Prep: The Console and the Kernel Shell — 45k · 46k

Session: Thu Nov 12, 1h45 · Exercises: 45k_console, 46k_shell · Prep time: ~20 min · Lecture: Week 12 · The Console, the Shell, and User Mode

What you will build

A keypress makes the UART raise source 10 and the PLIC deliver a supervisor external interrupt; your handler asks the PLIC which source fired, drains the waiting bytes into the ring buffer, and tells the PLIC it is done. Then the evaluate step of a kernel-mode REPL: split a line into words and send the first to one of four given commands (pwd, ls, cd, mkdir), or report it as not found. The tests check that a simulated interrupt lands its byte in the buffer and that mkdir docs then ls lists docs; then cargo run boots to a rv6$ prompt.

Concepts you need

• Claim, service, complete; two ways a console dies — Week 12 · Claim, service, complete · rv6 Architecture: Path 2, an S-mode device interrupt
• Top half, bottom half, the lock-free ring, wfi — Week 12 · A ring between two worlds
• A shell is a REPL, and here also the line discipline — Week 12 · A loop, and words
• Tokens are borrowed views; split_whitespace allocates nothing — Week 12 · A loop, and words
• Dispatch by match; output through the Out trait — Week 12 · One match, and &mut dyn Out · rv6 Architecture: Two shells
• Reading to extend, not to rebuild — Week 12 · Thursday · 46k The kernel shell

Read before class

What	Time
Week 12 · This week, then Thursday · 45k The console through Claim, service, complete	4 min
Week 12 · A ring between two worlds, with its four-slot trace	3 min
Week 12 · Thursday · 46k The kernel shell, both sections and the 47k extra-credit box	3 min
Week 12 · For the exam: The PLIC's four registers. Not needed today; on Midterm 2	1 min
rv6 Architecture: Path 2, an S-mode device interrupt and Two shells	4 min

Mental model

Type ls and Enter into a four-byte ring. The counters only grow; % 4 picks the slot:

                                BUF      HEAD  TAIL
'l', 's' -> two handler pushes  [ls..]    0     2    in the trap, SIE off
shell getc -> 'l', echo it      [ls..]    1     2    normal priority
shell getc -> 's', echo it      [ls..]    2     2    empty: HEAD == TAIL
shell getc -> wfi               [ls..]    2     2    halted, zero cycles
Enter -> handler push           [ls\r.]   2     3    wfi returns; "ls" is a line

The handler writes only BUF and TAIL, the reader only HEAD; a stale read errs safely, so one hart needs no lock, and a lock in the handler would deadlock with whatever it interrupted. The ring holds no echo, no erasing, no line: those are the shell's job, off the interrupt path.

Check yourself

1. Your handler buffers the byte but never writes the source number back to the PLIC. What do you see?
   Answer

   The first keypress works; later ones vanish. The PLIC still considers source 10 claimed and never delivers it again; nothing panics.
2. The opposite: it completes without reading the UART. Why is that different?
   Answer

   An interrupt storm: the byte still sits in the receive register, the level-triggered line stays high, and the PLIC re-raises it the moment the gateway re-arms; the kernel spins at 100% CPU.
3. The tokenizer yields &str slices into the line buffer. What stops the REPL from clearing the line too early?
   Answer

   The tokens borrow the line, so the borrow checker refuses line.clear() until the evaluate step returns.

What "done" looks like

oslings run is green, then oslings submit before you leave. Not green? Submit anyway — the tests that pass earn their share — then finish it at a make-up session (office hours, on the class network) for three quarters, or anywhere else for half, and submit again. Midterm 2 is next Thursday, Nov 19, and covers processes through user mode.

Extra credit today

47k_file_commands (+0.5): touch, cat, echo TEXT > FILE, rm, and rmdir. The redirect and rmdir are given as worked examples; the other three stitch together filesystem promises, and each decides which of the filesystem's facts count as errors. See Week 12 · Extra credit · 47k and Why the call is unlink.

If you finish early

Work Week 12 · Problem 1: The PLIC for a second hart and Problem 2: Predict the screen and the line on paper; the PLIC's registers come back on Midterm 2. Then read chapter 5 of the xv6 book, "Interrupts and device drivers," or start Friday's prep page, Prep: User Mode.