Framebuffer And Shell

Parse framebuffer information for text-mode rendering. Why it matters: The framebuffer info from the bootloader must be parsed before any pixels can be drawn. Focus on `init` in `kernel/src/fb/mod.rs` while you trace the snippet.

/// Initialise the framebuffer text console.
///
/// Called once from `kernel_main` before `task::run()`.  Extracts the raw
/// pointer and layout from `fb`, stores them in the global [`CONSOLE`], and
/// clears the screen.
///
/// # Safety
/// `fb` must be `&'static mut` (derived from `boot_info.framebuffer`).  The
/// caller must not access `boot_info.framebuffer` again after this call.
///
/// Returns `true` if the framebuffer was large enough to enable the text console.
pub fn init(fb: &'static mut FrameBuffer) -> bool {
    let info: FrameBufferInfo = fb.info();
    // Extract the raw mutable pointer from the &'static mut FrameBuffer.
    // SAFETY: fb is &'static mut so the pointer is valid for the kernel
    // lifetime.  We store it inside a Mutex<Option<FbConsole>> so it is
    // only ever accessed with the lock held, preventing aliased writes.
    let buf_ptr: *mut u8 = fb.buffer_mut().as_mut_ptr();
    // SAFETY: buf_ptr and info are both derived from &'static mut FrameBuffer.
    unsafe { init_from_parts(buf_ptr, info) }
}

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Implement line input with basic editing. Why it matters: Interactive use requires at minimum line-buffered input with backspace support. Focus on `read_line` in `userspace/shell/src/main.rs` while you trace the snippet.

/// Read a line from stdin, handling backspace and character echo.
/// Returns the number of valid bytes (excluding null terminator).
fn read_line(buf: &mut [u8; MAX_LINE]) -> usize {
    let mut pos = 0usize;
    loop {
        let mut byte = [0u8; 1];
        let n = read(STDIN_FILENO, &mut byte);
        if n <= 0 {
            // EOF or error — return what we have.
            if pos > 0 {
                return pos;
            }
            // Yield CPU briefly (~10ms) and retry.
            syscall_lib::nanosleep(1);
            continue;
        }

        let ch = byte[0];

        if ch == b'\n' || ch == b'\r' {
            // Echo newline (stdin_feeder sends \n but doesn't echo it).
            let _ = write(STDOUT_FILENO, b"\n");
            return pos;
        }

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