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Create a collection of C `#define` directives for all possible values that a three-bit flag can have, where the flag occupies the 3rd, 4th, and 5th bits (counting from 0 from the right) in a one-byte variable `char flags`.

Show C code that can:
1. Obtain the value of the flag.
2. Set it to one of the possible values.
3. Clear the flag (i.e., set it to `000`) without affecting the values of any other flags in the `flags` byte.

Answer :

Final answer:

This approach entails defining C macros for flag values, and using bitwise operations to perform flag manipulation. Specific functions or code segments obtain, set, and clear a 3-bit flag within a byte without disturbing other bits.

Explanation:

The task at hand is to create a collection of C #defines for the different values a three-bit flag can have within a one-byte variable in C, as well as to write code to manipulate these flags without affecting other bits. To accomplish this, bitwise operations are used.

Define Possible Flag Values

#define FLAG_VALUE_0 (0x0 << 2) // 000
#define FLAG_VALUE_1 (0x1 << 2) // 001
#define FLAG_VALUE_2 (0x2 << 2) // 010
#define FLAG_VALUE_3 (0x3 << 2) // 011
#define FLAG_VALUE_4 (0x4 << 2) // 100
#define FLAG_VALUE_5 (0x5 << 2) // 101
#define FLAG_VALUE_6 (0x6 << 2) // 110
#define FLAG_VALUE_7 (0x7 << 2) // 111

Obtain the Value of the Flag

unsigned char getFlagValue(unsigned char flags) {
return (flags >> 2) & 0x07;
}

Set the Flag to a Value

void setFlagValue(unsigned char *flags, unsigned char value) {
*flags &= ~(0x07 << 2); // Clear the flag bits
*flags |= (value & 0x07) << 2; // Set the flag bits
}

Clear the Flag

void clearFlag(unsigned char *flags) {
*flags &= ~(0x07 << 2); // Clear the flag bits
}

To achieve this, first, the possible flag values are defined using #define. The value of the flag is obtained using a combination of the right shift operator (>>) and bitwise AND (&) to isolate the relevant bits. To set the flag to a given value, bitwise AND with a mask clears the bits, then bitwise OR (|) sets the new value. Clearing the flag simply involves clearing the relevant bits using a mask with bitwise AND.

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