diff --git a/libcpuid/libcpuid.h b/libcpuid/libcpuid.h
index ebff735..163e4b8 100644
--- a/libcpuid/libcpuid.h
+++ b/libcpuid/libcpuid.h
@@ -822,7 +822,7 @@ struct msr_driver_t* cpu_msr_driver_open(void);
 /**
  * @brief Similar to \ref cpu_msr_driver_open, but accept one parameter
  *
- * This function works on Linux only
+ * This function works on certain operating system (GNU/Linux, FreeBSD)
  *
  * @param core_num specify the core number for MSR.
  *          The first core number is 0.
@@ -832,7 +832,7 @@ struct msr_driver_t* cpu_msr_driver_open(void);
  *          The error message can be obtained by calling \ref cpuid_error.
  *          @see cpu_error_t
  */
-struct msr_driver_t* cpu_msr_driver_open_core(int core_num);
+struct msr_driver_t* cpu_msr_driver_open_core(uint8_t core_num);
 
 /**
  * @brief Reads a Model-Specific Register (MSR)
diff --git a/libcpuid/rdmsr.c b/libcpuid/rdmsr.c
index d6d6678..e663a25 100644
--- a/libcpuid/rdmsr.c
+++ b/libcpuid/rdmsr.c
@@ -46,12 +46,11 @@ struct msr_driver_t* cpu_msr_driver_open(void)
 	return cpu_msr_driver_open_core(0);
 }
 
-struct msr_driver_t* cpu_msr_driver_open_core(int core_num)
+struct msr_driver_t* cpu_msr_driver_open_core(uint8_t core_num)
 {
 	char msr[32];
 	struct msr_driver_t* handle;
-	if(core_num < 0 && cpuid_get_total_cpus() <= core_num)
-	{
+	if (core_num >= cpuid_get_total_cpus())	{
 		set_error(ERR_INVCNB);
 		return NULL;
 	}
@@ -112,12 +111,11 @@ struct msr_driver_t* cpu_msr_driver_open(void)
 	return cpu_msr_driver_open_core(0);
 }
 
-struct msr_driver_t* cpu_msr_driver_open_core(int core_num)
+struct msr_driver_t* cpu_msr_driver_open_core(uint8_t core_num)
 {
 	char msr[32];
 	struct msr_driver_t* handle;
-	if(core_num < 0 && cpuid_get_total_cpus() <= core_num)
-	{
+	if (core_num >= cpuid_get_total_cpus()) {
 		set_error(ERR_INVCNB);
 		return NULL;
 	}