interrupts: IDT, PIC remap, PIT timer, PS/2 keyboard IRQ

- IDT with 256 entries, loaded via lidt (CS read from current context)
- PIC 8259A remapped to vectors 32-47, timer+keyboard IRQs unmasked
- PIT channel 0 at ~100 Hz (mode 2, rate generator)
- PS/2 keyboard IRQ (IRQ1 → vector 33): reads port 0x60, pushes byte
  to lock-free SPSC ring buffer
- Timer IRQ (IRQ0 → vector 32): sends EOI, increments tick counter
- ps2::poll_key() now reads from ring buffer with cli/sti+hlt for
  race-free sleep (instead of busy-polling the data port)
- build.rs: use idiomatic Result<_, Box<dyn Error>> return type
- tools/qemu.sh: fix SC2207 shellcheck warning

1 files changed, 21 insertions, 34 deletions

diff --git a/src/ps2.rs b/src/ps2.rs
index e972658..3a42c1a 100644
--- a/src/ps2.rs
+++ b/src/ps2.rs
@@ -179,47 +179,34 @@ fn drain_input() {
     }
 }
 
-/// Poll for a key event. Blocks until a key is pressed.
+/// Poll for a key event. Blocks via `hlt` until a key is pressed.
+/// The keyboard interrupt handler pushes raw scancodes to a ring buffer;
+/// this function pops and decodes them.
 pub fn poll_key() -> KeyEvent {
     loop {
-        let byte = read_data();
-        if let Some(ev) = process_byte(byte) {
-            return ev;
-        }
-    }
-}
+        // Disable interrupts so the buffer-empty → hlt window is atomic.
+        unsafe { core::arch::asm!("cli", options(nostack, preserves_flags)); }
 
-/// Non-blocking poll.
-pub fn try_poll_key() -> Option<KeyEvent> {
-    unsafe {
-        let status: u8;
-        core::arch::asm!("in al, dx", out("al") status, in("dx") STATUS_PORT,
-            options(nostack, preserves_flags));
-        if status & STAT_OUTPUT_FULL == 0 {
-            return None;
+        if let Some(byte) = crate::interrupts::read_scancode() {
+            unsafe { core::arch::asm!("sti", options(nostack, preserves_flags)); }
+            if let Some(ev) = process_byte(byte) {
+                return ev;
+            }
+        } else {
+            // Re-enable and halt.  If a keyboard IRQ arrived between the
+            // buffer check and `sti`, the CPU sees a pending interrupt
+            // and `hlt` returns immediately.
+            unsafe {
+                core::arch::asm!("sti; hlt", options(nostack, preserves_flags));
+            }
         }
-        let data: u8;
-        core::arch::asm!("in al, dx", out("al") data, in("dx") DATA_PORT,
-            options(nostack, preserves_flags));
-        process_byte(data)
     }
 }
 
-/// Read a byte from the keyboard data port (blocking).
-fn read_data() -> u8 {
-    unsafe {
-        loop {
-            let status: u8;
-            core::arch::asm!("in al, dx", out("al") status, in("dx") STATUS_PORT,
-                options(nostack, preserves_flags));
-            if status & STAT_OUTPUT_FULL != 0 {
-                let data: u8;
-                core::arch::asm!("in al, dx", out("al") data, in("dx") DATA_PORT,
-                    options(nostack, preserves_flags));
-                return data;
-            }
-        }
-    }
+/// Non-blocking poll — read from the interrupt ring buffer if available.
+pub fn try_poll_key() -> Option<KeyEvent> {
+    let byte = crate::interrupts::read_scancode()?;
+    process_byte(byte)
 }
 
 /// Process a single scancode byte. Returns Some(KeyEvent) when a visible