diff --git a/docs/API b/docs/API
index dfa6690260..a61408e6aa 100644
--- a/docs/API
+++ b/docs/API
@@ -227,7 +227,8 @@ Various
   int set_irq_level(int level)
 
      Sets the interrupt level (0 = lowest, 15 = highest) and returns the
-     previous level.
+     previous level. Note that you must shift the argument 4 bits to the left:
+     set_irq_level(level << 4);
 
   void queue_init(struct event_queue *q)
 
diff --git a/firmware/drivers/button.c b/firmware/drivers/button.c
index 66c7e978fb..cbb6d58f7b 100644
--- a/firmware/drivers/button.c
+++ b/firmware/drivers/button.c
@@ -30,6 +30,7 @@
 #include "adc.h"
 #include "serial.h"
 #include "power.h"
+#include "system.h"
 
 struct event_queue button_queue;
 
@@ -256,10 +257,10 @@ static int button_flip(int button)
  */
 void button_set_flip(bool flip)
 {
-    if (flip != flipped) /* not the curent setting */
+    if (flip != flipped) /* not the current setting */
     {
         /* avoid race condition with the button_tick() */
-        int oldlevel = set_irq_level(15);
+        int oldlevel = set_irq_level(HIGHEST_IRQ_LEVEL);
         lastbtn = button_flip(lastbtn); 
         flipped = flip;
         set_irq_level(oldlevel);
diff --git a/firmware/drivers/power.c b/firmware/drivers/power.c
index 563ff3e25c..621f53e11e 100644
--- a/firmware/drivers/power.c
+++ b/firmware/drivers/power.c
@@ -118,7 +118,7 @@ bool ide_powered(void)
 
 void power_off(void)
 {
-    set_irq_level(15);
+    set_irq_level(HIGHEST_IRQ_LEVEL);
 #ifdef HAVE_POWEROFF_ON_PBDR
     and_b(~0x10, &PBDRL);
     or_b(0x10, &PBIORL);
diff --git a/firmware/drivers/serial.c b/firmware/drivers/serial.c
index 5aae38e58c..2badf2e39f 100644
--- a/firmware/drivers/serial.c
+++ b/firmware/drivers/serial.c
@@ -172,7 +172,7 @@ static void screen_dump(void)
 
     serial_enable_tx();
 
-    level = set_irq_level(15);
+    level = set_irq_level(HIGHEST_IRQ_LEVEL);
     for(y = 0;y < LCD_HEIGHT/8;y++)
     {
         for(x = 0;x < LCD_WIDTH;x++)
diff --git a/firmware/export/system.h b/firmware/export/system.h
index 5886145d7b..038d9567e0 100644
--- a/firmware/export/system.h
+++ b/firmware/export/system.h
@@ -25,7 +25,6 @@
 
 extern void system_reboot (void);
 extern void system_init(void);
-extern int set_irq_level(int level);
 
 #define FREQ CPU_FREQ
 #define BAUDRATE 9600
@@ -73,6 +72,20 @@ extern int set_irq_level(int level);
 
 #ifndef SIMULATOR
 
+/****************************************************************************
+ * Interrupt level setting
+ * The level is left shifted 4 bits
+ ****************************************************************************/
+#define HIGHEST_IRQ_LEVEL (15<<4)
+static inline int set_irq_level(int level)
+{
+    int i;
+    /* Read the old level and set the new one */
+    asm volatile ("stc sr, %0" : "=r" (i));
+    asm volatile ("ldc %0, sr" : : "r" (level));
+    return i;
+}
+
 static inline short SWAB16(short value)
   /*
     result[15..8] = value[ 7..0];
diff --git a/firmware/kernel.c b/firmware/kernel.c
index 1c37f004a4..84fb438b49 100644
--- a/firmware/kernel.c
+++ b/firmware/kernel.c
@@ -118,7 +118,7 @@ void queue_post(struct event_queue *q, int id, void *data)
     int wr;
     int oldlevel;
 
-    oldlevel = set_irq_level(15);
+    oldlevel = set_irq_level(HIGHEST_IRQ_LEVEL);
     wr = (q->write++) & QUEUE_LENGTH_MASK;
 
     q->events[wr].id = id;
@@ -201,7 +201,7 @@ void IMIA0(void)
 int tick_add_task(void (*f)(void))
 {
     int i;
-    int oldlevel = set_irq_level(15);
+    int oldlevel = set_irq_level(HIGHEST_IRQ_LEVEL);
 
     /* Add a task if there is room */
     for(i = 0;i < MAX_NUM_TICK_TASKS;i++)
@@ -221,7 +221,7 @@ int tick_add_task(void (*f)(void))
 int tick_remove_task(void (*f)(void))
 {
     int i;
-    int oldlevel = set_irq_level(15);
+    int oldlevel = set_irq_level(HIGHEST_IRQ_LEVEL);
 
     /* Remove a task if it is there */
     for(i = 0;i < MAX_NUM_TICK_TASKS;i++)
diff --git a/firmware/mp3_playback.c b/firmware/mp3_playback.c
index 422a0e6822..a775ba4ff5 100644
--- a/firmware/mp3_playback.c
+++ b/firmware/mp3_playback.c
@@ -338,7 +338,7 @@ static void postpone_dma_tick(void)
 #ifdef HAVE_MAS3587F
 void demand_irq_enable(bool on)
 {
-    int oldlevel = set_irq_level(15);
+    int oldlevel = set_irq_level(HIGHEST_IRQ_LEVEL);
     
     if(on)
     {
diff --git a/firmware/system.c b/firmware/system.c
index 3ec56f7c15..cb0cb29778 100644
--- a/firmware/system.c
+++ b/firmware/system.c
@@ -318,18 +318,6 @@ void system_reboot (void)
 		  "r"(*(int*)0),"r"(4));
 }
 
-/****************************************************************************
- * Interrupt level setting
- ****************************************************************************/
-int set_irq_level(int level)
-{
-    int i;
-    /* Read the old level and set the new one */
-    asm volatile ("stc sr, %0" : "=r" (i));
-    asm volatile ("ldc %0, sr" : : "r" (level << 4));
-    return (i >> 4) & 0x0f;
-}
-
 void UIE (unsigned int pc) /* Unexpected Interrupt or Exception */
 {
     bool state = true;