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Almost all Numerical Computation arithmetic is performed using
IEEE 754-1985 Standard for Binary Floating-Point Arithmetic.
The two formats that we deal with in practice are the 32 bit and
64 bit formats. You need to know how to get the format you desire
in the language you are programming. Complex numbers use two values.
older
C Java Fortran 95 Fortran Ada 95 MATLAB
------ ------ ---------------- ------- ---------- -------
32 bit float float real real float N/A
64 bit double double double precision real*8 long_float 'default'
complex
32 bit 'none' 'none' complex complex complex N/A
64 bit 'none' 'none' double complex complex*16 long_complex 'default'
'none' means not provided by the language (may be available as a library)
N/A means not available, you get the default.
IEEE Floating-Point numbers are stored as follows:
The single format 32 bit has
1 bit for sign, 8 bits for exponent, 23 bits for fraction
The double format 64 bit has
1 bit for sign, 11 bits for exponent, 52 bits for fraction
There is actually a '1' in the 24th and 53rd bit to the left
of the fraction that is not stored. The fraction including
the non stored bit is called a significand.
Floating point numbers are sign magnitude. Invert the sign bit to negate.
Some example numbers and their bit patterns:
decimal
stored hexadecimal sign exponent fraction significand
The "1" is not stored |
|
31 30....23 22....................0 |
1.0
3F 80 00 00 0 01111111 00000000000000000000000 1.0 * 2^(127-127)
0.5
3F 00 00 00 0 01111110 00000000000000000000000 1.0 * 2^(126-127)
0.75
3F 40 00 00 0 01111110 10000000000000000000000 1.1 * 2^(126-127)
0.9999995
3F 7F FF FF 0 01111110 11111111111111111111111 1.1111* 2^(126-127)
0.1
3D CC CC CD 0 01111011 10011001100110011001101 1.1001* 2^(123-127)
63 62...... 52 51 ..... 0
1.0
3F F0 00 00 00 00 00 00 0 01111111111 000 ... 000 1.0 * 2^(1023-1023)
0.5
3F E0 00 00 00 00 00 00 0 01111111110 000 ... 000 1.0 * 2^(1022-1023)
0.75
3F E8 00 00 00 00 00 00 0 01111111110 100 ... 000 1.1 * 2^(1022-1023)
0.9999999999999995
3F EF FF FF FF FF FF FF 0 01111111110 111 ... 1.11111* 2^(1022-1023)
0.1
3F B9 99 99 99 99 99 9A 0 01111111011 10011..1010 1.10011* 2^(1019-1023)
|
sign exponent fraction |
before storing subtract bias
Note that an integer in the range 0 to 2^23 -1 may be represented exactly.
Any power of two in the range -126 to +127 times such an integer may also
be represented exactly. Numbers such as 0.1, 0.3, 1.0/5.0, 1.0/9.0 are
represented approximately. 0.75 is 3/4 which is exact.
Some languages are careful to represent approximated numbers
accurate to plus or minus the least significant bit.
Other languages may be less accurate.
/* flt.c just to look at .o file with hdump */
void flt() /* look at IEEE floating point */
{
float x1 = 1.0f;
float x2 = 0.5f;
float x3 = 0.75f;
float x4 = 0.99999f;
float x5 = 0.1f;
double d1 = 1.0;
double d2 = 0.5;
double d3 = 0.75;
double d4 = 0.99999999; The "1" not stored
double d5 = 0.1; |
} |
31 30....23 22....................0 |
3F 80 00 00 0 01111111 00000000000000000000000 1.0 * 2^(127-127)
3F 00 00 00 0 01111110 00000000000000000000000 1.0 * 2^(126-127)
3F 40 00 00 0 01111110 10000000000000000000000 1.1 * 2^(126-127)
3F 7F FF 58 0 01111110 11111111111111101011000 1.1111* 2^(126-127)
3D CC CC CD 0 01111011 10011001100110011001101 1.1001* 2^(123-127)
63 62...... 52 51 ..... 0
3F F0 00 00 00 00 00 00 0 01111111111 000 ... 000 1.0 * 2^(1023-1023)
3F E0 00 00 00 00 00 00 0 01111111110 000 ... 000 1.0 * 2^(1022-1023)
3F E8 00 00 00 00 00 00 0 01111111110 100 ... 000 1.1 * 2^(1022-1023)
