diff --git a/Makefile b/Makefile
index 8024a4c..0ae82bd 100644
--- a/Makefile
+++ b/Makefile
@@ -14,6 +14,7 @@ CFLAGS+= -fno-common -ffunction-sections -fdata-sections
 CFLAGS+= -g -gdwarf-2
 CFLAGS+= -Wall
 
+
 LDFLAGS+= ${CFLAGS}
 LDFLAGS+= --static
 #LDFLAGS+= -nostartfiles
@@ -32,7 +33,8 @@ OBJS+= main.o
 OBJS+= syscall.o
 OBJS+= usartu.o
 OBJS+= scheduler.o
-
+OBJS+= semoper.o
+OBJS+= semaphore.o
 
 main.elf: $(OBJS)
 	$(TARGET)-gcc $(^F) $(LDFLAGS) -o $@
@@ -42,7 +44,7 @@ main.elf: $(OBJS)
 	$(TARGET)-gcc $(CFLAGS) -c -o $@ $<
 
 %.o: %.S
-	$(TARGET)-as $(ASFLAGS) -o $@ $<
+	$(TARGET)-gcc $(CFLAGS) -c -o $@ $<
 
 %.bin: %.elf
 	$(TARGET)-objcopy -O binary $< $@
diff --git a/main.c b/main.c
index 7df08d3..55b5668 100644
--- a/main.c
+++ b/main.c
@@ -18,6 +18,7 @@
 
 #include "scheduler.h"
 #include "usartu.h"
+#include "semoper.h"
 
 
 void delay(uint32_t n) {
@@ -81,8 +82,11 @@ void task3(void) {
 }
 
 void task4(void) {
+    static volatile int32_t t4;
+
     while (true) {
-        printf("task 4 %d\r\n", g_uptime);
+        sem_post32(&t4, (int32_t)1);
+        printf("task 4 %d %lu\r\n", g_uptime, t4);
         delay(3300);
     };
 }
diff --git a/semaphore.c b/semaphore.c
index f6b885a..a8e0032 100644
--- a/semaphore.c
+++ b/semaphore.c
@@ -3,21 +3,19 @@
  */
 
 #include "semaphore.h"
+#include "semoper.h"
 
-typedef struct {
-    int value;
-} sem_t;
-
-int sem_init(sem_t* sem, int value) {
+void sem_init(sem_t* sem, int32_t value) {
     sem->value = value;
 }
 
-int sem_wait(sem_t* sem) {
-    while(sem->value > 0);
-    sem->value--;
+int32_t sem_wait(sem_t* sem) {
+    //while(sem->value > 0);
+    //sem->value--;
+    return sem_wait32(&(sem->value), (int32_t)1);
 }
 
-
-int sem_post(sem_t* sem) {
-    sem->value++;
+int32_t sem_post(sem_t* sem) {
+    //sem->value++;
+    return sem_post32(&(sem->value), (int32_t)1);
 }
diff --git a/semaphore.h b/semaphore.h
index 0cf2b8e..c8d005c 100644
--- a/semaphore.h
+++ b/semaphore.h
@@ -3,10 +3,12 @@
  */
 
 
+#include <stdint.h>
+
 typedef struct {
-    int value;
+    int32_t value;
 } sem_t;
 
-int sem_init(sem_t* sem, int value);
-int sem_wait(sem_t* sem);
-int sem_post(sem_t* sem);
+void sem_init(sem_t* sem, int32_t value);
+int32_t sem_wait(sem_t* sem);
+int32_t sem_post(sem_t* sem);
diff --git a/semoper.S b/semoper.S
new file mode 100644
index 0000000..561f1f2
--- /dev/null
+++ b/semoper.S
@@ -0,0 +1,31 @@
+.thumb
+.syntax unified
+
+.text
+
+.globl  sem_post32
+.type   sem_post32, %function
+
+sem_post32:
+1:
+        ldrex           r2, [r0]
+        add             r2, r2, r1
+        strex           r3, r2, [r0]
+        teq             r3, #0
+        bne             1b
+        mov             r0, r2
+        bx              lr
+
+
+.globl  sem_wait32
+.type   sem_wait32, %function
+
+sem_wait32:
+1:
+        ldrex           r2, [r0]
+        sub             r2, r2, r1
+        strex           r3, r2, [r0]
+        teq             r3, #0
+        bne             1b
+        mov             r0, r2
+        bx              lr
diff --git a/semoper.h b/semoper.h
new file mode 100644
index 0000000..23d2d10
--- /dev/null
+++ b/semoper.h
@@ -0,0 +1,13 @@
+/*
+ * Copyright 2022 Oleg Borodin  <borodin@unix7.org>
+ */
+
+#ifndef SEMOPER_H_QWERTY
+#define SEMOPER_H_QWERTY
+
+#include <stdint.h>
+
+int32_t sem_post32(volatile int32_t *addr, int32_t value);
+int32_t sem_wait32(volatile int32_t *addr, int32_t value);
+
+#endif