Of course, in each case you don't have to use a variable to specify the mask you could just use a numeric literal.

For example instead of:

int flag = 0xFFFF;
int result = flag ^ mask;

you can write:

int result = flag ^ 0xFFFF;

Also, if you want to update the flag rather than derive a new result, you can use:

&=
|= 

and:

^= 

to perform the update directly.

For example, instead of:

int flag = flag ^ 0xFFFF;

you can use:

int flag ^=  0xFFFF;

To summarize:

• create a mask that has 1 at the position of each bit you want to change.

• OR the mask with the flag to set just those bits.

• AND the NOT of the mask to unset just those bits.

• XOR the mask to flip just those bits.

Bits in the mask that are 0 are unaffected by any of the operations.

Using Masks

What sorts of things do you use masking operations for?

Often a low-level API will require that particular bits in a status word are set or unset to make it operate in a particular way. Similarly, hardware registers are generally organized into groups of bits that you have to change to control the device.

A very common application is to extract the RGB color information from a single int representing the color of a pixel. For example:

int RGBcolor=0x010203;
int B=RGBcolor & 0x0000FF;
int G=RGBcolor & 0x00FF00;
int R=RGBcolor & 0xFF0000;

takes an RGB value and splits it up into its components using appropriate masks. The result is that you end up with 0x010000 stored in R, 0x000200 in G and 0x000003 in B. Notice that the value of B is correct, but R and G are incorrect - the bits need shifting to the right. This brings us to the use of the shift operators.

Not in this extract:

• Shifting Values
• Rotate
• Testing a Bit
• Endianism
• Some Examples
      Options
      Overflow Free Average
      Computing a Checksum

Summary

• There are four bitwise logical operators &, |, ^ and ~. These should not be confused with the logical operators &&, || and !.

• The bitwise logical operators behave differently on signed and unsigned types.

• A mask is a way of controlling which bits a bitwise operation affects.

• There are two shift operators: the << arithmetic or logical shift left and >> an arithmetic shift right.

• Arithmetic shift left corresponds to multiplying an integer type by 2 and an arithmetic shift right divides by 2.

• Arithmetic shift right isn't exactly the same as integer division by 2 for negative values as it rounds towards negative infinity.

• There is no rotate operator in C, but you can construct one.

• Masks can also be used to test the state of a bit or group of bits.

• There is no standard for the order in which parts of a multi-byte value are stored. Little endian stores the low-order bits first and big endian stores the high-order bits first.

• Harry Fairhead is the also the author of Applying C For The IoT With Linux, Raspberry Pi IoT in C, Raspberry Pi IoT In C Using Linux Drivers and  Micro:bit IoT in C 2ed. 

Related Articles

Remote C/C++ Development With NetBeans

Raspberry Pi And The IoT In C

Getting Started With C/C++ On The Micro:bit

Fundamental C: Getting Closer To The Machine

Now available as a paperback and ebook from Amazon.

1. About C
     Extract Dependent v Independent
                 & Undefined Behavior 
2. Getting Started With C Using NetBeans
3. Control Structures and Data
4. Variables
     Extract Variables
5. Arithmetic  and Representation
     Extract Arithmetic and Representation
6. Operators and Expression
     Extract: Expressions
     Extract Side Effects, Sequence Points And Lazy Evaluation
     First Draft of Chapter: Low Down Data
7. Functions Scope and Lifetime
8. Arrays
     Extract  Simple Arrays
     Extract  Ennumerations
9. Strings
     Extract  Simple Strings
     Extract: String I/O ***NEW!!
10. Pointers
      Extract  Starting Pointers
      Extract  Pointers, Cast & Type Punning
11. Structs
      Extract Basic Structs
      Extract Typedef
    
12. Bit Manipulation
      Extract Basic Bits
      Extract Shifts And Rotates 
    
13. Files
     Extract Files
     Extract Random Access Files 

14. Compiling C – Preprocessor, Compiler, Linker
     Extract Compilation & Preprocessor
    

Also see the companion volume: Applying C

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