reading certain bits from a byte

pretend my char is 85 (01010101)
if I only want to read the last 3 bits (010...) from the char and use it as a separate value, how do I do it?
since 010 the result would be 2, I know the value of the bits are found at 0x80, 0x40, and 0x20.
any one know how to do this?

Combination of bit masking and shifting.

Code:

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#include <iostream>

int main() {
        int test=85;
        std::cout << ((test & (0x80 | 0x40 | 0x20)) >> 5) << std::endl;
        return 0;
}

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0x80 | 0x40 | 0x20 is 0xE0, just shown like that to see where it comes from.

SilentStrike,

How does '>> 5' work?

I understand what you're doing up the point of >> 5. The first part, (test & (0x80 | 0x40 | 0x20)), should return 01000000b. Judging by the output, I'm guessing that '>> 5' shifts everything to the right by 5 bits.

fletch

You can use bitfields to manipulate specific bits:

Code:

---

#include <iostream>

using namespace std;

typedef struct
{
    unsigned int bit0:1;
    unsigned int bit1:1;
    unsigned int bit2:1;
    unsigned int bit3:1;

    unsigned int bit4:1;
    unsigned int bit5:1;
    unsigned int bit6:1;
    unsigned int bit7:1;
} bitfield_s;

typedef union
{
    bitfield_s bitfield;
    unsigned char byte;
} byte_u;

int main ()
{
    byte_u var;

    var.byte = 0xBA;

    cout << "bit 0: " << var.bitfield.bit0 << endl;
    cout << "bit 6: " << var.bitfield.bit6 << endl;
   
    return 0;
}

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Quote:

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I understand what you're doing up the point of >> 5. The first part, (test & (0x80 | 0x40 | 0x20)), should return 01000000b. Judging by the output, I'm guessing that '>> 5' shifts everything to the right by 5 bits.

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That's exactly what it does.

okay, but what if I only want to read the first 3 bits of the byte (0x01, 0x02, and 0x04) and use it as a separate value?

Code:

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int test1, test2;

test1 = 85;
test2 = ((test1 & 0xE0) >> 5);

std::cout << "test1 = " << test1 << std::endl;        // 85
std::cout << "test2 = " << test2 << std::endl;        // 2

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