what.asm

default rel
bits 64

segment .text
global main

extern _CRT_INIT
extern ExitProcess

    ; Define constants to refer to the function arguments as offsets from RSP/RBP
    a equ    0
    b equ    8

foo:
    push    rbp                 ; Set up a stack frame for the function
    mov    rbp, rsp             ; Continuing the linked list here, by pointing rbp to this stack frame
    sub    rsp, 32              ; Must align on 16 byte boundary

    mov    [rbp + a], rcx       ; save parameter a (rcx)
    mov    [rbp + b], rdx       ; save parameter a (rdx)

    
    leave                       ; Undo the stack frame (it sets rsp to rbp, then pops rbp.)
    ret                         ; return to calling procedure (main).

main:
    call _CRT_INIT                 ; Needed since our entry point is not _DllMainCRTStartup. See https://msdn.microsoft.com/en-us/library/708by912.aspx

    push    rbp                 ; Set up a stack frame
    mov    rbp, rsp             ; base pointer now points to stack frame
    sub    rsp, 32              ; Leave room for shadow parameters (4 for windows)

    mov    rcx, 100             ; First parameter to function
    mov    rdx, 200

    call    foo           ; Invoke function

    xor eax, eax                ; return 0
    leave

    call ExitProcess            ; On Windows, terminates the process.