fib2.asm

%include "os_dependent_stuff.asm"
global fib
global _fib
fib:
_fib:
  cmp rdi, 1
  jle return_one; if the relationship is "less than or equal", goto return_one
  push rdi      ; save n
  mov rsi, rdi
  inc rsi       ; rsi now contains n+1
  shl rsi, 3    ; multiply by 8 to get the number of bytes, since integers take up 8 bytes each
  mov rdi, 0
  mov r8, -1
  mov r9, 0
  mov r10, MAP_PRIVATE | MAP_ANON
  mov rdx, PROT_READ | PROT_WRITE
  mov rax, SYSCALL_MMAP
  syscall
  ; assume allocation was sucessful, and buffer is now in rax
  mov rsi, rax ; rsi will be our working buffer
  pop rcx      ; rcx contains n
  mov qword [rsi+rcx*8], 1
  dec rcx      ; rcx contains n-1
  mov qword [rsi+rcx*8], 1
.loop:
  dec rcx
  ; at this point, rcx is the index to compute
  ; computation consists of adding the rcx+1th element and rcx+2th element
  mov rax, [rsi+(rcx+1)*8]
  mov rdx, [rsi+(rcx+2)*8]
  add rax, rdx
  mov [rsi+rcx*8], rax
  test rcx, rcx
  jg .loop
; done
  push rax
; TODO: free the memory
  pop rax
  ret

return_one:
  mov rax, 1
  ret