What is 256 negative power 1 over 2?

Assumption:  You are asking for   256^-1 / 2

= (2^8)^-1 / 2^1

2^(8*-1) / 2^1

2^-8   *  2^-1

2^(-8+ -1)

2^-9   or  1/512  or 1/(2^9)  or 2^-9