Thread: Size of Assembly instructions

Hello. I'm not sure where to post this, but here goes.

I need to read instructions one by one from a binary, but the fact that IA-32 instructions are not fixed-size makes it difficult. Is there some resource available on the Internet that has the size information for every opcode?

Here's an example

Code:

//I could read the first byte and determine the length of the rest of the instrucition if I had some sort of a reference.

B8 78 56 34 12          mov eax, 0x12345678
//B8 would suggest that it's an instruction that moves a dword into eax and I'd have to read additional 4 bytes to complete the instruction

6A 3E                push 0x3e
//6A, the opcode to push a byte value on the stack, would mean that I only need to read one more byte

So, is there a list with all the opcodes and their instruction size? Thanks!

There's libraries for dealing with x86 opcodes, but I don't know if they can do that. A Synaptic search of "opcode" return libasm0, that might be useful.

Or you could do it the fun way... Here's a good ref: coder32 edition | X86 Opcode and Instruction Reference 1.11

Intel® 64 and IA-32 Architectures Software Developer's Manuals

You will want to take a look at parts 2A and 2B.

Thanks for the links, although it still seems to be a lot of work and hassle to get the size from that information (that's what I'll resort to if nothing better shows up). Is there a summary somewhere with the size of each opcode's instruction in bytes?

Laziness will get you know where. Did you even bother to take a look at the referenced documents?

Hint: Part 2B, Appendix A. I can assure you, that's the most concise, clear, not to mention complete overview you will find, anywhere.

Yes, I had seen the documents before I started this thread. I was just hoping to find ready-made list of all opcodes taking 1 byte, all opcodes taking 2 bytes and so on. If there's no such resource, I'll work with the Intel manual, thank you for that.

Originally Posted by Overlord

Yes, I had seen the documents before I started this thread. I was just hoping to find ready-made list of all opcodes taking 1 byte, all opcodes taking 2 bytes and so on. If there's no such resource, I'll work with the Intel manual, thank you for that.

I am not sure why I even bother answering, but here we go, straight from the reference I gave you:

"A.3 ONE, TWO, AND THREE-BYTE OPCODE MAPS"

The document then goes on to give you:

"Tabel A-2. One-byte Opcode Map: (00H - F7H) *"

And so on.

As already mentioned by Sebastiani, disassembling a piece of code is a lot more involved for the majority of cases.

Now get off your lazy butt and take a look at the references you have been given.