ASM Community

Hi there. Simple question. I'm looking for some pure C or C++ code that does what the RCR code below in ASM does. Is this possible? Perhaps a simple function "int RCR(int value, int rotateNr)"?

_asm
{
mov eax,val1
mov cl,al
rcr eax, cl
mov val1,eax
}

There have been proposed some rotation operators to C but as for now, no major compiler supports them. So the easiest, but of course not the best, way to add ror to our code is to define some clever macros.

#define ROR(x) x=((((unsigned)x)>>1)|((x&1)?(1<<((sizeof(x)*8)-1)):0))

Decent compilers with optimizations turned on should compile "ROR(x);" to simple ror instruction (at least Visual C++ 2008 detects these kinds of code of produces single 'ror'). Adding the support for carry flag is not trivial. But you can define a variable which will emulate the carry flag:

#define RCR(x, c) x=((((unsigned)x)>>1)|((c&1)?(1<<((sizeof(x)*8)-1)):0)|((c=(x&1))&0))

This way it'll work on machines that don't have the carry flag.

And then you use it like:

int a=7;
ROR(a);

// ...

char b=3, c;

c = initial_value_for_our_carry;
RCR(b, c);
// here our carry contains modified value

I haven't tested it so there may be errors.