[feat] 100Hz task switch

kernel/interrupt/pit.cpp

@@ -0,0 +1,48 @@
+#include <pit.h>
+#include <common.h>
+#include <pic.h>
+#include <serial.h>
+
+static inline void outb(UINT16 port, UINT8 val) {
+    ASM("outb %0, %1" : : "a"(val), "Nd"(port));
+}
+
+static volatile UINT64 g_ticks = 0;
+static timer_callback_t g_tick_handler = NULL;
+
+extern "C" void pit_irq_handler(void) {
+    g_ticks++;
+    if (g_tick_handler) {
+        g_tick_handler();
+    }
+}
+
+void pit_init(void) {
+    serial_write("PIT: initializing channel 0 at ");
+    serial_write_hex(PIT_TICK_HZ);
+    serial_write(" Hz\n");
+
+    UINT32 divisor = PIT_BASE_FREQ / PIT_TICK_HZ;
+
+    // Command byte: channel 0, lobyte/hibyte, rate generator, binary
+    outb(PIT_COMMAND_PORT, 0x36);
+
+    // Send divisor (low byte first, then high byte)
+    outb(PIT_CHANNEL0_DATA, (UINT8)(divisor & 0xFF));
+    outb(PIT_CHANNEL0_DATA, (UINT8)((divisor >> 8) & 0xFF));
+
+    // Unmask IRQ 0 (timer)
+    pic_unmask_irq(0);
+
+    serial_write("PIT: divisor = ");
+    serial_write_hex(divisor);
+    serial_write("\n");
+}
+
+void pit_set_tick_handler(timer_callback_t handler) {
+    g_tick_handler = handler;
+}
+
+UINT64 pit_get_ticks(void) {
+    return g_ticks;
+}