Vitis HLS supports fixed-point types that allow fractional arithmetic to be easily handled. The advantage of fixed-point arithmetic is shown in the following example.
ap_fixed<11, 6> Var1 = 22.96875; // 11-bit signed word, 5 fractional bits ap_ufixed<12,11> Var2 = 512.5; // 12-bit word, 1 fractional bit ap_fixed<16,11> Res1; // 16-bit signed word, 5 fractional bits Res1 = Var1 + Var2; // Result is 535.46875
have different precisions, the fixed-point type ensures that the decimal
point is correctly aligned before the operation (an addition in this
case), is performed. You are not required to perform any operations in
the C code to align the decimal point.
The type used to store the result of any fixed-point arithmetic operation must be large enough (in both the integer and fractional bits) to store the full result.
If this is not the case, the
- overflow handling (when the result has more MSBs than the assigned type supports)
- quantization (or rounding, when the result has fewer LSBs than the assigned type supports)
ap_[u]fixed type provides various options on
how the overflow and quantization are performed. The options are discussed