My first x86-x64 ASM program

main.s

;#---------------------
;#  GNU Assembler file
;#  Function calls demo
;#---------------------
.intel_syntax noprefix
.global _start
.text
_start:
mov rbp, rsp
mov rdi, 5
mov rsi, 10
call add
mov rsi, rax
call print_val
call exit

;# rsi: the string to measure its length
;# rax: the measured length
strlen:
xor rax, rax
xor rbx, rbx
1:
mov bl, byte ptr [rsi + rax]
cmp bl, 0
je 2f
inc rax
jmp 1b
2:
ret


;# rdi: a
;# rsi: b
;# rax: the calculated value
add:
mov rax, rdi
add rax, rsi
ret

;# rdi: write to
;# rsi: value to convert
itoa:
mov rax, rsi
mov rbx, 0x0a
xor rcx, rcx

1:
xor rdx, rdx
div rbx
add rdx, 0x30
mov byte ptr [rdi + rcx], dl
inc rcx
cmp rax, 0
jne 1b
mov byte ptr [rdi + rcx], 0
mov rsi, rdi
lea rdi, [rdi + rcx - 1]
call strrev
ret

;# rsi: string to reverse
;# rdi: the end of the string to reverse
strrev:
cmp rsi, rdi
jae 1f
mov al, [rsi]
mov bl, [rdi]
mov [rsi], bl
mov [rdi], al
inc rsi
dec rdi
jmp strrev
1:
ret


;# rdi: dist
;# rsi: src
;# rax: the length of copied bytes
strcpy:
xor rax, rax
1:
mov bl, byte ptr [rsi + rax]
cmp bl, 0
je 2f
mov byte ptr [rdi + rax], bl
inc rax
jmp 1b
2:
ret

;# rsi: value to print
print_val:
push rbp
mov rbp, rsp
sub rsp, 32
buf = -31
buf_len = -32

;# store the value
push rsi

;# copy prefix msg
lea rdi, [rbp + buf]
lea rsi, msg
call strcpy
mov [rbp + buf_len], al

;# restore the value
pop rsi

;# convert value to string and append to the buffer
lea rdi, [rbp + buf]
movzx rax, byte ptr [rbp + buf_len]
add rdi, rax
call itoa

;# measure the new buffer length
lea rsi, [rbp + buf]
call strlen

;# write the buffer to the stdout
mov rdi, 1
mov dl, al
call write

mov rsp, rbp
pop rbp
ret

;# rdi: fd
;# rsi: to write
;# rdx: length in bytes
write:
mov rax, 1
syscall
ret

exit:
mov rax, 60
xor rdi, rdi
syscall

.data
msg: .asciz "Value: "