Allocating Memory in NASM

mem.nasm

global _main
    extern _ExitProcess@4
    extern _MessageBoxA@16
    extern _GetProcessHeap@0
    extern _HeapAlloc@12

    section .text

_main:
    ; Place heap in eax
    call _GetProcessHeap@0

    ; Allocate 5 bytes and point to eax
    push 5
    push 8
    push eax
    call _HeapAlloc@12

    mov byte [eax], 65
    mov byte [eax+1], 66
    mov byte [eax+2], 67
    mov byte [eax+3], 68
    mov byte [eax+4], 0

    ; Display message
    push 0
    push _title
    push _msg
    push 0
    call _MessageBoxA@16

    push 0
    call _ExitProcess@4

_msg: db "Hello", 0
_title: db "Title", 0