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Binary representation of stored integer of fi object

Asked by Charles Jackel on 18 Mar 2013

I am using a lookup table with values ranging from 0 to pi/4.

These values are all less than one, i.e. 0 to .7854.

I am using the bin function to get the eight MSBs of a 16 bit number.

I noticed that bin(fi(.5000,0,16)) = 1000 0000 0000 0000 but bin(fi(.4555, 0, 16)) = 1110 1001 0011 0111

with bin(fi(.2500,0,16)) = 1000 0000 0000 0000.

Why do .5 and .25 have the same binary representation? Why does .4555 have a larger sized binary representation?

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Charles Jackel

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2 Answers

Answer by Titus Edelhofer on 18 Mar 2013
Accepted answer

Hi,

if you only specify the word length and not the fraction length you get two different objects:

>> fi(0.25, 0, 16)
ans = 
  0.2500
          DataTypeMode: Fixed-point: binary point scaling
            Signedness: Unsigned
            WordLength: 16
        FractionLength: 17
>> fi(0.5, 0, 16)
ans = 
    0.5000
          DataTypeMode: Fixed-point: binary point scaling
            Signedness: Unsigned
            WordLength: 16
        FractionLength: 16

Note the difference on the FractionLength.

Now if you want to have numbers between 0 and 1, so FractionLength 16, you get what I think you are looking for:

>> bin(fi(0.5, 0, 16, 16))
ans =
1000000000000000
>> bin(fi(0.25, 0, 16, 16))
ans =
0100000000000000

Titus

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Titus Edelhofer
Answer by Charles Jackel on 18 Mar 2013

Thank you! That helped a lot!

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Charles Jackel

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