Comments by "Mikko Rantalainen" (@MikkoRantalainen) on "Prime React: Fast Inverse Square Root — A Quake III Algorithm" video.

  1. If I remember correctly floating point operation was a single FPU instruction but it took A LOT of CPU clocks to complete. Quake (the first version) did one FPU division operation at the same time as CPU was doing 16 multiplications and 16 additions. 486 was way slower to compute the floating point division which is why it was really slow to render Quake graphics. Square root is even slower operation than division in FPU. One could argue that original Quake was running on two cores on a single core CPU (the CPU and FPU could compute things at parallel on Pentium CPU). The fast square root algorithm does division + square root using one bit bitshift operation, three multiplications and do additions (identical to substraction with the sign reversed) so it was about 5 times faster than a single division on FPU, never mind the square root operation.
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