; We need to tell it about main when using c lib functions
; else it will complain about it during linking
global main

; c library symbols for the compiler (linker doesn't care)
extern printf
extern atoi

section .data
	; Strings to printf'
	fmt: db '%d + %d = %d',0xA,0x0 ; append with newline and null terminator
	usageMsg: db 'usage: %s first_number second_number',0xA,0x0

section .text
; Note that this will be called by some external function (actually,
; the compiler will generate its own _start that eventually calls this)
main:
	push rbp
	mov rbp, rsp
	push rbx
	push r12
	push r13
	push r14
	push r15

	; main takes an int and a char pointer as arguments
	mov r12, rdi
	mov r13, rsi

	; This segfaults when moved below the line "jne .fail"
	; Why?
	; Because it stores the float registers on the stack
	; as well. This doesn't seem to be particularly well
	; documented.
	sub rsp, 8 ; allocate 8 bytes = 2 integers

	cmp r12, 3 ; argc == 3
	jne .fail

	mov rdi, qword [r13 + 8] ; argv[1]
	call atoi
	mov dword [rsp], eax ; store integer 1

	mov rdi, qword [r13 + 16] ; argv[2]
	call atoi
	mov dword [rsp + 4], eax ; store integer 2

	; add the two numbers
	mov ecx, dword [rsp]
	add ecx, dword [rsp + 4]

	; print the result
	mov rdi, fmt
	mov esi, [rsp]
	mov edx, [rsp + 4]
	call printf

	; main returns an integer
	xor rax, rax
	jmp .end
.fail:
	mov rdi, usageMsg
	mov rsi, [r13] ; argv[0] = the executable name
	call printf
	mov rax, 1
.end:
	pop r15
	pop r14
	pop r13
	pop r12
	pop rbx
	mov rsp, rbp
	pop rbp

	ret