[C++/C/ASM][tutorial] __asm in C++/C. Basics.

Written by me (Matek,registered as Charlie on RaGEZONE) on 25th June 2008 ;)

Here it is, my first tutorial.

Most of you have seen a lot of codes and scripts containing '__asm'. You
could wonder what it is for in a C++ code.
The answer is simple, it's called 'inline assembly' and is used to extend
power and abilities of regular C++ applications. That's enough for an introduction I guess.
MSDN says it all:
Because the assembly code can be mixed inline with C or C++ statements, it
can do tasks that are cumbersome or impossible in C or C++.

I'm not going to explain assembly in no way here. I want to show you the
way to use asm in C++ code and how to mix them up.

Let's move on.

1. Syntax.

As you can see below there's nothing much to do. Type __asm and put assembly
code between curly braces { }.

Code:

---

__asm {

...code...

}

---

You could start every line (or instruction) with __asm but it's senseless
unless you're using a single instruction ( mov eax, ebx ). It would look
like this:

Code:

---

__asm ...code...
__asm ...code...

//or...

__asm ...code...  __asm ...code...

//senseless no?



//The code below is fine

__asm add eax,ebx

cout << "We used a single instruction.";

---

It's not hard to notice that doing it this way is just a waste of your time
and there's a possibility that you forget to start every line with it.

2. Mixing C++/C with __asm.

In __asm block of code you can use ALMOST all C++/C symbols. This means
that you can use variable names, function names and labels within __asm block.
However you have to keep in mind that symbols' names cannot be the same as
MASM reserved words such as instruction names (eg PUSH). All functions
you are referring to must be declared before the beginning of __asm block.

3. C++/C data in __asm.

A great convenience of inline assembly is the ability to refer to C or C++ v
ariables by name. The easiest way to describe it is using an example so here it is:

Code:

---

int VariableStoringMyAge;
VariableStoringMyAge = 15;

//Here a short one-line __asm block starts and look! Our int variable is
right there. It adds VariableStoringMyAge's value (15) to eax and stores
the sum in it.

__asm add eax,VariableStoringMyAge

---

I hope it's clear and easy to understand.

NOTE: However (nothing can be wonderful ;( ) if any class or structure
share the same member name (it's not unique because
there are two of them), you must place a variable or typedef name right
before the period (.) operator. No example because it's only a note! (:p)

I think it's enough for my first tutorial and it explains the basics.
Now go learn assembly and use it in your C++/C codes :)

yours nefykenny/Matek.

Credits:
Me.
MSDN.
Google.

Feel free to evaluate but don't you dare to
flame me!

Nice work ;), added the [tutorial] tag.

I'm getting into ASM too, lately. Pretty hard to grasp it in the beginning, but you can build some efficient things with it ;)

Thanks ;)

That's true. It's very good on its own but added to C++ works as a nice addon and gives the C language more abilities as mentioned in the tut.

some more asm xD:

JMP - go to offset / jump
MOV- save value
What are the most used Registers ?
EAX,EBX,ECX,EDX,EBP,ESI,EDI
what is EIP ? - Program Entry Point (where program starts)
saving value example:
MOV EAX,1
now eax is 1
MOV EAX,5 - eax is 5
MOV EBX,EAX - now EBX is 5
MOV ECX,EBX - now ECX is 5 ;D
JMP 0x00000001 - example of jumping to offset

note - i dont know if this will work with the inline asm in VC++

Such basics should do.

You are partially wrong. MOV isn't 'save value' only.

Code:

---

mov eax,ebx
// it moves ebx to eax and saves it in eax.

---

Quote:

---

Originally Posted by penihop

some more asm xD:

JMP - go to offset / jump
MOV- save value
What are the most used Registers ?
EAX,EBX,ECX,EDX,EBP,ESI,EDI
what is EIP ? - Program Entry Point (where program starts)
saving value example:
MOV EAX,1
now eax is 1
MOV EAX,5 - eax is 5
MOV EBX,EAX - now EBX is 5
MOV ECX,EBX - now ECX is 5 ;D
JMP 0x00000001 - example of jumping to offset

note - i dont know if this will work with the inline asm in VC++

---

Depends on which compiler and processor you use, using MingW32 I have different commands.

Quote:

---

Originally Posted by Charlie

Such basics should do.

You are partially wrong. MOV isn't 'save value' only.

Code:

---

mov eax,ebx
// it moves ebx to eax and saves it in eax.

---

---

btw, why u type 2 times " _ " ? whats the difference between 1 _ and 2" __ "
i do like this
_asm
{
//code
}

__asm is the present version of asm. However:

Quote:

---

Originally Posted by MSDN

For compatibility with previous versions, _asm is a synonym for __asm.

---

Simply saying, _asm is an older version of __asm and newer version of asm (without underscores).