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).