雙精度浮點數

雙精度浮點數英語:)是计算机使用的一種資料型別。比起單精度浮點數僅有 32 位元(4字节),雙精度浮點數使用 64 位元(8字节) 來儲存一個浮點數[1]。 它可以表示二進位制的53有效數字,其可以表示的数字的绝对值范围为

格式

  • (符號):用來表示正負號
  • (指數):用來表示次方數
  • (尾數):用來表示精確度

符号

0代表數值為正,1代表數值為負。

指數

共有11個位元 , 使用「偏移表示法」, 有2個例外分別為

  1. 「11個位元皆為0」
  2. 「11個位元皆為1」

並且以1023為偏移標準,表示實際指數為0,因此指數範圍為 -1022 到 +1023:

指數 000167ff16 具有特殊意義:

000000000002 = 00016當尾數為0時為±0,尾數不為0時為非正規形式的浮點數

111111111112 = 7ff16當尾數為0時為∞,尾數不為0時為NaN

尾數

二進位的「科學記號」,數字被表示為:

二進位的「科學記號」(a×2n)的a的範圍是大於等於1而小於2,例如:

  • 二進位制的 可以規格化為 ,儲存時尾数只需要儲存1101即可。
  • 二進位制的 可以規格化為 ,儲存時尾數只需要儲存10011即可。

小結

根據以上的敘述,一個雙精度浮點數所代表的數值為:

例子

0 01111111111 00000000000000000000000000000000000000000000000000002 ≙ 3FF0 0000 0000 000016 ≙ +20 × 1 = 1
0 01111111111 00000000000000000000000000000000000000000000000000012 ≙ 3FF0 0000 0000 000116 ≙ +20 × (1 + 2−52) ≈ 1.0000000000000002, the smallest number > 1
0 01111111111 00000000000000000000000000000000000000000000000000102 ≙ 3FF0 0000 0000 000216 ≙ +20 × (1 + 2−51) ≈ 1.0000000000000004
0 10000000000 00000000000000000000000000000000000000000000000000002 ≙ 4000 0000 0000 000016 ≙ +21 × 1 = 2
1 10000000000 00000000000000000000000000000000000000000000000000002 ≙ C000 0000 0000 000016 ≙ −21 × 1 = −2
0 10000000000 10000000000000000000000000000000000000000000000000002 ≙ 4008 0000 0000 000016 ≙ +21 × 1.12 = 112 = 3
0 10000000001 00000000000000000000000000000000000000000000000000002 ≙ 4010 0000 0000 000016 ≙ +22 × 1 = 1002 = 4
0 10000000001 01000000000000000000000000000000000000000000000000002 ≙ 4014 0000 0000 000016 ≙ +22 × 1.012 = 1012 = 5
0 10000000001 10000000000000000000000000000000000000000000000000002 ≙ 4018 0000 0000 000016 ≙ +22 × 1.12 = 1102 = 6
0 10000000011 01110000000000000000000000000000000000000000000000002 ≙ 4037 0000 0000 000016 ≙ +24 × 1.01112 = 101112 = 23
0 01111111000 10000000000000000000000000000000000000000000000000002 ≙ 3F88 0000 0000 000016 ≙ +2−7 × 1.12 = 0.000000112 = 0.01171875 (3/256)
0 00000000000 00000000000000000000000000000000000000000000000000012 ≙ 0000 0000 0000 000116 ≙ +2−1022 × 2−52 = 2−1074
≈ 4.9406564584124654 × 10−324 (Min. subnormal positive double)
0 00000000000 11111111111111111111111111111111111111111111111111112 ≙ 000F FFFF FFFF FFFF16 ≙ +2−1022 × (1 − 2−52)
≈ 2.2250738585072009 × 10−308 (Max. subnormal double)
0 00000000001 00000000000000000000000000000000000000000000000000002 ≙ 0010 0000 0000 000016 ≙ +2−1022 × 1
≈ 2.2250738585072014 × 10−308 (Min. normal positive double)
0 11111111110 11111111111111111111111111111111111111111111111111112 ≙ 7FEF FFFF FFFF FFFF16 ≙ +21023 × (1 + (1 − 2−52))
≈ 1.7976931348623157 × 10308 (Max. Double)
0 00000000000 00000000000000000000000000000000000000000000000000002 ≙ 0000 0000 0000 000016 ≙ +0
1 00000000000 00000000000000000000000000000000000000000000000000002 ≙ 8000 0000 0000 000016 ≙ −0
0 11111111111 00000000000000000000000000000000000000000000000000002 ≙ 7FF0 0000 0000 000016 ≙ +∞ (positive infinity)
1 11111111111 00000000000000000000000000000000000000000000000000002 ≙ FFF0 0000 0000 000016 ≙ −∞ (negative infinity)
0 11111111111 00000000000000000000000000000000000000000000000000012 ≙ 7FF0 0000 0000 000116 ≙ NaN (sNaN on most processors, such as x86 and ARM)
0 11111111111 10000000000000000000000000000000000000000000000000012 ≙ 7FF8 0000 0000 000116 ≙ NaN (qNaN on most processors, such as x86 and ARM)
0 11111111111 11111111111111111111111111111111111111111111111111112 ≙ 7FFF FFFF FFFF FFFF16 ≙ NaN (an alternative encoding of NaN)
0 01111111101 01010101010101010101010101010101010101010101010101012
= 3fd5 5555 5555 555516 ≙ +2−2 × (1 + 2−2 + 2−4 + ... + 2−52)
1/3
0 10000000000 10010010000111111011010101000100010000101101000110002
= 4009 21fb 5444 2d1816 ≈ pi

参考文献

  1. Stanley B. Lippman, Josée Lajoie, Barbara E. Moo. . 碁峰資訊. 2020: 第33頁. ISBN 978-986-502-172-6.

參閱

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