Bitwise Shifting in C

Question

I've recently decided to undertake an SMS project for sending and receiving SMS though a mobile.

The data is sent in PDU format - I am required to change ASCII characters to 7 bit GSM alphabet characters. To do this I've come across several examples, such as http://www.dreamfabric.com/sms/hello.html

This example shows Rightmost bits of the second septet, being inserted into the first septect, to create an octect.

Bitwise shifts do not cause this to happen, as >> will insert to the left, and << to the right. As I understand it, I need some kind of bitwise rotate to create this - can anyone tell me how to move bits from the right handside and insert them on the left?

Thanks,

Answer 1

Here is a quick algorithm to do that:

int b1, bnext;
int mod;
int pand;
char *b; // this is your byte buffer, with message content
int blen; // this is your byte buffer length
char sb[160];
int totchars = 0;

b1 = bnext = 0;
for (int i=0; i < blen; i++) {
    mod = i%7;
    pand = 0xff >> (mod + 1);
    b1 = ((b[i] & pand) << mod) | bnext;
    bnext = (0xff & b[i]) >> (7 - mod);
    sb[totchars++] = (char)b1;
    if (mod == 6) {
        sb[totchar++] = (char)bnext;
        bnext = 0;
    }
}
sb[totchar] = 0;

It will convert 7bit compressed buffers to regular ASCII char arrays in C.

Answer 2

can anyone tell me how to move bits from the right handside and insert them on the left?

There are indirect ways in C but I'd simply do it like this:

void main()
{
    int x = 0xBAADC0DE;
    __asm
    {
        mov eax, x;
        rol eax, 1;
    }
}

This will rotate (not shift!) the bits to the left (one step). "ror" will rotate to the right.