0x00000000: 89442404      : mov dword [esp + 0x4], eax

10001001 01 000 100 00 100 100 00000100                                                                                                                         

10001001|                                                      opcode `mov`

10001001|                                                      operand code `Ev_Gv`

        |       100                                            `mmm` field selects sib access

        |   000                                                `eax` (`rrr` selects register number 0)

        |01                                                    1-byte displacement

        |                  100                                 `esp` (`bbb` selects register number 4)

        |              100                                     iii selects no index register

        |                      00000100                        displacement: 0x4

        |

        opcode ends/operands begin (typically)