Decimal64 floating-point format

Floating-point precisions
IEEE 754: 16-bit: Half (binary16) 32-bit: Single (binary32), decimal32 64-bit: Double (binary64), decimal64 128-bit: Quadruple (binary128), decimal128 Other: Minifloat · Extended precision Arbitrary precision

In computing, decimal64 is a decimal floating-point computer numbering format that occupies 8 bytes (64 bits) in computer memory. It is intended for applications where it is necessary to emulate decimal rounding exactly, such as financial and tax computations.

Decimal64 supports 16 decimal digits of significand and an exponent range of −383 to +384, i.e. ±0.000000000000000×10^⁻³⁸³ to ±9.999999999999999×10^³⁸⁴. (Equivalently, ±0000000000000000×10^⁻³⁹⁸ to ±9999999999999999×10^³⁶⁹.) Because the significand is not normalized, most values with less than 16 significant digits have multiple possible representations; 1×10²=0.1×10³=0.01×10⁴, etc. Zero has 768 possible representations (1536 if you include both signed zeros).

Decimal64 floating point is a relatively new decimal floating-point format, formally introduced in the 2008 version of IEEE 754.

Representation of decimal64 values

IEEE 754 allows two alternative representation methods for decimal64 values. The standard does not specify how to signify which representation is used, for instance in a situation where decimal64 values are communicated between systems.

In one representation method, based on binary integer decimal, the significand is represented as binary coded positive integer.

The other, alternative, representation method is based on densely packed decimal for most of the significand (except the most significant digit).

Both alternatives provide exactly the same range of representable numbers: 16 digits of significand and 3×2⁸ = 768 possible exponent values.

In both cases, the most significant 4 bits of the significand (which actually only have 10 possible values) are combined with the most significant 2 bits of the exponent (3 possible values) to use 30 of the 32 possible values of a 5-bit field. The remaining combinations encode infinities and NaNs.

If the leading 4 bits of the significand is between 0 and 7, the number begins as follows

s 00 xxxx   Exponent begins with 00, significand with 0mmm
s 01 xxxx   Exponent begins with 01, significand with 0mmm
s 10 xxxx   Exponent begins with 10, significand with 0mmm

If the leading 4 bits of the significand are binary 1000 or 1001 (decimal 8 or 9), the number begins as follows:

s 1100 xx   Exponent begins with 00, significand with 100m
s 1101 xx   Exponent begins with 01, significand with 100m
s 1110 xx   Exponent begins with 10, significand with 100m

The following bits (xxx in the above) encode the additional exponent bits and the remainder of the most significant digit, but the details vary depending on the encoding alternative used. There is no particular reason for this difference, other than historical reasons in the eight-year long development of IEEE 754-2008.

The final combinations are used for infinities and NaNs, and are the same for both alternative encodings:

s 11110 x  ±Infinity (see Extended real number line)
s 111110   quiet NaN (sign bit ignored)
s 111111   signaling NaN (sign bit ignored)

In the latter cases, all other bits of the encoding are ignored. Thus, it is possible to initialize an array to NaNs by filling it with a single byte value.

Binary integer significand field

This format uses a binary significand from 0 to 10¹⁶−1 = 9999999999999999 = 2386F26FC0FFFF₁₆ = 100011100001101111001001101111110000001111111111111111₂. The encoding can represent binary significands up to 10×2⁵⁰−1 = 11258999068426239 = 27FFFFFFFFFFFF₁₆, but values larger than 10¹⁶−1 are illegal (and the standard requires implementations to treat them as 0, if encountered on input).

As described above, the encoding varies depending on whether the most significant 4 bits of the significand are in the range 0 to 7 (0000₂ to 0111₂), or higher (1000₂ or 1001₂).

If the 2 bits after the sign bit are "00", "01", or "10", then the exponent field consists of the 10 bits following the sign bit, and the significand is the remaining 53 bits, with an implicit leading 0 bit:

s 00eeeeeeee (0)TTTtttttttttttttttttt tttttttttttttttttttttttttttttttt
s 01eeeeeeee (0)TTTtttttttttttttttttt tttttttttttttttttttttttttttttttt
s 10eeeeeeee (0)TTTtttttttttttttttttt tttttttttttttttttttttttttttttttt

This includes subnormal numbers where the leading significand digit is 0.

If the 4 bits after the sign bit are "1100", "1101", or "1110", then the 10-bit exponent field is shifted 2 bits to the right (after both the sign bit and the "11" bits thereafter), and the represented significand is in the remaining 51 bits. In this case there is an implicit (that is, not stored) leading 3-bit sequence "100" in the true significand.

s 11 00eeeeeeee (100)Ttttttttttttttttttt tttttttttttttttttttttttttttttttt
s 11 01eeeeeeee (100)Ttttttttttttttttttt tttttttttttttttttttttttttttttttt
s 11 10eeeeeeee (100)Ttttttttttttttttttt tttttttttttttttttttttttttttttttt

The "11" 2-bit sequence after the sign bit indicates that there is an implicit "100" 3-bit prefix to the significand. Compare having an implicit 1 in the significand of normal values for the binary formats. Note also that the "00", "01", or "10" bits are part of the exponent field.

