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Inline Assembler Overview

Microsoft Specific

The inline assembler lets you embed assembly-language instructions in your C and C++ source programs without extra assembly and link steps. The inline assembler is built into the compiler — you don't need a separate assembler such as the Microsoft Macro Assembler (MASM).

Because the inline assembler doesn't require separate assembly and link steps, it is more convenient than a separate assembler. Inline assembly code can use any C or C++ variable or function name that is in scope, so it is easy to integrate it with your program's C and C++ code. And because the assembly code can be mixed with C and C++ statements, it can do tasks that are cumbersome or impossible in C or C++ alone.

The __asm keyword invokes the inline assembler and can appear wherever a C or C++ statement is legal. It cannot appear by itself. It must be followed by an assembly instruction, a group of instructions enclosed in braces, or, at the very least, an empty pair of braces. The term "__asm block" here refers to any instruction or group of instructions, whether or not in braces.

The following code is a simple __asm block enclosed in braces. (The code is a custom function prolog sequence.)

// asm_overview.cpp
// processor: x86
void __declspec(naked) main()
{
    // Naked functions must provide their own prolog...
    __asm {
        push ebp
        mov ebp, esp
        sub esp, __LOCAL_SIZE
    }
    
    // ... and epilog
    __asm {
        pop ebp
        ret
    }
}

Alternatively, you can put __asm in front of each assembly instruction:

__asm push ebp
__asm mov  ebp, esp
__asm sub  esp, __LOCAL_SIZE

Since the __asm keyword is a statement separator, you can also put assembly instructions on the same line:

__asm push ebp   __asm mov  ebp, esp   __asm sub  esp, __LOCAL_SIZE

END Microsoft Specific

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