Using mask in bit manipulations...

Hi,

In bit manipulations using bitwise operators, what is a mask and why is it needed?

thnx

say you have a program that if the user whats a particular option on

you could use a boolean variable for this or you ould use "bit flags"

where ase each bit of say an int represents a flag

bit 0 is one thing on or off
bit 1 is something else

if you wanted to test this values you would "Mask" out the other bts in the int like so

[Code]

#include <stdio.h>
#define OPTION 0x01

int main(void)
{
char ch = 0;
int flags = 0;

printf("Hit a Key To Toggle the Bit Flag On Hit 'o' to turn it off\n");
if((ch = getchar()) != 'o')
flags |= OPTION; // set the first bit or our option flag on
else
flags &= ^OPTION; // set the Option flag off

if(flags & OPTION)
printf("The Option is Enabled!");

return 0;
}

forgot to end my code tag.

Code:

---


#include <stdio.h>
#define OPTION 0x01

int main(void)
{
    char ch = 0;
    int flags = 0;

    printf("Hit a Key To Toggle the Bit Flag On Hit 'o' to turn it off\n");
    if((ch = getchar()) != 'o')
        flags |= OPTION; // set the first bit or our option flag on
    else
        flags &= ^OPTION;  // set the Option flag off

    if(flags & OPTION)
      printf("The Option is Enabled!");

    return 0;
}

---

keep getting loged out...

So if i am thinking correct, the mask can be anything to use in different situations, but it's used usually with AND right?

thnx

yup.

>So if i am thinking correct, the mask can be anything to use in
>different situations, but it's used usually with AND right?

The reason why an AND is used is that

0 AND 0 = 0
0 AND 1 = 0
1 AND 0 = 0
1 AND 1 = 1

So if you have the byte

1101:0110

And you want to test if bit 3 is set (i.e. is equal to 1), then you can define the mask

0000:1000

And then AND the mask with the byte, which results in

0000:0000

So bit 3 is not set.

Code:

---

byte = 0xD6;
mask = 0x08;

if ((byte & mask) > 0)
    /* bit 3 is set */

---

it's useful for saving space. 1 char = 8 bits.
i think OR can be used to set bits. set a mask, like

Code:

---

0010 0001, OR
0100 0011 and you get
0110 0011.

---

i've only seen xor used in encripting. i remember something called eqv(in basic i think), but i've forgotten what it was. can someone remind me?

Use XOR if you want to switch bits (1->0, 0->1)

0 XOR 0 = 0
0 XOR 1 = 1
1 XOR 0 = 1
1 XOR 1 = 0

0100:1101 XOR 0000:1111 would produce 0100:0010