diff --git a/thread.c b/thread.c
index c15c36fda4..a622f4bf4f 100644
--- a/thread.c
+++ b/thread.c
@@ -1247,7 +1247,7 @@ rb_thread_sleep(int sec)
 }
 
 static void
-rb_thread_schedule_limits(unsigned long limits_us)
+rb_thread_schedule_limits(uint32_t limits_us)
 {
     thread_debug("rb_thread_schedule\n");
     if (!rb_thread_alone()) {
@@ -2085,7 +2085,7 @@ rb_threadptr_execute_interrupts(rb_thread_t *th, int blocking_timing)
 	}
 
 	if (timer_interrupt) {
-	    unsigned long limits_us = TIME_QUANTUM_USEC;
+	    uint32_t limits_us = TIME_QUANTUM_USEC;
 
 	    if (th->priority > 0)
 		limits_us <<= th->priority;
diff --git a/vm_core.h b/vm_core.h
index c414498270..f379e9ce20 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -816,7 +816,7 @@ typedef struct rb_thread_struct {
 #ifdef USE_SIGALTSTACK
     void *altstack;
 #endif
-    unsigned long running_time_us;
+    uint32_t running_time_us; /* 12500..800000 */
     VALUE name;
 } rb_thread_t;