2 IIR2
         Name: IIR2.ASM
         Type: Assembler Macro
      Version: 1.0
 Date Entered:  15-Jul-87
  Last Change:  15-Jul-87

  Description: Direct Form Second Order All Pole Filter with Scaling

 This macro implements a second order all pole IIR filter. The
 DSP56000/1 scaling modes are used to provide coefficient scaling.
 This allows coefficients with magnitudes less than two.  This 
 is similar in operation to the IIR1 macro.

 When the A or B register is read, the value is shifted
 left such that the value of the filter state is multiplied by 2.
 Since the coefficients are divided by two, the scale factors cancel.
 This allows coefficients in filters to have magnitudes bounded by
 2 which is typical of second order sections.

 The difference equation for the filter is:

 y(n)= 2 [x(n)  +  a(1)y(n-1)  +  a(2)y(n-2)]
          ---      ----           ----
           2        2               2
 with z transform:

  Y(z)                1
 ----- =  -------------------------
  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
      T   = sample period


An example network to realize such a filter is shown in below.

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

      Second Order Direct Form All Pole IIR Filter

 The coefficients in the example are arbitrary. The memory
 map for the filter is shown below:

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

         --------------------
    Y:   |  a(1)  |   a(2)  |
         |  .8/2  |  -.3/2  | Filter Coefficients
         --------------------
             ^
             |
             r4

        Memory Map for the Second Order All Pole Filter

 The modulo registers m0  and  m4  are  set  to  -1  for  linear
 arithmetic.

 For an example of how to use this macro, see the test
 program IIR2T.ASM