fsqrt.txt

[ARRL.无线电通讯手册].The.Arrl.Handbook.For.Radio.Communications.-.2007

fsqrt:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Entry conditions: 32-bit argument in AM ;
;   register.                             ;
; Exit conditions: 16-bit square root in  ;
;   d register.                           ;
; Modified: D, E, X, Y                    ;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

      ldy  #0000     ;start with 0 count
      tmxed          ;arg in x:e:d
      xgde           ;lsw in e, msw in d
      tsta           ;msb=0?
      beq  fazero    ;yes, normalize
      aiy  #$01      ;1 shift more
      lsrd           ;divide by 2
      rore           ;
      lsrd           ;next divide by 2
      bita #$ff      ;done?
      beq  fdzero    ;yes
      rore           ;finish divide
      aiy  #$01      ;1 shift more
      lsrd           ;divide by 2 again
      rore           ;
      lsrd           ;next divide by 2
      bita #$ff      ;done?
      beq  fdzero    ;yes
      rore           ;finish divide
      aiy  #$01      ;1 shift more
      lsrd           ;divide by 2
      rore           ;
      lsrd           ;next divide by 2
                     ;
fdzero: rore         ;normalized argument in d:e
      asld           ;;;mult by 2 for table address
      ldx  #roottabl+2 ;base address of root table
      adx            ;;;add offset
      ldd  0,x       ;get nearest root from table
      subd -2,x      ;minus the lower root
      lsre           ;make the remainder a signed fraction
      fmuls          ;b*(xa+1-xa)
      adce -2,x      ;b*(xa+1-xa)+xa
      xgdy           ;# of shifts, k, in d
      addd #-4       ;d-4 in d
      negd           ;4-d in d
      beq  fddone    ;this is 0
      lsre           ;denormalize the result
      addd #-1       ;next
      beq  fddone    ;
      lsre           ;shift back
      addd #-1       ;next
      beq  fddone    ;
      lsre           ;shift back
                     ;
fddone: xgde         ;result in d
      rts            ;out
                     ;
fazero: cmpb #$3f    ;done normalizing?
      bhi  fadz      ;yes, k=0
      aiy  #$01      ;one shift more
      asle           ;divide by 2
      rold           ;
      asle           ;divide by 2 again
      rold           ;done
      cmpb #$3f      ;done normalizing?
      bhi  fadz      ;yes, k=2
      aiy  #$01      ;one shift more
      asle           ;divide by 2
      rold           ;
      asle           ;and again
      rold           ;done
      cmpb #$3f      ;done?
      bhi  fadz      ;yes, k=4
      aiy  #$01      ;one shift more
      asle           ;divide by 2
      rold           ;
      asle           ;and again
      rold           ;done, k=6
                     ;
fadz:  asld          ;;;mult by 2 to be table address
      ldx  #roottabl+2 ;table base address
      adx            ;;;add offset
      ldd  0,x       ;get table word
      subd -2,x      ;(xa+1-xa)
      lsre           ;make remainder a signed fraction
      fmuls          ;b*(xa+1-xa)
      adce -2,x      ;b*(xa+1-xa)+xa
      lsre           ;divide by 16 right off, since arg<224
      lsre           ;
      lsre           ;
      lsre           ;OK
      xgdy           ;# of shifts, k, in d
      tstd           ;=0?
      beq  fazone    ;done
      lsre           ;denormalize the result, divide by 2
      addd #-1       ;next
      beq  fazone    ;done
      lsre           ;divide by 2
      addd #-1       ;next
      beq  fazone    ;done
      lsre           ;divide by 2
                     ;
fazone: xgde         ;result in d
      rts            ;out

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