Single precision
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In computing, single precision is a computer numbering format that occupies one storage location in computer memory at a given address. A single-precision number, sometimes simply a single, may be defined to be an integer, fixed point, or floating point.
Modern computers with 32-bit words (single precision) provide 64-bit double precision. Single precision floating point is an IEEE 754 standard for encoding floating point numbers that uses 4 bytes.
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[edit] Single precision memory format
Sign bit: 1 Exponent width: 8 Significand precision: 23 (24 implicit)
The format is written with an implicit most-significant bit with value 1 unless the written exponent is all zeros. Thus only 23 bits of the fraction mantissa appear in the memory format but the total precision is 24 bits (better than 7 decimal digits, log10(224)).
[edit] Exponent encodings
Emin (0x01) = -126 Emax (0xfe) = 127 Exponent bias (0x7f) = 127
The true exponent = written exponent - exponent bias
0x00 and 0xff are reserved exponents 0x00 is used to represent zero and denormals 0xff is used to represent infinity and NaNs
All bit patterns are valid encodings.
[edit] Single precision examples in hexadecimal
3f80 0000 = 1
c000 0000 = -2
7f7f ffff ~ 3.4028234 x 1038 (Max Single)
3eaa aaab ~ 1/3
By default, 1/3 rounds up instead of down like double precision, because of the even number of bits in the significand. So the bits beyond the rounding point are 1010... which is more than 1/2 of a unit in the last place.
0000 0000 = 0 8000 0000 = -0
7f80 0000 = Infinity ff80 0000 = -Infinity
[edit] Converting from single precision to human readable form
We start with the hexadecimal representation of the value, 41c80000, in this example, and convert it to binary
41c8 000016 = 0100 0001 1100 1000 0000 0000 0000 00002
then we break it down into three parts; sign bit, exponent and mantissa.
Sign bit: 0 Exponent: 1000 00112 = 8316 = 131 Mantissa: 100 1000 0000 0000 0000 00002 = 48000016
We then add the implicit 24th bit to the mantissa
Mantissa: 1100 1000 0000 0000 0000 00002 = C8000016
and decode the exponent value by subtracting 127
Raw exponent: 8316 = 131 Decoded exponent: 131 - 127 = 4
Each of the 24 bits of the mantissa, bit 23 to bit 0, represents a value, starting at 1 and halves for each bit, as follows
bit 23 = 1 bit 22 = 0.5 bit 21 = 0.25 bit 20 = 0.125 bit 19 = 0.0625 . .
The mantissa in this example has three bits set, bit 23, bit 22 and bit 19. We can now decode the mantissa by adding the values represented by these bits.
Decoded mantissa: 1 + 0.5 + 0.0625 = 1.5625
Then we need to multiply with the base, 2, to the power of the exponent to get the final result
1.5625 * 24 = 25
Thus
41c8 0000 = 25
[edit] See also
- half precision – single precision – double precision – quadruple precision
- Floating point
