.386
.model flat, stdcall
.data
bigEndian BYTE 12h, 34h, 56h, 78h
littleEndian DWORD ?
.code
start:
    ;write your code here
    mov al, [bigEndian+3]
    mov ah, [bigEndian]
    mov [bigEndian], al
    mov [bigEndian+3], ah
    mov al, [bigEndian+2]
    mov ah, [bigEndian+1]
    mov [bigEndian+1], al
    mov [bigEndian+2], ah
    mov eax, DWORD PTR bigEndian
    mov littleEndian, eax

    ret
    END start