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It depends whether You can or cannot use floating point operations.
If You can do that, simply multiply the floating point value by 2^(number_of_fractional_bits) and convert the result to integer. In C there's a libm function ldexp() as far as I remember, which provides fast multiplication of floating point numbers by powers of two. It works by manipulating number's exponent value directly.
If You want to do this without messing with floating point math, just extract the mantisa, put '1' bit on the left, shift the number left or right according to it's exponent, and finaly multiply it with 1 or -1 depending on sign bit.
To be more precise, if Your mantisa length is M bits, e.g 23 for single precision ieee floating point numbers (24 bits with 'hidden 1', for double precision - ask google, and You have F fractional bits in Your fixed point numbers, then You need to shift the mantisa (with '1' as MSb included) (M - F) bits to the right, if (M-F) is negative, then You need to shift it to the left.
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