Cannot find entry symbol _start...

[YourSurrogateGod]

I'm using NASM. When I try to link the program (using this command "ld -s -o test test.o"), I get the below warning. As a result of that, when I run the progam it seg-faults. Does anyone have an idea what I'm screwing up?

Warning:

ld: warning: cannot find entry symbol _start; not setting start address

Code:

; test.asm

; this is where the variables are stored.
SECTION .DATA
  hello:     db 'Hello world!',10
  helloLen:  equ $-hello

; the beginning of the executing code.
SECTION .TEXT
  GLOBAL _START

; this is a routine that will be called when the program starts running.
_START:

  ; Write 'Hello world!' to the screen
  mov eax, 4           ; 'write' system call
  mov ebx, 1           ; file descriptor 1 = screen
  mov ecx, hello       ; string to write
  mov edx, helloLen    ; length of string to write
  int 80h              ; call the kernel

  ; Terminate program
  mov eax, 1           ; 'exit' system call
  mov ebx, 0           ; exit with error code 0, we want to exit without any errors.
  int 80h              ; call the kernel