/*---------------------------------------------------------------------------
tblllkup.asm    - Table Lookup with Interpolation
-----------------------------------------------------------------------------
Description:

Performs a Table Lookup with Interpolation to approximate f(x) from a table
of values. The interpolation formula used is:

                                      x - X0 
    f'(x) = { ( F[X+1] - F[X] )  *  ( ------  - X ) } + F[X]
                                        S

    where:
        x       = input value x
        f'(x)   = the approximation of f(x)
        Xi      = the x value for the ith element in table
        F[i]    = is the ith element of the table 
                = f(Xi) exactly
        S       = x spacing in the table
        X0      = x corresponding to F[0]
        X       = floor (( x - X0 ) / S ) 

-----------------------------------------------------------------------------
Program Characteristics:

Calling Values:
    f0      = x
    f7      = 1/S
    f15     = X0
    i0      = start of table
    l0      = length of table
    m0      = 1
    l1      = 2

Registers Altered:
    f1-f4, m1

MODE1 Register Bits:
    TRUNC   = 1

Return Value:    f1 = interpolated f(x)

Computation Time = 11 cycles 
                 = 440ns @ 25MHz

Note: 11 cycles includes the DAG hold after load of m1 register.
-----------------------------------------------------------------------------
Author:     Jim Donahue, Analog Devices DSP Division
Revised:    19-AUG-91
---------------------------------------------------------------------------*/

.GLOBAL tbllkup;

#define     X       f0          /* s, the input value                      */
#define     XF      f2          /* (float) floor(X)                        */
#define     X0      f15         /* F(0), first value in table              */
#define     IS      f7          /* 1/S inverse of table stepsize in X      */
#define     IDX     r3          /* (int) floor(X)                          */
#define     INTRP   f2          /* interpolation factor (F(i)-->F(i+1)     */
#define     FI      f4          /* F(x)                                    */
#define     FI1     f1          /* F(x+1)                                  */
#define     RES     f1          /* the interpolated result                 */

.SEGMENT /pm    pm_code;
tbllkup:    
        X = X - X0, i0 = b0;
        X = X * IS;
        IDX = fix X;
        XF = float IDX, m1 = IDX;
        INTRP = X - XF, modify(i0,m1);
        FI = dm(i0,m0);
        FI1 = dm(i0,m0);
        rts (db), RES = FI1 - FI;
            RES = RES * INTRP;
            RES = RES + FI;
.ENDSEG;