Note that the leading bits of the significand field do not encode the most significant decimal digit; they are simply part of a larger pure-binary number. For example, a significand of 8000000000000000 is encoded as binary 11100011010111111010100100110001101000000000000000000, with the leading 4 bits encoding 7; the first significand which requires a 54th bit is 2⁵³ = 9007199254740992.

In the above cases, the value represented is

(−1)^sign × 10^{exponent−398} × significand

If the four bits after the sign bit are "1111" then the value is an infinity or a NaN, as described above:

s 11110 xx...x    ±infinity
s 111110 x...x    a quiet NaN
s 111111 x...x    a signalling NaN

Densely packed decimal significand field

In this version, the significand is stored as a series of decimal digits. The leading digit is between 0 and 9 (3 or 4 binary bits), and the rest of the significand uses the densely packed decimal encoding.

Unlike the binary integer significand version, where the exponent changed position and came before the significand, this encoding combines the leading 2 bits of the exponent and the leading digit (3 or 4 bits) of the significand into the five bits that follow the sign bit.

This eight bits after that are the exponent continuation field, providing the less-significant bits of the exponent.

The last 50 bits are the significand continuation field, consisting of 5 10-bit "declets". Each declet encodes three decimal digits using the DPD encoding.

If the first two bits after the sign bit are "00", "01", or "10", then those are the leading bits of the exponent, and the three bits after that are interpreted as the leading decimal digit (0 to 7):

s 00 TTT (00)eeeeeeee (TTT)[tttttttttt][tttttttttt][tttttttttt][tttttttttt][tttttttttt]
s 01 TTT (01)eeeeeeee (TTT)[tttttttttt][tttttttttt][tttttttttt][tttttttttt][tttttttttt]
s 10 TTT (10)eeeeeeee (TTT)[tttttttttt][tttttttttt][tttttttttt][tttttttttt][tttttttttt]

If the 4 bits after the sign bit are "1100", "1101", or "1110", then the second two bits are the leading bits of the exponent, and the last bit is prefixed with "100" to form the leading decimal digit (8 or 9):

s 1100 T (00)eeeeeeee (100T)[tttttttttt][tttttttttt][tttttttttt][tttttttttt][tttttttttt]
s 1101 T (01)eeeeeeee (100T)[tttttttttt][tttttttttt][tttttttttt][tttttttttt][tttttttttt]
s 1110 T (10)eeeeeeee (100T)[tttttttttt][tttttttttt][tttttttttt][tttttttttt][tttttttttt]

The remaining two combinations (11110 and 11111) of the 5-bit field are used to represent ±infinity and NaNs, respectively.

The DPD/3BCD transcoding for the declets is given by the following table. b9...b0 are the bits of the DPD, and d2...d0 are the three BCD digits.

Densely packed decimal encoding rules^[1]

b9	b8	b7	b6	b5	b4	b3	b2	b1	b0	d2	d1	d0	Values encoded	Digit pattern
a	b	c	d	e	f	0	g	h	i	0abc	0def	0ghi	(0–7) (0–7) (0–7)	Three small digits
a	b	c	d	e	f	1	0	0	i	0abc	0def	100i	(0–7) (0–7) (8–9)	Two small digits, one large
a	b	c	d	e	f	1	0	1	i	0abc	100f	0dei	(0–7) (8–9) (0–7)
a	b	c	d	e	f	1	1	0	i	100c	0def	0abi	(8–9) (0–7) (0–7)
a	b	c	1	0	f	1	1	1	i	0abc	100f	100i	(0–7) (8–9) (8–9)	One small digit, two large
a	b	c	0	1	f	1	1	1	i	100c	0abf	100i	(8–9) (0–7) (8–9)
a	b	c	0	0	f	1	1	1	i	100c	100f	0abi	(8–9) (8–9) (0–7)
x	x	c	1	1	f	1	1	1	i	100c	100f	100i	(8–9) (8–9) (8–9)	Three large digits

The 8 decimal values whose digits are all 8s or 9s have four codings each. The bits marked x in the table above are ignored on input, but will always be 0 in computed results. (The 8×3 = 24 non-standard encodings fill in the gap between 10³=1000 and 2¹⁰=1024.)

In the above cases, with the true significand as the sequence of decimal digits decoded, the value represented is

$(-1)^\text{signbit}\times 10^{\text{exponentbits}_2-398_{10}}\times \text{truesignificand}_{10}$

See also

• IEEE Standard for Floating-Point Arithmetic (IEEE 754)
• ISO/IEC 10967, Language Independent Arithmetic
• Primitive data type

References

1. ^ Cowlishaw, M. F. (2000-10-03). "Summary of Densely Packed Decimal encoding". http://speleotrove.com/decimal/DPDecimal.html. Retrieved 2008-09-10.