section .text

global addsub

addsub:
    push ebp
    mov ebp, esp

    push bx

    mov eax, 0

    mov bx, word [ebp+16] ;Operation
    mov ax, word [ebp+8] ;op1

    cmp bx, 0x2b
    jne substract

    add ax, word [ebp+12] ;op2
    jmp return

substract: sub ax, word [ebp+12] ;op2

return:
    pop bx
    mov esp, ebp
    pop ebp
    ret

section .data

operation dw 2