diff --git a/src/kernel/cpu/timer.c b/src/kernel/cpu/timer.c
index 5acb253..7f5fa31 100644
--- a/src/kernel/cpu/timer.c
+++ b/src/kernel/cpu/timer.c
@@ -14,7 +14,7 @@
 
 volatile uint32_t apic_ticks_per_us = 1;
 
-static void pit_setup(uint8_t ms) {
+static void pit_setup(uint64_t us) {
   // Disable the pc speaker output
   uint8_t spkr = inb(PC_SPKR);
   spkr &= ~0x1;
@@ -24,7 +24,7 @@ static void pit_setup(uint8_t ms) {
   outb(PIT_CMD, 0x80 | 0x30);
 
   // Set up CH2 for counting down from a number of ms
-  uint64_t count = PIT_FREQ * ms/1000;
+  uint64_t count = PIT_FREQ * us/1000000;
   outb(PIT_CH2, count & 0xFF);
   outb(PIT_CH2, (count >> 8) & 0xFF);
 }
@@ -41,29 +41,33 @@ static void pit_wait() {
 }
 
 static uint64_t calibrate_apic_timer() {
-  pit_setup(10);
+  uint64_t wait_time = 10000;
+  pit_setup(wait_time);
 
+  // Divide bus clock by 16
   APIC_REG(APIC_TIMER_DIVIDER) = 0xB;
-  APIC_REG(APIC_LVT(LVT_TIMER)) = 1<<16; // Mask
+  // Bit 16: Mask interrupt
+  APIC_REG(APIC_LVT(LVT_TIMER)) = 1<<16;
   APIC_REG(APIC_TIMER_INIT) = 0xFFFFFFFF;
 
   pit_wait();
 
   uint32_t ticks = APIC_REG(APIC_TIMER_CURRENT);
-  return (0xFFFFFFFF-ticks)/10/1000;
+  return (0xFFFFFFFF-ticks)/wait_time;
 }
 
 void reset_apic_timer(uint64_t us) {
+  // Divide bus clock by 16
   APIC_REG(APIC_TIMER_DIVIDER) = 0xB;
   APIC_REG(APIC_TIMER_INIT) = us*current_cpu->timer_freq;
-  APIC_REG(APIC_LVT(LVT_TIMER)) = 0x0 | IRQ_INTERRUPT(IRQ_TIMER);
+  // Bit 17: Periodic timer mode
+  APIC_REG(APIC_LVT(LVT_TIMER)) = 1<<17 | IRQ_INTERRUPT(IRQ_TIMER);
 }
 
 volatile int ctr = 0;
 
 registers *apic_timer_handler(registers *r) {
   current_cpu->timer_ticks++;
-  reset_apic_timer(1000);
   irq_ack();
   return r;
 }