3F EF FF FF FA A1 9C 47 0 01111111110 111 ... 1.11111* 2^(1022-1023)
3F B9 99 99 99 99 99 9A 0 01111111011 1001 ..1010 1.10011* 2^(1019-1023)
|
sign exponent fraction |
subtract bias
decimal binary fraction / decimal exponent IEEE normalize
A = 3.5 * 10^6 a = 11.1 * 2^6 1.11 * 2^7
B = 2.5 * 10^5 b = 10.1 * 2^5 1.01 * 2^6
A+B 3.50 * 10^6 a+b 11.10 * 2^6 1.110 * 2^7
+ 0.25 * 10^6 + 1.01 * 2^6 + 0.101 * 2^7
_____________ ______________ ------------
3.75 * 10^6 100.11 * 2^6 10.011 * 2^7
normalize 1.0011 * 2^8
A-B 3.50 * 10^6
- 0.25 * 10^6
-------------
3.25 * 10^6
A*B 3.50 * 10^6
* 2.5 * 10^5
-------------
8.75 * 10^11
A/B 3.5 *10^6 / 2.5 *10^5 = 1.4 * 10^1
The mathematical basis for floating point is simple algebra
The common uses are in computer arithmetic and scientific notation
given: a number x1 expressed as 10^e1 * f1
then 10 is the base, e1 is the exponent and f1 is the fraction
example x1 = 10^3 * .1234 means x1 = 123.4 or .1234*10^3
or in computer notation 0.1234E3
In computers the base is chosen to be 2, i.e. binary notation
for x1 = 2^e1 * f1 where e1=3 and f1 = .1011
then x1 = 101.1 base 2 or, converting to decimal x1 = 5.5 base 10
Computers store the sign bit, 1=negative, the exponent and the
fraction in a floating point word that may be 32 or 64 bits.
The operations of add, subtract, multiple and divide are defined as:
Given x1 = 2^e1 * f1
x2 = 2^e2 * f2 and e2 <= e1
x1 + x2 = 2^e1 *(f1 + 2^-(e1-e2) * f2) f2 is shifted then added to f1
x1 - x2 = 2^e1 *(f1 - 2^-(e1-e2) * f2) f2 is shifted then subtracted from f1
x1 * x2 = 2^(e1+e2) * f1 * f2
x1 / x2 = 2^(e1-e2) * (f1 / f2)
an additional operation is usually needed, normalization.
if the resulting "fraction" has digits to the left of the binary
point, then the fraction is shifted right and one is added to
the exponent for each bit shifted until the result is a fraction.
if the resulting "fraction" has zeros immediately to the right of
the binary point, then the fraction is shifted left and one is
subtracted from the exponent for each bit shifted until there
is a non zero digit to the right of the binary point.
Numeric examples: (exponents are decimal integers, fractions are decimal)
(normalized numbers have 1.0 > fraction >= 0.5)
x1 = 2^4 * 0.5 or x1 = 8.0
x2 = 2^2 * 0.5 or x2 = 2.0
x1 + x2 = 2^4 * (.5 + 2^-(4-2) * .5) = 2^4 * (.5 + .125) = 2^4 * .625
x1 - x2 = 2^4 * (.5 - 2^-(4-2) * .5) = 2^4 * (.5 - .125) = 2^4 * .375
not normalized, multiply fraction by 2, subtract 1 from exponent
= 2^3 * .75
x1 * x2 = 2^(4+2) * (.5*.5) = 2^6 * .25 not normalized
= 2^5 * .5 normalized
x1 / x2 = 2^(4-2) * (.5/.5) = 2^2 * 1.0 not normalized
= 2^3 * .5 normalized
IEEE 754 Floating Point Standard
A few minor problems, e.g. the square root of all complex numbers
are in the right half of the complex plane and thus the real
part of the square root should never be negative. As a concession
to early hardware, the standard define the sqrt(-0) to be -0
rather than +0. Several places the standard uses the word should.
If a standard is specifying something, the word shall is typically used.
Basic decisions and operations for floating point add and subtract:
The decisions indicated above could be used to design the control
component shown in the data path diagram below:
Some graphics boards have large computing capacity and
some are releasing the specs so programmers can use the
computing capacity.
nVidia example
Fortunately, CMSC 411 does not require VHDL for floating point,
just the ability to manually do floating point add, subtract,
multiply and divide. (Examples in class on board.)
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