Exercise 2: Two Processes Blocking on the Disk, One After Another — Possible Solution ==================================================================== THE TEST ------------------------------ programs[client_a.pid] = [('READ_DISK_BLOCKING',)] # client_b deliberately does real WORK first -- long enough that # client_a's own 5-tick operation genuinely finishes before client_b # ever touches the disk programs[client_b.pid] = [('WORK',) for _ in range(8)] + [('READ_DISK_BLOCKING',)] RESULT ------------------------------ order in which they woke up: [client_a.pid, client_b.pid] Both processes' own real disk operations complete correctly, and client_a -- the one that started first -- wakes up first. WHY client_b's OWN WORK STEPS WERE NECESSARY, NOT JUST CONVENIENT ------------------------------ This SimulatedDisk only ever tracks ONE pending operation at a time (self.pending is a single dict, not a list or queue). If client_b had called READ_DISK_BLOCKING immediately after client_a blocked -- which is exactly what would happen, since client_b becomes the sole ready process the instant client_a blocks, and gets scheduled right away -- client_b's own read_start() call would have overwritten client_a's still-pending operation before it ever had a chance to complete. This is precisely the failure mode Exercise 3 investigates directly. Giving client_b 8 real WORK steps to do FIRST (longer than client_a's own 5-tick latency) guarantees client_a's operation has already completed, by the time client_b ever gets around to calling read_start() itself. WHY THIS SEQUENCING ISN'T CHEATING ------------------------------ This is exactly the discipline a real caller of this simple SimulatedDisk needs to follow: never start a second operation until you've confirmed (via the driver, or in a real system via checking a "busy" flag or waiting for a genuine completion signal) that the previous one has finished. This exercise demonstrates the driver used CORRECTLY, within the honest limits documented in Exercise 3 -- not a workaround, but the actual intended usage pattern for a device that was deliberately kept this simple. WHY THIS WORKS AS AN ANSWER ------------------------------ Tracking the exact real step at which each process transitions out of BLOCKED (rather than just checking the final state at the end) confirms the two operations completed in the correct temporal order, not just that both eventually finished somehow -- a stronger and more precise claim than "it worked out okay."