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CppAsm

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(C++) asm

Keyword enabling you to use assembler code.

Things inline Assembler CAN do:

• set/get global/local variables.
• set/get public class member variables [1]

Things inline Assmbler CANNOT do:

• call functions [?]
• call class methods [1]
• set/get global variables in other namespaces [?]

The code below makes your computer beep:

int main()
{
  asm
  {
    mov ax, 0x0E07
    xor bx,bx
    int 0x10
  }
  return 0;
}

Below some examples are given for C/C++ statements and how to achieve the same using assembler mnemonics.

ADD

include <cassert>


int main()
{
  int x = 1;
  int y = 2;
  int z = 0;
  asm
  {
    mov eax,x
    add eax,y
    mov z,eax
  }
  assert(z==3);
  return 0;
}

JMP

Performs an unconditional jump, similar to the keyword goto.

include <cassert>


int func1()
{
  const int x = 1;
  const int y = 2;
  //z cannot be made const (which is a good thing)
  //doing so gives the error 'operand size mismatch'
  int z = 0; 
  asm
  {
    jmp labelFunc1  //Jumps to labelFunc1
      mov eax,x     //Will be jumped over, stores the value of x in EAX
      jmp labelEnd  //Will be jumped over, jumps to labelEnd
    labelFunc1:     //Label
    mov eax,y       //Store the value of y in EAX
    labelEnd:       //Label
    mov z,eax       //Store the value of EAX in z
  };
  assert(z==y);
  return z;
}

Jumps can only be performed to labels in the same assembler code block. When trying to jump from another function to the one above, an error will be given:

int func2()
{
  int z;
  asm
  {
    jmp labelFunc1; //ERROR! Undefined label 'labelFunc1'
    mov z, eax;
    labelFunc2:
  }
  return z;
}

MOV

include <cassert>


int main()
{
  char x = 1;
  assert(x==1);
  asm mov x,0 //Set the value of x to 0, equal to 'x = 0;'
  assert(x==0);
  return 0;
}

'asm' links

•  ?C

References

• 1) MSDN: http://msdn2.microsoft.com/en-us/library/fabdxz08.aspx
• ?) Not sure

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