@@ -48,6 +48,34 @@ __asm__ volatile ("sti");
 }
 
 /*!
+ * Pushes the contents of the EFLAGS register onto the stack.
+ */
+__attribute__((always_inline)) inline void pushf(void) {
+    __asm__ volatile ("pushf");
+}
+
+/*!
+ * Pops the contents of the EFLAGS register from the top of the stack.
+ */
+__attribute__((always_inline)) inline void popf(void) {
+    __asm__ volatile ("popf");
+}
+
+/*!
+ * Pushes the contents of the general purpose registers onto the stack.
+ */
+__attribute__((always_inline)) inline void pusha(void) {
+    __asm__ volatile ("pusha");
+}
+
+/*!
+ * Pops the contents of the general purpose registers off the top of the stack.
+ */
+__attribute__((always_inline)) inline void popa(void) {
+    __asm__ volatile ("popa");
+}
+
+/*!
  * CPUID
  */
 __attribute__((always_inline)) inline void cpuid(uint32_t eax, uint32_t *ebx, uint32_t *ecx, uint32_t *edx) {

@@ -7,8 +7,14 @@ #define OS_INTERRUPTS_H
 
 #include <stdint.h>
 
-#define INTERRUPT(name, frame) __attribute__((interrupt)) void name(INTERRUPT_FRAME *frame)
-#define EXCEPTION(name, frame, err_code) __attribute__((interrupt)) void name(INTERRUPT_FRAME *frame, uword_t err_code)
+#define INTERRUPT(name, frame) __attribute__((interrupt, optimize("O0"))) void name(INTERRUPT_FRAME *frame)
+#define EXCEPTION(name, frame, err_code) __attribute__((interrupt, optimize("O0"))) void name(INTERRUPT_FRAME *frame, uword_t err_code)
+
+#define INT_GATE (0b10001110)
+#define TRAP_GATE (0b10001111)
+#define SYSCALL_INTERRUPT (0b11101110)
+
+#define INTERRUPT_COUNT 0x31
 
 /*!
  * Represents a single entry of the IDT.
@@ -30,5 +36,6 @@
 typedef uint32_t uword_t;
 
 int interrupts_init(void);
+void install_interrupt(int num, void* func, uint16_t selector, uint8_t type);
 
 #endif

@@ -39,11 +39,11 @@
 /*!
  * Sends an end-of-interrupt-signal to the corresponding PIC.
  */
-inline void pic1_eoi(void) {
+__attribute__((always_inline)) inline void pic1_eoi(void) {
     outb(PIC_END_OF_INTERRUPT, PIC1_COMMAND);
 }
 
-inline void pic2_eoi(void) {
+__attribute__((always_inline)) inline void pic2_eoi(void) {
     outb(PIC_END_OF_INTERRUPT, PIC2_COMMAND);
     outb(PIC_END_OF_INTERRUPT, PIC1_COMMAND);
 }

@@ -36,4 +36,6 @@ * \param freq_div Desired frequency divider.
  */
 void pit_setup_channel(PIT_CHANNEL channel, PIT_MODE mode, uint16_t freq_div);
 
+void pit_init(void);
+
 #endif

@@ -19,4 +19,9 @@ * Initializes the scheduler.
  */
 int sched_init(void);
 
+/*!
+ * Starts the scheduler.
+ */
+int sched_start(void);
+
 #endif

@@ -1,10 +1,8 @@
 #include "cedos/interrupts.h"
 #include "cedos/drivers/console.h"
 #include "cedos/pic.h"
-
-#define INT_GATE (0b10001110)
-#define TRAP_GATE (0b10001111)
-#define SYSCALL_INTERRUPT (0b11101110)
+#include "cedos/core.h"
+#include "cedos/pit.h"
 
 #define array_sizeof(array) (sizeof(array)/sizeof(array[0]))
 
@@ -50,14 +48,15 @@ printk("PIC2 INTERRUPT\n");
     pic2_eoi();
 }
 
-IDT_ENTRY IDT[31];
+IDT_ENTRY IDT[INTERRUPT_COUNT];
+void (*INT_HANDLERS[INTERRUPT_COUNT])(void);
 
-void install_interrupt(int index, void* func, uint16_t selector, uint8_t type) {
-    IDT[index].offset_0 = (uint16_t)((uint32_t)func);
-    IDT[index].selector = selector;
-    IDT[index].__zero = 0;
-    IDT[index].type_attr = type;
-    IDT[index].offset_16 = (uint16_t)((uint32_t)(func) >> 16);
+void install_interrupt(int num, void* func, uint16_t selector, uint8_t type) {
+    IDT[num].offset_0 = (uint16_t)((uint32_t)func);
+    IDT[num].selector = selector;
+    IDT[num].__zero = 0;
+    IDT[num].type_attr = type;
+    IDT[num].offset_16 = (uint16_t)((uint32_t)(func) >> 16);
 }
 
 struct {
@@ -69,7 +68,7 @@ IDT
 };
 
 int interrupts_init(void) {
-    for (uint32_t i = 0; i < array_sizeof(IDT); i++) {
+    for (uint32_t i = 0; i < INTERRUPT_COUNT; i++) {
         if (i == 0x03) {
             install_interrupt(i, breakpoint_isr, 0x08, INT_GATE);
         } else if (i == 0x08) {

