;===========================================================
;
; File Name     :qinv1.asm
; 
; Originator    :Digital Control Systems Group 
;                Texas Instruments 
; 
; Description   :This file contain source code to find the reciprocal of 
;                fixed point number, The output is a 32 bit number                          
; Date          : 7/11/2000
;==========================================================
; 
; 
; Routine Name  : Generic Function      
; Routine Type  : C Callable
; 
; Description   :
; long int qinv1(int x) 
; 
; Input format:  Signed number in Q(x) format (x<16)
; Output format: Signed number in Q(31-x) format (32 bit result)
; 
; Example 1: 
; Input format:  Signed Q15 format  (Fractional Number)
; Output format: Signed Q16  format 
;
; Example 2:
; Input format:  Signed Q0 format   (Integer Number)
; Output format: Signed Q31
;                                     
; Example 3: 
; Input format:  Signed Q13 format
; Output format: Signed Q18 format
;=================================================================
; Function Local Frame
;=================================================================
;   |_______|
;   |_______|<- Stack Pointer
;   |_______|<- Computation     (FP+1)
;   |_______|<- Frame ptr       (FP) 
;   |_______|<- Old FP
;   |_______|<- Return Address of the Caller 
;   |_______|<- Formal parameter x in signed m.n format
;==================================================================

; Module definition for external referance
            .def    _qinv1
            
__inv1_frs  .set    00002h          ; Local frame size for this routine         

_qinv1:
            POPD    *+              ; Store the Return Address in stack
            SAR     AR0,*+          ; Store the Caller's Frame Pointer
            SAR     AR1,*
            LAR     AR0,#__inv1_frs     
            LAR     AR0,*0+,AR2     ; Create Local frame for cos function
            
            SETC    OVM             ; To saturate ABS value of 0x8000 to 0x7fff                        
            SETC    SXM             
            
            LAR     AR2,#0FFFDh      
            MAR     *0+             ; Modify AR2 to point to x, which is in 16.0
            LACC    *,16,AR3        ; Load the input to the ACCH
            
            LAR     AR3,#01h
            MAR     *0+
            
            BCND    calculate,NEQ   ; If input is zero, return with ZERO
            LACC    #0000h
            B       return   
            
calculate:  BCND    positive, GT
            ABS                     ; Obtain the absolute value of x<0

positive:   SACH    *               ; Store |x| in 16.0
            LACC    #01h,15         ; #1 in Q15 format

            RPT     #15
            SUBC    *               ; Generate first 16 bits of Quotient
            
            MAR     *,AR0
            SACL    *,0,AR3         ; Store the Quotient
            AND     #0ffffh,16      ; Mask the quotient 
            
            RPT     #15             ; Generate another 16 quotient bits
            SUBC    *               ; by dividing the remainder from the
                                    ; first stage.   
            
            AND     #0ffffh         ; Mask the remainder
            MAR     *,AR0
            ADD     *,16,AR2        ; Combine the Quotient bits
            ABS                     ; To convert 8000 0000 to 7fff ffff 
                
isnot1:     BIT     *,0
            BCND    return,NTC
            NEG
            
return:     CLRC    OVM
            MAR     *,AR1           
            SBRK    #(__inv1_frs+1); Clear the local frame
            LAR     AR0,*-          ; Retrive Caller's frame pointer
            PSHD    *               ; Push the return address to TOS
            RET                     ; Return to the caller