Issue when converting between multiple data types

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Basically you are trying to do this:

 LHS(msb + lsb downto 0) <= RHS(msb + lsb downto lsb) ;

which doesn't work as the LHS is wider than the RHS.

Using for-loop won't work either as a for-loop needs constant boundaries as well.

You could use a barrel shifter to dynamically align data.

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It would work if you loop through the whole of the output and see if the current bit should be assigned a specific bit from the bit_field.