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Float is a 32-bit data type representing the single precision floating-point format, in IEEE 754-1985 called single, in IEEE 754-2008 the 32-bit base 2 format is officially referred to as binary32. Due to normalization the true significand includes an implicit leading one bit unless the exponent is stored with all bits zeros (0x00) or ones (0xff) which are reserved for Denormal numbers. Thus only 23 bits of the significand are stored but the total precision is 24 bits (≈7.225 decimal digits). Exponent bias is 0x7f.


Float example.svg

Single precision floating-point format

x86 Float Instruction Sets

Recent x86 and x86-64 processors provide x87, SSE and 3DNow! (AMD only, shared with MMX/x87) floating point instruction sets. 3DNow! and SSE are SIMD instructions with vectors of two or four floats. Since SSE is not obligatory for x86-32, 32-bit operating systems rely on x87. x86-64 64-bit operating systems may use the faster SSE instructions, but so far only 64-bit compiler for 64-bit Windows emit those instructions implicitly for floating point operations [1] . SSE instructions can be mixed with x87 or 3DNow! and are explicitly available through (inline) Assembly or intrinsics of various C-Compilers.

Integer to Float Conversion


To convert a signed or unsigned integer to float, two x87 instructions are needed, FILD and FSTP working on the x87 floating point stack [2] .

FILD The FILD instruction converts a signed-integer in memory to double-extended-precision (80-bit) format and pushes the value onto the x87 register stack. The value can be a 16-bit, 32-bit, or 64- bit integer value. Signed values from memory can always be represented exactly in x87 registers without rounding.

FSTP The FSTP instruction pops the x87 stack after copying the value. The instruction FSTP ST(0) is the same as popping the stack with no data transfer. If the specified destination is a single-precision or double-precision memory location, the instruction converts the value to the appropriate precision format. It does this by rounding the significand of the source value as specified by the rounding mode determined by the RC field of the x87 control word and then converting to the format of destination. It also converts the exponent to the width and bias of the destination format.


CVTDQ2PS Converts four packed signed doubleword integers in the source operand (second operand) to four packed single-precision floating-point values in the destination operand (first operand).

CVTPI2PS Converts two packed signed doubleword integers in the source operand (second operand) to two packed single-precision floating-point values in the destination operand (first operand).


PI2FD Converts packed 32-bit integer values to packed floating-point, single-precision values

  • Mnemonic: PI2FD mm1, mm2
  • Intrinsic: _m_pi2fd

BitScan Purpose

Integer to Float conversion can be used as base 2 logarithm of a power of two value of a 32-bit signed or unsigned integer, which might even base of a 64-bit bitscan [3] . The 23 lower significant bits are always zero, the exponent contains the biased bitindex:

i 2^i as hexstring tofloat as hexstring exponent - 127
0 0x00000001 0x3f800000 0
1 0x00000002 0x40000000 1
2 0x00000004 0x40800000 2
3 0x00000008 0x41000000 3
4 0x00000010 0x41800000 4
5 0x00000020 0x42000000 5
6 0x00000040 0x42800000 6
7 0x00000080 0x43000000 7
8 0x00000100 0x43800000 8
9 0x00000200 0x44000000 9
10 0x00000400 0x44800000 10
11 0x00000800 0x45000000 11
12 0x00001000 0x45800000 12
13 0x00002000 0x46000000 13
14 0x00004000 0x46800000 14
15 0x00008000 0x47000000 15
16 0x00010000 0x47800000 16
17 0x00020000 0x48000000 17
18 0x00040000 0x48800000 18
19 0x00080000 0x49000000 19
20 0x00100000 0x49800000 20
21 0x00200000 0x4a000000 21
22 0x00400000 0x4a800000 22
23 0x00800000 0x4b000000 23
24 0x01000000 0x4b800000 24
25 0x02000000 0x4c000000 25
26 0x04000000 0x4c800000 26
27 0x08000000 0x4d000000 27
28 0x10000000 0x4d800000 28
29 0x20000000 0x4e000000 29
30 0x40000000 0x4e800000 30
31 0x80000000 0x4f000000 31
31 0x80000000 0xcf000000 31

See also


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