I had a very annoying problem with select failing with EINVAL in X. I
traced it to microtime(), and hence gettimeofday() sometimes returning
a bad tv_usec value. Here is a patch:

--- ../../../../../src/sys/arch/pc532/pc532/clock.c	Thu Mar 20 22:19:01 1997
+++ clock.c	Tue Mar 25 22:30:02 1997
@@ -181,10 +181,13 @@
 	/* check for timer overflow (ie; clock int disabled) */
 	if (count > 0 && (ipend & (1<<IR_CLK))) {
 		tvp->tv_usec += tick;
-		if (tvp->tv_usec > 1000000) {
-			tvp->tv_usec -= 1000000;
-			tvp->tv_sec++;
-		}
+	}
+	if (tvp->tv_usec >= 1000000) {
+		tvp->tv_usec -= 1000000;
+		tvp->tv_sec++;
+	}
+	if (tvp->tv_usec < 0 || tvp->tv_usec > 1000000) {
+		printf("microtime bad tv_usec: %ld\n", tvp->tv_usec);
 	}
 }