example call a routine and show the result

show_sum

section	.text
    global _start         ;must be declared for using gcc
_start:	;tell linker entry point
	mov	ecx,'4'
	sub     ecx, '0'
	mov 	edx, '5'
	sub     edx, '0'
	call    sum     ;call sum procedure
	mov 	[res], eax

	; show display message
	mov	ecx, msg
	mov	edx, len
	mov	ebx,1	;file descriptor (stdout)
	mov	eax,4	;system call number (sys_write)
	int	0x80	;call kernel

	; show sum result number
	mov	ecx, res
	mov	edx, 1
	mov	ebx, 1	;file descriptor (stdout)
	mov	eax, 4	;system call number (sys_write)
	int	0x80	;call kernel

	; show return in console 
	mov	ecx, msg2
	mov	edx, len2
	mov	ebx,1	;file descriptor (stdout)
	mov	eax,4	;system call number (sys_write)
	int	0x80	;call kernel

	mov	eax,1	;system call number (sys_exit)
	int	0x80	;call kernel
sum:
   mov     eax, ecx
   add     eax, edx
   add     eax, '0'
   ret
section .data
msg db "The sum is:", 0xA,0xD
len equ $- msg 

msg2 db "", 0xA,0xD
len2 equ $- msg2

segment .bss
res resb 1

$ ./demo
The sum is:
9