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In computer programming, an arithmetic shift is a shift operator, sometimes known as a signed shift (though it is not restricted to signed operands). The two basic types are the arithmetic left shift and the arithmetic right shift. For binary numbers it is a bitwise operation that shifts all of the bits of its operand; every bit in the operand is simply moved a given number of bit positions, and the vacant bit-positions are filled in. Instead of being filled with all 0s, as in logical shift, when shifting to the right, the leftmost bit (usually the sign bit in signed integer representations) is replicated to fill in all the vacant positions (this is a kind of sign extension). Some authors prefer the terms "sticky right-shift" and "zero-fill right-shift".〔 Thomas R. Cain and Alan T. Sherman. ("How to break Gifford's cipher" ). Section 8.1: "Sticky versus Non-Sticky Bit-shifting". Cryptologia. 1997. 〕 Arithmetic shifts can be useful as efficient ways of performing multiplication or division of signed integers by powers of two. Shifting left by ''n'' bits on a signed or unsigned binary number has the effect of multiplying it by 2''n''. Shifting right by ''n'' bits on a two's complement ''signed'' binary number has the effect of dividing it by 2''n'', but it always rounds down (towards negative infinity). This is different from the way rounding is usually done in signed integer division (which rounds towards 0). This discrepancy has led to bugs in more than one compiler. For example, in the x86 instruction set, the SAR instruction (arithmetic right shift) divides a signed number by a power of two, rounding towards negative infinity. However, the IDIV instruction (signed divide) divides a signed number, rounding towards zero. So a SAR instruction cannot be substituted for an IDIV by power of two instruction nor vice versa. == Formal definition == The formal definition of an arithmetic shift, from Federal Standard 1037C is that it is: :A shift, applied to the representation of a number in a fixed radix numeration system and in a fixed-point representation system, and in which only the characters representing the fixed-point part of the number are moved. An arithmetic shift is usually equivalent to multiplying the number by a positive or a negative integral power of the radix, except for the effect of any rounding; compare the logical shift with the arithmetic shift, especially in the case of floating-point representation. An important word in the FS 1073C definition is "usually". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「arithmetic shift」の詳細全文を読む スポンサード リンク
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