akr	2012-02-23 19:52:51 +0900 (Thu, 23 Feb 2012)

  New Revision: 34762

  http://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=rev&revision=34762

  Log:
    update comment.

  Modified files:
    trunk/bignum.c

Index: bignum.c
===================================================================
--- bignum.c	(revision 34761)
+++ bignum.c	(revision 34762)
@@ -349,6 +349,14 @@
  * for each 0 <= i < num_longs.
  * So buf is little endian at whole on a little endian machine.
  * But buf is mixed endian on a big endian machine.
+ *
+ * The buf represents negative integers as two's complement.
+ * So, the most significant bit of the most significant word,
+ * (buf[num_longs-1]>>(SIZEOF_LONG*CHAR_BIT-1)),
+ * is the sign bit: 1 means negative and 0 means zero or positive.
+ *
+ * If given size of buf (num_longs) is not enough to represent val,
+ * higier words (including a sign bit) are ignored.
  */
 void
 rb_big_pack(VALUE val, unsigned long *buf, long num_longs)
@@ -391,7 +399,7 @@
     }
 }
 
-/* See rb_big_pack comment for endianness of buf. */
+/* See rb_big_pack comment for endianness and sign of buf. */
 VALUE
 rb_big_unpack(unsigned long *buf, long num_longs)
 {

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