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

  Description: Direct Form FIR Filter Assembler Macro

 This macro implements a direct form FIR filter with an arbitrary
 number of coefficient taps.  The difference equation for a four
 tap filter is shown below:

       y(n)= b(0)x(n) + b(1)x(n-1) + b(2)x(n-1) + b(3)x(n-3)

 with z transform:

       Y(z)               -1         -2           -3
      ----- = b(0) + b(1)z    + b(2)z      + b(3)z
       X(z)

 where:
      x(n) = input sample at time nT
      y(n) = output sample at time nT
      T    = sampling period
      b(n) = filter impulse response at time nT

 This difference equation can be implemented  with  the  discrete
 time filter structure shown:

      Input
    x(n) >-----------1/z---------1/z---------1/z-------
                |           |           |             |
               0.1         0.3        -0.1           0.2
             b(0) \    b(1) |      b(2) |       b(3) /
                   \        \___     ___/           /
                    \           \   /              /
                     ----------->(+)<--------------
                                  |-----------------> y(n) Output
                             

  The coefficients in the example are arbitrary.  For the
  above filter, the  memory  map  for  the  filter states
  (past input samples) and coefficients (b) are shown below.


           r0         m0=ntaps-1   (=3, mod 4)
           |
           v
       ---------------------------------------
  X:   |  New   |   Old   |   Old   |   Old  | Filter States
       | x(n)   | x(n-1)  | x(n-2)  | x(n-3) |
       ---------------------------------------

       ---------------------------------------
  Y:   |  b(0)  |   b(1)  |   b(2)  |  b(3)  | Filter Coefficients
       |  .1    |   .3    |  -.1    |  .2    |
       ---------------------------------------
           ^
           |
           r4        m4=ntaps-1    (=3, mod 4)

               Memory Map for the FIR Filter


 For an example of how to use the filter macro, see the test program
 FIRT.ASM.