How are hex sequence translated to assembly without ambiguity?

Question

8B EC 56 8B F4 68 00 70 40 00 FF 15 BC 82 40   

A senquence like above can be segmented in various ways,each segment can be translated to correspon

Answer 1

If you open a binary in a hex editor, copy a portion of data and paste in a disassembler, you will very probably not copy a complete instruction. Let's use your example.. in a Windows XP 32bits SP3 English, if I assembly 8B EC 56 8B F4 68 00 70 40 00 FF 15 BC 82 40 I'll get:

Hex dump          Command
8BEC              mov ebp,esp
56                push esi
8BF4              mov esi,esp
68 00704000       push 407000
FF15 BC824000     call dword ptr ds:[4082bc]

As you can see it assembled completely different then the answer of the other guys below...

Now let's pretend that instead of assembling 8B EC 56 8B F4 68 00 70 40 00 FF 15 BC 82 40
you added C0 opcode at the beginning C0 8B EC 56 8B F4 68 00 70 40 00 FF 15 BC 82 40

Now check below what a single byte did in our instructions:

Hex dump             Command
C08B EC568BF4 68     ror byte ptr ds:[ebx+f48b56ec],68
0070 40              add byte ptr ds:[eax+40],dh
00FF                 add bh,bh
15 BC824000          adc eax,4082bc

As you can see it was completely modified!

The processor know where to start and what arguments to the instruction to take by the opcode instruction. In the first example our first opcode was 8B so it knows that it can be followed by another byte. If this byte is EC so the instruction ends here, and it means mov ebp, esp.

In our second example it start with C0 and it can be followed by another byte, meaning another instruction. Then C08B is the instruction, and EC568BF4 68 is the argument.

Imagine that inside the processor has a huge (but nano) circuit, that behave like a chain of ifs (conditions), that depending on the value of the hexcode (or opcode) it knows "where to go" or "how to behave".