Question

section .data
fun:
   add word[L+2],0x203
L: mov eax,0x1020304 ; this instruction including operand stored in 5 byte in memory
   ret

what the “fun” function do? why eax will have a new value 0x1040604

Answer 1

Ok, you have a mov eax, 0x1020304, with a 5-byte encoding (there is a 6-byte encoding as well, but let’s ignore that). So it looks like this (remember – little endian):

B8 04 03 02 01

Where L refers to the B8 and L+2 refers to the 03. Now if you add 0x0203 to it (remember – little endian), the 03 is added to the 03 and the 02 is added to the 02, so you get this:

B8 04 06 04 01

Which is:

mov eax, 0x01040604