4. Endianness: Byte Ordering in Computer Memory

• Consider a small program, little_endian.asm .

• Assembler fragment of little_endian.lst listing file shows generated data and code:

   00000000                .DATA
   00000000 EE FF                        byte0   BYTE    0EEh, 0FFh
   00000002 1234                        word2   WORD    1234h
   00000004 56789ABC                    var4    DWORD   56789ABCh
   00000008 00000000                    var8    DWORD   0
  
   00000000                .CODE
   00000000                _start:
   00000000  B8 00000002 R            mov eax, OFFSET word2
   00000005  A3 00000008 R            mov [var8], eax
   0000000A  C3                        ret             ; Exit program
  
  

• DUMPBIN output for this program yields:

  C:\>DUMPBIN /DISASM little_endian.exe
  Dump of file E:\little_endian.exe
  File Type: EXECUTABLE IMAGE
  __start:
    00301000: B8 02 40 30 00     mov         eax,304002h
    00301005: A3 08 40 30 00     mov         dword ptr ds:[00304008h],eax
    0030100A: C3                 ret
  

• Did you notice something strange about highlighted opcodes?

  • The byte sequence that belongs to the 32-bit displacement seems out of order: instead of expected

        00 30 40 08
    

    we see a reversed sequence,

        08 40 30 00.