%

/isqrt {
    /n exch def
    /k n -1 bitshift def
    /x false def
    {
        /k1 k n k idiv add -1 bitshift def
        k1 k ge {
            /x true def
        } if
        /k k1 def
        x { exit } if
    } loop
    k
} def

(test isqrt) =
10 isqrt 3 eq { (Pass) } { (Fail) } ifelse print ( ) print
27 isqrt 5 eq { (Pass) } { (Fail) } ifelse print ( ) print
85 isqrt 9 eq { (Pass) } { (Fail) } ifelse print ( ) print
101 isqrt 10 eq { (Pass) } { (Fail) } ifelse =