Question

Write an ARM assembly language subroutine (named nfibo) to calculate and return the n-th Fibonacci number. Fibonacci numbers (or a Fibonacci sequence) are a series of numbers with a property that the next number in the series is a sum of previous two numbers. Starting the series from 0, 1 as the first two numbers we have 0, 1, (0 + 1) = 1, (1 + 1) = 2, (1 + 2) = 3, (2 + 3) = 5, (3 + 5) = 8, (5 + 8) = 13, (8 + 13) = 21, (13 + 21) = 34. The subroutine, nfibo, accepts n as an input in R0 and returns the corresponding Fibonacci number at the n-th position in R0. Don't try to use an array to map as Fibonacci is an infinite series. This programming can be done with loops, or a recursive subroutine. Recursive subroutine is shorter and easier to write then loops, but both are doable, you can choose either method. As always the subroutine should preserve all the registers except the return/output register. For example if n = 3, return value would be 2 if n = 6. return value would be 8

Answer 1

nfibo proc
  PUSH EBP          ; save previous frame pointer
  MOV  EBP, ESP     ; set current frame pointer

  MOV  EAX, [EBP+8] ; get argument N
  CMP  EAX, 1       ; N<=1?
  JA   Recurse      ; no, compute it recursively
  MOV  ECX, 1       ; yes, Fib(1)--> 1
  JMP  exit

Recurse:
  DEC  EAX          ; = N-1
  MOV  EDX, EAX     ; = N-1
  PUSH EDX          ; save N-1
  PUSH EAX          ; set argument = N-1
  CALL nfibo    ; compute Fib(N-1) to ECX
  POP  EAX          ; pop N-1
  DEC  EAX          ; = N-2
  PUSH ECX          ; save Fib(N-1)
  PUSH EAX          ; set argument = N-2
  CALL nfibo    ; compute Fib(N-2) to ECX
  POP  EAX          ; = Fib(N-1)
  ADD  ECX, EAX     ; = Fib(N-1)+FIB(N-2)
exit:
  MOV  ESP,EBP      ; reset stack to value at function entry 
  POP  EBP          ; restore caller's frame pointer
  RET               ; and return