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Real floating-point type, usually referred to as a double-precision floating-point type. Actual properties unspecified (except minimum limits); however, on most systems, this is the IEEE 754 double-precision binary floating-point format (64 bits). This format is required by the optional Annex F "IEC 60559 floating-point arithmetic".
The representation has a limited precision. For example, only 15 decimal digits can be represented with a 64-bit real. If a very small floating-point number is added to a large one, the result is just the large one. The small number was too small to even show up in 15 or 16 digits of resolution, and the computer effectively discards it.
For example, a packed decimal value encoded with the bytes 12 34 56 7C represents the fixed-point value +1,234.567 when the implied decimal point is located between the fourth and fifth digits: 12 34 56 7C 12 34.56 7+ The decimal point is not actually stored in memory, as the packed BCD storage format does not provide for it.
If a decimal string with at most 6 significant digits is converted to the IEEE 754 single-precision format, giving a normal number, and then converted back to a decimal string with the same number of digits, the final result should match the original string. If an IEEE 754 single-precision number is converted to a decimal string with at least 9 ...
The value encoded is (−1) s ×10 q ×c. In both formats the range of possible values is identical, but they differ in how the significand c is represented. In the decimal encoding, it is encoded as a series of p decimal digits (using the densely packed decimal (DPD) encoding).
Floating-point constants may be written in decimal notation, e.g. 1.23. Decimal scientific notation may be used by adding e or E followed by a decimal exponent, also known as E notation, e.g. 1.23e2 (which has the value 1.23 × 10 2 = 123.0). Either a decimal point or an exponent is required (otherwise, the number is parsed as an integer constant).
The full decimal significand is then obtained by concatenating the leading and trailing decimal digits. The 10-bit DPD to 3-digit BCD transcoding for the declets is given by the following table. b 9 … b 0 are the bits of the DPD, and d 2 … d 0 are the three BCD digits.
BCD numbers can be represented in two ways in integer registers: packed decimal and unpacked decimal. Packed (4 bits) In packed decimal representation a decimal digit is stored in one nibble. The values 10 to 15 are not used. [2] Unpacked (8 bits) In unpacked decimal representation a decimal digit is stored in one byte. The values 10 to 255 are ...