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## Re: [avr-gcc-list] Checking carry flag and Rotate operation in C.

 From: Andy H Subject: Re: [avr-gcc-list] Checking carry flag and Rotate operation in C. Date: Sun, 15 Jun 2008 20:45:00 -0400 User-agent: Thunderbird 2.0.0.14 (Windows/20080421)

The simple answer us that you cant. Thougg we could do with this in library and/or gcc patterns (builtin rotate)
```
This is close:

unsigned char foo(unsigned char b)
{
if (b & 128)
{
b  <<= 1;
b ^= 0b00011101;
}
else
{
b<<=1;
}
return b;
}

unsigned char bar(unsigned char b)
{
if (b & 128)
{
b ^= 0b0001110;
b <<= 1;
b |= 1;
}
else
{
b<<=1;
}
return b;
}

Jonathan Blanchard wrote:
```
```Hi,

I got two question about programming with AVR-GCC. Both are related to
finding a way to generate a specific output in assembler.

First, how do you create the rotate operation in C. Specifically how
can the ROL and ROR can be generated.

Secondly I have this piece of code where b is a 16 bit unsigned integer :

b = b << 1;
if( b & 256 )
b = b ^ 0b100011101;

To optimize that I only need to check if b overflow at the left shift
operation by checking the carry flag. I'm trying to find a way to do
that in C. Right now I'm using the following piece of inline assembly
to do the trick :

asm volatile(

"LSL %0     \n\t"
"BRCC 1f    \n\t"
"EOR %0, %1 \n\t"
"1:""           \n\t"
:"+d" (b)
:"r"  (PPoly)
```
);
```
In this last piece of code b is a 8 bit unsigned integer and PPoly is
a 8 bit unsigned integer with the value 0b00011101. I'm just curious
to know if it's possible to achieve the same result only by using C.

Jonathan Blanchard

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```