fldpi ; Load pi into st(0).
  pushd 180
  fild DWORD PTR [esp] ; Load 180 into st(0).  Pi is now st(1).
  add esp,4
  fdiv ; Divide pi by 180, store result in st(0).
  ; old value = 90
  fld oldvalue ; Load the old FOV into st(0).  Move the divided result into st(1).
  fmul ; Multiply the old FOV value by st(1) and store it in st(0).
  pushd 2
  fild DWORD PTR [esp] ; Load 2 into st(0). Multiplied result is now st(1).
  add esp,4
  fdiv ; Divide the multiplied result by 2 and store it in st(0).
  fptan ; Calculate the tangent of st(0).  Store 1 in st(0) and the calculated angle in st(1).
  fstp st ; Pop 1 off of the stack.
  fstp newvalue ; Store st(0).