; FIR_asm_func.asm 
; asm function called from C to implement fixed-point FIR 
; A4 = x[n] address, B4 = h[0] address, A6 = filter order N 
; input samples organized as x(n)...x(n-N)
; coefficients as h[0]...h[N]     

                .def    _FIR_asm_func
_FIR_asm_func:                          ; asm function called from C
                MV     .L1    A6,A1     ; setup loop count in A1
                ZERO   .S1    A8        ; init A8 for accumulation      
                LDH    .D1    *A4++,A2  ; x[n]
                LDH    .D2    *B4++,B2  ; h[0]
                NOP           4
                MPY    .M1x   A2,B2,A7  ; A7=x[n]*h[0]
                NOP 
                ADD    .L1    A7,A8,A8  ; accumlate in A8

LOOP:                                   ; start of FIR loop  
                MV     .L1    A2,A3     ; used to update input vector x
                LDH    .D1    *A4,A2    ; A2=x[n-k] k=1,...,N
                LDH    .D2    *B4++,B2  ; B2=h[k]   k=1,...,N
                NOP           4
                STH    .D1    A3,*A4++  ; update input vector, inc x[k]
                MPY    .M1x   A2,B2,A7  ; A7=h[k]*x[n-k]
                NOP 
                ADD    .L1    A7,A8,A8  ; accumlate in A8               
                SUB    .S1    A1,1,A1   ; decrement loop count A1

  [A1]  B      .S2    LOOP              ; branch to loop if A1 # 0
                NOP               5

                MV     .L1    A8,A4      ; result returned in A4
                B      .S2    B3         ; return addr to calling routine
                NOP               5