getfib:
;input: BX = the desired fibonacci number (in other words, the "n" in "F(n)")
;       DS must point to the segment where the fibonacci table is stored
;outputs to DX:AX (DX = high word, AX = low word)
  push ds
    cmp bx,41  ;bounds check
    ja IndexOutOfBounds
    shl bx,1
    shl bx,1 ;multiply by 4, since this is a table of dwords
    mov ax,@code
    mov ds,ax
    mov si,offset fib
    mov ax,[ds:si]    ;fetch the low word into AX
    mov dx,2[ds:si]   ;fetch the high word into DX
  pop ds
  ret


IndexOutOfBounds:
     stc             ;set carry to indicate an error
     mov ax,0FFFFh   ;return FFFF as the error code
   pop ds
   ret

;table of the first 41 fibonacci numbers
fib dword 0, 1, 1, 2, 3, 5, 8, 13
    dword 21, 34, 55, 89, 144, 233, 377, 610
    dword 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657
    dword 46368, 75025, 121393, 196418, 317811, 514229, 832040, 1346269
    dword 2178309, 3524578, 5702887, 9227465, 14930352, 24157817, 39088169, 63245986
    dword 102334155