@@ -1,6 +1,7 @@
 #include "cedos/drivers/console.h"
 #include "cedos/interrupts.h"
 #include "cedos/pic.h"
+#include "cedos/pit.h"
 #include "cedos/scheduler.h"
 #include "cedos/mm/paging.h"
 #include "cedos/core.h"
@@ -12,13 +13,17 @@ printk("Core functions initialized.\n");
 
     printk("Initializing PIC...");
     pic_init();
+    printk("done.\n");
+
+    printk("Initializing PIT...");
+    pit_init();
     printk("done.\n");
     
     printk("Initializing interrupts...");
     interrupts_init();
     printk("done.\n");
 
-    printk("Activating interrupts...");
+    printk("Setting up timer interrupts...");
     sti();
     printk("done.\n");
 
@@ -37,13 +42,19 @@ cpuid(0, &res[0], &res[2], &res[1]);
     res[3] = 0;
     printk("CPU manufacturer: %s\n", res);
 }
+
+extern uint8_t* IDT;
 
 int os_main(void) {
+    //pic_unmask_interrupt(0);
     infodump();
 
-    kpanic("SIMULATED KERNEL PANIC");
+    printk("Starting scheduler.");
+    sched_start();
+
+    //kpanic("SIMULATED KERNEL PANIC");
 
     // won't be executed
-    printk("Main procedure terminating.\n");
+    //printk("Main procedure terminating.\n");
     return 0;
 }

@@ -27,7 +27,7 @@ outb(0x01, PIC1_DATA);
     outb(0x01, PIC2_DATA);
     nop(); nop(); nop(); nop();
 
-    // clear interrupt masks
+    // mask all interrupts
     outb(0xff, PIC1_DATA);
     outb(0xff, PIC2_DATA);
 
@@ -42,7 +42,7 @@ */
 int pic_unmask_interrupt(int irq) {
     uint16_t port;
 
-    if (irq >= 8) {
+    if (irq < 8) {
         port = PIC1_DATA;    
     } else {
         port = PIC2_DATA;
@@ -64,7 +64,7 @@ */
 int pic_mask_interrupt(int irq) {
     uint16_t port;
     
-    if (irq >= 8) {
+    if (irq < 8) {
         port = PIC1_DATA;    
     } else {
         port = PIC2_DATA;

@@ -1,10 +1,12 @@
 #include "cedos/pit.h"
 #include "assembly.h"
+#include "cedos/pic.h"
 
 #define PIT_ACCESS_MODE_LO_HI (0b11 << 4)
 #define PIT_COMMAND_PORT ((uint16_t)0x43)
 
 void pit_setup_channel(PIT_CHANNEL channel, PIT_MODE mode, uint16_t freq_div) {
+    pushf();
     cli();
     outb(((uint8_t)channel << 6) | PIT_ACCESS_MODE_LO_HI | (uint8_t)mode, PIT_COMMAND_PORT);
     nop(); nop(); nop(); nop();
@@ -13,5 +15,10 @@ outb((uint8_t)(freq_div & 0xFF), 0x40 + (uint16_t)channel);
     nop(); nop(); nop(); nop();
     
     outb((uint8_t)(freq_div >> 8), 0x40 + (uint16_t)channel);
-    sti();
+    popf();
+}
+
+void pit_init(void) {
+    pic_unmask_interrupt(0);
+    return 1;
 }

@@ -3,6 +3,11 @@ #include "cedos/process.h"
 #include "cedos/mm/paging.h"
 #include "cedos/drivers/console.h"
 #include "cedos/core.h"
+#include "cedos/interrupts.h"
+#include "cedos/pit.h"
+#include "cedos/pic.h"
+#include "assembly.h"
+#include "cedos/drivers/speaker.h"
 
 PROCESS* get_slot(void) {
     static PROCESS free_slots[4];
@@ -27,17 +32,37 @@
     return add_process(p);
 }
 
+INTERRUPT(sched_interrupt, frame) {
+    printk("--#");
+
+    // reset the timer
+    pit_setup_channel(PIT_CHANNEL_0, PIT_MODE_0, 0xFFFF);
+
+    uint32_t big_ben[] = { 415, 330, 370, 247, 247, 370, 415, 330 };
+    static int i = 0;
+
+    bios_sp.make_sound(big_ben[(i / 8) % (sizeof(big_ben)/sizeof(big_ben[0]))], 1);
+    i++;
+
+    pic1_eoi();
+}
+
 int sched_init(void) {
     // TODO: create and start idle process
 
     //sched_exec(create_empty_page_dir(), (void*)0, (void*)0);
+
+    // install timer interrupt
+    install_interrupt(0x20, sched_interrupt, 0x08, INT_GATE);
+
     return 1;
 }
 
-int sched_dispatcher(void) {
-
+int sched_start(void) {
+    // perform the first timer interrupt manually
+    INT(0x20);
 }
 
-INTERRUPT(sched_interrupt, frame) {
+int sched_dispatcher(void) {
 
 }