2 IIR5
         Name: IIR5.ASM
         Type: Assembler Macro
      Version: 1.0
 Date Entered:  15-Jul-87
  Last Change:  15-Jul-87

  Description: Second Order Direct Canonic Filter with Scaling

  This macro implements a second order filter with a minimum of
  delay elements.  The scaling mode of the DSP56000/1 is used to
  directly implement coefficients with a magnitude less than two.

  The difference equation relating the output to the input is:

      y(n) =2 [a(1)y(n-1) + a(2)y(n-2) + x(n) + b(1)x(n-1) + b(2)x(n-2)]
               ---          ---          ---    ---          ---
                2            2            2      2            2

   with z transform:

                          -1       -2    
        Y(z)     1 + b(1)z  + b(2)z
      ------- = ----------------------
        X(z)              -1       -2
                 1 - a(1)z  - a(2)z

   where:
    x(n)  = input sample at time nT
    y(n)  = output of the filter at time nT
    a(n)  = filter coefficient n (magnitude less than two)
    b(n)  = filter coefficient n (magnitude less than two)
      T   = sample period


  In this example, it is  assumed  that  b(0)=1.   A  network  to
  implement this filter is shown below.

     Input                 w(n)
 x(n) >----(+)--------------------------(+)-----> y(n)
            ^                |             ^      Output
            |     a(1)      1/z    b(1)    |
           (+)<-- 0.8/2 -----|---- 0.4/2 -->(+)
            ^                v             ^
            |     a(2)      1/z    b(2)    |
           (+)<-- -0.3/2 ----|--- -0.6/2 -->(+)


 The coefficients in the example are arbitrary.  For the
 above filter, the memory map  for  the  filter states and 
 the coefficients is shown below.

           r0
           |
           v
       -------------------
  X:   |        |        |
       |2*w(n-1)|2*w(n-2)| Filter States
       -------------------

       -------------------------------------
  Y:   |  a(1)  |  a(2)  |  b(1)  |  b(2)  |
       | .8/2   | -.3/2  | .4/2   | -.6/2  | Filter Coefficients
       -------------------------------------
           ^
           |
           r4        m0=3 (mod 4)

  Note that the coefficients for the filter are stored divided by
  two.  When the filter states are saved, they are scaled up by two.
  Since the coefficients are scaled down by two, the scale factors
  cancel when the filter states are multiplied by the coefficients.

  The address register r0 is initialized pointing to  the  first
  filter  state  and  r4  is  initialized pointing to the filter
  coefficients.  Modulo register m0 is  set  to  -1  to  provide
  linear  arithmetic.   Modulo register m4 is set to 3 providing
  modulo 4 arithmetic.

  For an example of how to use this filter see the test program
  IIR5T.ASM