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Midterm 2

Thursday, November 19 — in class, full period. No exercise session on Friday, November 20.

Format

Pencil and paper, closed book. One permitted reference: the Cheatsheet, printed. No electronic devices.

Scope

Cumulative in concepts, but weighted heavily toward 34k–48k — processes through user mode. Anything from Midterm 1 may reappear as a building block; nothing will be asked that depends only on Midterm 1 material. Worth 15% of the course grade.

Processes through the console (34k–45k)

  • Context switching: swtch, which registers and why only those, #[repr(C)], and the double switch through the per-CPU scheduler context
  • Scheduling: mechanism vs policy, round robin, and the survey — FCFS, SJF, priority, MLFQ, CFS; fairness, quantum, throughput, starvation
  • Concurrency: how a race arises, atomicity, test-and-set vs compare-and-swap, Acquire/Release, the RAII guard, Send/Sync, deadlock, lock ordering, and why a kernel disables interrupts while holding a spinlock
  • Semaphores, P/V, the lost-wakeup problem, bounded buffers
  • The kernel heap: what #[global_allocator] is and what Box/Vec/Arc cost
  • Turning the MMU on: the bootstrap paradox, identity mapping, the satp encoding, sfence.vma, the TLB
  • Filesystems: inodes vs directories, why the name lives in the directory, path resolution
  • Devices: registers, status flags, polling, volatile
  • Boot order as a dependency graph
  • Traps: M/S/U, exceptions vs interrupts, the M→S handoff and its six CSRs, stvec/sepc/scause/sstatus/stval, sret
  • Timer interrupts, the CLINT, and why preemption needs a timer
  • Device interrupts, the PLIC's four-register protocol, the console ring buffer

Shell and user mode (46k–48k)

  • The shell as a REPL and the command table
  • User mode: privilege levels, the trampoline page and why it must be mapped at the same virtual address in both tables, the trapframe, ecall

Not on this exam: exec, file descriptors, fork/wait, pipes. Those are the final's territory.

What the questions look like

Same three shapes as Midterm 1 — trace the registers, decode the bits, order the steps — plus two that are specific to this material:

Find the race. Given two concurrent sequences, identify an interleaving that breaks an invariant, and say what makes it safe.

Trace the trap. Given an ecall or a timer interrupt, walk what the hardware does and what the software does at each step, naming the CSR involved.

How to prepare

  1. Reread 35k, 37k, 39k, 43k, 48k — the five that carry the most examinable material.
  2. Redraw from memory: the double context switch, the kernel address space after satp is set, the trap path from ecall to sret, the PLIC handshake.
  3. Do Practice Set 2 on paper.
  4. Use rv6 Architecture as the map; it has all three trap paths as diagrams.