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| 22 | <title>Bits Shifted In (GNU C Language Manual)</title>
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Shift-Operations.html" rel="up" title="Shift Operations">
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| 33 | <link href="Shift-Caveats.html" rel="next" title="Shift Caveats">
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| 57 | <body lang="en">
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| 58 | <span id="Bits-Shifted-In"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Shift-Caveats.html" accesskey="n" rel="next">Shift Caveats</a>, Up: <a href="Shift-Operations.html" accesskey="u" rel="up">Shift Operations</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Shifting-Makes-New-Bits"></span><h4 class="subsection">6.7.1 Shifting Makes New Bits</h4>
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| 64 |
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| 65 | <p>A shift operation shifts towards one end of the number and has to
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| 66 | generate new bits at the other end.
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| 67 | </p>
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| 68 | <p>Shifting left one bit must generate a new least significant bit. It
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| 69 | always brings in zero there. It is equivalent to multiplying by the
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| 70 | appropriate power of 2. For example,
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| 71 | </p>
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| 72 | <div class="example">
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| 73 | <pre class="example">5 << 3 <span class="roman">is equivalent to</span> 5 * 2*2*2
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| 74 | -10 << 4 <span class="roman">is equivalent to</span> -10 * 2*2*2*2
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| 75 | </pre></div>
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| 76 |
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| 77 | <p>The meaning of shifting right depends on whether the data type is
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| 78 | signed or unsigned (see <a href="Signed-and-Unsigned-Types.html">Signed and Unsigned Types</a>). For a signed
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| 79 | data type, it performs “arithmetic shift,” which keeps the number’s
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| 80 | sign unchanged by duplicating the sign bit. For an unsigned data
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| 81 | type, it performs “logical shift,” which always shifts in zeros at
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| 82 | the most significant bit.
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| 83 | </p>
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| 84 | <p>In both cases, shifting right one bit is division by two, rounding
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| 85 | towards negative infinity. For example,
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| 86 | </p>
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| 87 | <div class="example">
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| 88 | <pre class="example">(unsigned) 19 >> 2 ⇒ 4
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| 89 | (unsigned) 20 >> 2 ⇒ 5
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| 90 | (unsigned) 21 >> 2 ⇒ 5
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| 91 | </pre></div>
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| 92 |
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| 93 | <p>For negative left operand <code>a</code>, <code>a >> 1</code> is not equivalent to
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| 94 | <code>a / 2</code>. They both divide by 2, but ‘<samp>/</samp>’ rounds toward
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| 95 | zero.
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| 96 | </p>
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| 97 | <p>The shift count must be zero or greater. Shifting by a negative
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| 98 | number of bits gives machine-dependent results.
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| 99 | </p>
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| 100 |
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| 101 |
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| 102 |
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| 103 | </body>
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| 104 | </html>
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