wmdots/old_dos_demo/trig.s

   1 ;=========================================================================
   2 ;Trig functions for GFX routines   Copyright (c) 1995 by Mark I Manning IV
   3 ;=========================================================================
   4
   5 CosTab          equ SinTab+0800 ;A Right angle foreward in table!
   6
   7 ;-------------------------------------------------------------------------
   8
   9 ;Return Sin and Cos of angle AX in variables of same name
  10
  11 SinCos:
  12  and ax,0fff            ;4096 degrees = 0 degrees
  13  mov bx,ax              ;Put into an indexing reg
  14  add bx,bx
  15  mov ax,SinTab[bx]      ;Get Sin(N)
  16  mov Sin,ax             ;Bah no MOV Sin,[BX]... Crap processor!
  17
  18 ;Note.
  19 ; Can NOT use mov ax,SinTab[2*ebx] because in following code BX may go
  20 ;  negative and 2* a negative number = DOUBLY NEGATIVE!
  21
  22  cmp bx,01800           ;Was Sin(N) in last quadrant of circle?
  23  if nl sub bx,02000     ;Cos(N) must be in first quadrant then
  24  mov bx,CosTab[bx]      ;Get Cos(n)
  25  mov Cos,bx             ;Exits with Sin in AX and Cos in BX too!
  26  ret
  27
  28 ;=========================================================================
  29