per wikipedia "The stored exponents 00 and FF are interpreted specially." and the formula to use is (−1)^(signbits)×2^(−126)×0.(significandbits)
805c00f0 =>
8 0 5 c 0 0 f 0 =>
1000 0000 0101 1100 0000 0000 1111 0000 =>
1 00000000 101 1100 0000 0000 1111 0000 =>
-1 x 2^(-126) x 0.10111000000000011110000 =>
-1 x 2^(-126) x (2^-1 + 2^-3 + 2^-4 + 2^-5 + 2^-16 + 2^-17 + 2^-18 + 2^-19) =>
-1 * (2^-127 + 2^-129 + 2^-130 + 2^-131 + 2^-142 + 2^-143 + 2^-144 + 2^-145) =>
-8.449202e-39