{
    Subroutine to compute the inverse square root 
    of x to single-precision (24 bits mantissa)
    using the 1/sqrt(x) approximation.

            X = isqrtsgl(Y) = 1/sqrtsgl(Y)

        Calling Registers
    F0 = Y Input Value
    F8 = 3.0
    F1 = 0.5

        Result Registers
    F0 = isqrtsgl(Y)

        Altered Registers
    F0, F4, F12

        Computation Time
    9 Cycles

    Version 0.02    7/6/90      Gordon A. Sterling
}

#include "asm_glob.h"

.SEGMENT/PM     Assembly_Library_Code_Space;

.PRECISION=MACHINE_PRECISION;

.GLOBAL isqrtsgl;

isqrtsgl:       F4=RSQRTS F0;                   {Fetch seed}
                F12=F4*F4;                      {F12=X0^2}
                F12=F12*F0;                     {F12=C*X0^2}
                F4=F1*F4, F12=F8-F12;           {F4=.5*X0, F10=3-C*X0^2}
                F4=F4*F12;                      {F4=X1=.5*X0(3-C*X0^2)}
                F12=F4*F4;                      {F12=X1^2}
                RTS(DB), F12=F12*F0;            {F12=C*X1^2}
                F4=F1*F4, F12=F8-F12;           {F4=.5*X1, F10=3-C*X1^2}
                F0=F4*F12;                      {F4=X2=.5*X1(3-C*X1^2)}
.ENDSEG;