diff --git a/man2/ioperm.2 b/man2/ioperm.2
index 917c10774..f78db7d1d 100644
--- a/man2/ioperm.2
+++ b/man2/ioperm.2
@@ -61,7 +61,9 @@ Before Linux 2.6.8,
 only the first 0x3ff I/O ports could be specified in this manner.
 For more ports, the
 .BR iopl (2)
-system call had to be used.
+system call had to be used (with a
+.I level
+argument of 3).
 Since Linux 2.6.8, 65,536 I/O ports can be specified.
 
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