diff --git a/libraries/Desktop/include/avr/io.h b/libraries/Desktop/include/avr/io.h
index 695adec7e3..fa58db23a1 100644
--- a/libraries/Desktop/include/avr/io.h
+++ b/libraries/Desktop/include/avr/io.h
@@ -15,6 +15,10 @@ extern "C" volatile uint8_t __iomem[1024];
 
 #define _SFR_IO8(addr)  __iomem[addr]
 
+extern "C" volatile uint8_t SREG;
+
+#define _interrupts_are_blocked() ((SREG&0x80)==0)
+
 #define __ATmegaxx0__
 #include "iom2560.h"
 
diff --git a/libraries/Desktop/support/Arduino.cpp b/libraries/Desktop/support/Arduino.cpp
index a0c6c8e017..8671e7860d 100644
--- a/libraries/Desktop/support/Arduino.cpp
+++ b/libraries/Desktop/support/Arduino.cpp
@@ -11,6 +11,7 @@
 extern "C" {
 
 volatile uint8_t __iomem[1024];
+volatile uint8_t SREG = 0x80;
 
 unsigned __brkval = 0x2000;
 unsigned __bss_end = 0x1000;
@@ -18,19 +19,13 @@ unsigned __bss_end = 0x1000;
 // disable interrupts
 void cli(void)
 {
-	sigset_t set;
-        sigemptyset(&set);
-        sigaddset(&set, SIGALRM);
-        sigprocmask(SIG_BLOCK,&set,NULL);
+	SREG &= ~0x80;
 }
 
 // enable interrupts
 void sei(void)
 {
-	sigset_t set;
-        sigemptyset(&set);
-        sigaddset(&set, SIGALRM);
-        sigprocmask(SIG_UNBLOCK,&set,NULL);
+	SREG |= 0x80;
 }
 
 void pinMode(uint8_t pin, uint8_t mode)
diff --git a/libraries/Desktop/support/sitl.cpp b/libraries/Desktop/support/sitl.cpp
index 80900bfe12..363f7840ad 100644
--- a/libraries/Desktop/support/sitl.cpp
+++ b/libraries/Desktop/support/sitl.cpp
@@ -228,12 +228,10 @@ static void sitl_simulator_output(void)
 static void timer_handler(int signum)
 {
 	static uint32_t last_update_count;
-	static bool running;
 
-	if (running) {
+	if (_interrupts_are_blocked()) {
 		return;
 	}
-	running = true;
 	cli();
 
 #ifndef __CYGWIN__
@@ -271,13 +269,11 @@ static void timer_handler(int signum)
 	if (update_count == 0) {
 		sitl_update_gps(0, 0, 0, 0, 0, false);
 		sei();
-		running = false;
 		return;
 	}
 
 	if (update_count == last_update_count) {
 		sei();
-		running = false;
 		return;
 	}
 	last_update_count = update_count;
@@ -297,7 +293,6 @@ static void timer_handler(int signum)
 	ADCSRA &= ~_BV(ADSC);
 
 	sei();
-	running = false;
 }