Changing the number of bits for a value

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Korie Wagner 2020-8-6

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https://ww2.mathworks.cn/matlabcentral/answers/576286-changing-the-number-of-bits-for-a-value

评论： Fangjun Jiang 2020-8-11

Is there a way to put information into a specified number of bits? Specifically, the output of my code will be two separate values, one needs to be a 27-bit integer and the other needs to be a 12-bit integer. They are both currently doubles. Does matlab support dataypes with unconventional numbers of bits?

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Answer 1

Fangjun Jiang 2020-8-6

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https://ww2.mathworks.cn/matlabcentral/answers/576286-changing-the-number-of-bits-for-a-value#answer_476248

help fixdt

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Korie Wagner 2020-8-11

I am still confused about how to utilize this function. If I were to save the variable "a = 5" and I wanted it to be a 27-bit integer, would this function work to update the datatype and how exactly would I implement this change?

Fangjun Jiang 2020-8-11

I thought of Simulink without hesitation. For MATLAB, see Walter's answer. For Simulink

a=Simulink.Parameter;
a.Value=5;
a.DataType='fixdt(0,27,0)';

Then, drag a Constant block and specify the value as "a". Show "Port Data Types". It will show data type as ufix27.

Or, define your own data type and apply it everywhere

MyUint27=fixdt(0,27,0);
a.DataType='MyUint27';

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Answer 2

Walter Roberson 2020-8-11

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https://ww2.mathworks.cn/matlabcentral/answers/576286-changing-the-number-of-bits-for-a-value#answer_478299

https://www.mathworks.com/help/fixedpoint/ref/embedded.fi.html#mw_97a6439f-a398-4f2a-8767-d7ddeae8d7a9

You can create a numerictype() object that is a description of a fixedpoint datatype. When you need to convert a non-fixedpoint number use fi() passing in the value and the numerictype template.

Once a value is converted to fixedpoint then you can use arithmetic operations on it at need.

Note that individual fixedpoint objects will need the next major power of 2 bits to store in memory in MATLAB. A 27 bit number will need 32 bits to store plus have additional overhead due to having to go through class methods at the MATLAB level.

There are two major reasons to use fixedpoint objects at the MATLAB level:

you need to interface to something external that has that representation; or
you are creating code to be deployed to DSP or FPGA, where you might not have a floating point core and where you might literally be representing a value by that many bits at the hardware level. No point in using a larger die to hold additional gates that are not going to be used for the application.