Opcodes and instructions..?

Yo.

I was wondering, the other day when I had CE open. All the instructions are determined by the bytes I think? For example, the no-operation instruction would be 0x90 in hexadecimal.

I'm here to know if instructions like dec inc jmp call etc, have their own specific combination of bytes to make the instruction.

Another question:

If there are combinations of bytes, how would you do something like:

Code:

call 0xFFFFFFF

How would you determine what address to call?
OR
Something like:

Code:

mov eax,100h

Would all of this be determined by a combination of bytes?

Sorry if this is stupid, also, don't know if this should be in the assembly section.

Originally Posted by Davidm44

Well yeh. I imagine they all have their own opcodes, what those are specifically I'm not sure.

Another question:

If there are combinations of bytes, how would you do something like:

Code:

call 0xFFFFFFF

Well I imagine the use of certain opcodes let the processor know you are making a call to an address location. Since every asm instructions is well an instruction you see that the opcode has to come first. that's how it knows it accessing an address and not a value.

How would you determine what address to call?
OR
Something like:

Code:

mov eax,100h

Would all of this be determined by a combination of bytes?

Sorry if this is stupid, also, don't know if this should be in the assembly section.

its not a stupid question, I like to talk about asm... wish I knew more. I have a lot to learn too... Hopefully someone more experienced can feel in the details. I didnt do a good job explaining. :l

I didnt do a good job explaining. :l

You did a great job explaining. I just wanted to do a little bit of researching so I can make my own version of Dark Bytes memory viewer in CE.

Now I just need the exact specifications for the instructions.

^^^^^That's a lie actualy, i'm probably still far from being able make such a thing.

Originally Posted by Davidm44

^^^^^That's a lie actualy, i'm probably still far from being able make such a thing.

Well you got that right... memory isn't stored as simply as you might think. There's paged and unpaged, memory, but you should be able to get the executable from its stored location on disk... I'm not sure how you would go about reading it.

Originally Posted by why06

I don't quite understand what you're saying.

Reading the memory specifying what the instructions would be?

Originally Posted by Davidm44

I don't quite understand what you're saying.

Reading the memory specifying what the instructions would be?

To find out what code is currently being executed I mean, and actually stopping it from executing long enough to see where your at in the asm code, but I guess all you want to do is look at it?

Oh yeah, baby steps.

First I want to see it.
Then maybe later on when I have a better understanding of assembly I can try going further. Maybe duplicate the memory viewer in CE.

Opcodes and instructions..?

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