps to the kernel, which runs the scheduler.
3. Without the timer interrupt, a malicious or buggy program could loop forever, monopolising the CPU.
4. OSTEP §7.5 implicitly assumes this hardware: STCF is preemptive 'whenever a new job enters the system'.
5. Option B blocks any scheduling decision. Option C is COOPERATIVE scheduling (early Windows, MacOS 9). Option D is wrong — without timer interrupts, the kernel can't reclaim the CPU.
_Source: OSTEP Ch 6 (Limited Direct Execution) + Ch 7 (cited prerequisite), p. 6._
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