Apply by doing:
        cd /usr/src
        patch -p0 < 001_zsconsole.patch

And then rebuild your kernel.

Index: sys/arch/sparc/dev/zs.c
===================================================================
RCS file: /cvs/src/sys/arch/sparc/dev/zs.c,v
retrieving revision 1.26
retrieving revision 1.26.2.1
diff -u -r1.26 -r1.26.2.1
--- sys/arch/sparc/dev/zs.c	2000/07/14 20:27:37	1.26
+++ sys/arch/sparc/dev/zs.c	2000/11/10 20:18:41	1.26.2.1
@@ -957,7 +957,7 @@
 #endif
 		ienab_bis(IE_ZSSOFT);
 	}
-	return (intflags & ZSHARD_WAS_SERVICED) ? 1 : 0 /* -1 XXX */;
+	return (intflags & ZSHARD_WAS_SERVICED) ? 1 : -1 /* XXX */;
 }
 
 static int