Bit flags

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Definition

Bit flags are constants which each denotes a single unique case in one situation and can be combined to form different, unique cases. They are called bit flags, because when bits are used to denote a cases, we observe they are indeed single and unique and can be combined to form different unique combinations.

Bit flags are often used as integers:

Bit pattern - Integer value
0001 - 1
0010 - 2
0100 - 4
1000 - 8

These can be combined to form, for example:

Bit pattern - Integer value
1001 - 9
0110 - 6
1110 - 14
0101 - 5

Example

When we look at blit flags for example, we see the values:

Constant - Value - Description
B_HMIRROR - 1 - Blit the graph horizontally mirrored.
B_VMIRROR - 2 - Blit the graph vertically mirrored.
B_TRANSLUCENT - 4 - Blit the graph with half transparency.
B_ALPHA - 8 - Blit the graph in some way. (What does this do exactly?)
B_ABLEND - 16 - Blit the graph using additive blending (nice effect for fire).
B_SBLEND - 32 - Blit the graph using subtractive blending (nice effect for ghosting).
B_NOCOLORKEY - 128 - Blit the transparent parts of the graph as black.

These are all single unique cases and can be combined to form different unique cases. For example, when we want a translucent, horizontally mirrored blit operation with use of additive blending, we would do:

B_HMIRROR | B_TRANSLUCENT | B_ABLEND
1         | 4             | 16       = 21

Because the bits are unique, the addition operator can also be used. But when we consider there would be a constant called B_HVMIRROR, which has the value B_HMIRROR|B_VMIRROR (3), the addition operator can't be used all the time:

B_HMIRROR | B_HVMIRROR == 1 | 3 == 3
B_HMIRROR + B_HVMIRROR == 1 + 3 == 4

In conclusion, use the bor operator when dealing with bit flags to be on the safe side.

Usage Example

Process main()
Private
    unsigned byte options;
Begin

    /* Manipulate bit flags */

    // To set options, use = (the bits with 1's will be targeted)
    options = 01000001b;

    // To add options, use | (the bits with 1's will be targeted)
    options |= 00000001b;

    // To remove options, use & and ~ (the bits with 1's will be targeted)
    options &= ~00010000b;
    // same: options &= 11101111h (the bits with 0's will be targeted)

    // To switch options, use ^ (the bits with 1's will be targeted)
    options ^= 10000000b;

    /* Manipulate bit flags using the << operator (these are equivalent to the last three) */

    options|= 1<<0; // position 0 is now ON
    options&= ~(1<<4); // position 4 is now OFF
    options^= (1<<7); // position 7 is now SWITCHED

    /* Checking of bit flags */

    // Check if a certain option is set (the bits with 1's will be checked)
    if((options&10000000b)==10000000b)
    end

    // Check if a certain option is not set (the bits with 1's will be checked)
    if((options&10000000b)==0)
    end

    // Check if multiple options are set (the bits with 1's will be checked)
    if((options&10000001b)==10000001b)
    end

    // Check if multiple options are not set (the bits with 1's will be checked)
    if((options&00000110b)==0)
    end

    // Check if at least one of multiple options is set (the bits with 1's will be checked)
    if((options&00000110b)!=0)
    end

    // Check if at least one of multiple options is not set (the bits with 1's will be checked)
    if((options&00000110b)!=00000110b)
    end

    /* Checking of bit flags using the << operator */

    // Check if a certain option is set
    if((options&(1<<7))==(1<<7)) // position 7 is ON
    end

    // Check if a certain option is not set
    if((options&(1<<7))==0) // position 7 is OFF
    end

End

Used in example: Bitwise Operators