Bitwise Operators
Bitwise operators are much like the regular mathematics operators (+, * etc.) but are defined in terms of the individual bits of their operands. The full list of bitwise operators, with a brief summary of its operation:
- NOT: Invert all bits
- AND: True if both inputs are true
- OR: True if one or both inputs are true
- XOR: True if exactly one input is true
- EQV: True if both inputs are the same
- IMP: True unless first input is true and second is false
Syntax
With the exception of NOT, all the bitwise operators take two operands:
- result = value1 AND value2
NOT goes before the value it operates on:
- result = NOT value1
If value1 or value2 are non-integer numeric types, they are rounded to the nearest integer.
Description
Bitwise operators work by comparing the corresponding bits in each of the input values to generate a single bit in the output value. The operators differ in how they do the comparison. The table below shows the output bit for each pair of input bits:
Table 4: The logical operations and its results.
In this table, A and B are the Expressions to invert or combine.
Both may be results of former Boolean evaluations.
┌────────────────────────────────────────────────────────────────────────┐
│ Logical Operations │
├───────┬───────┬───────┬─────────┬────────┬─────────┬─────────┬─────────┤
│ A │ B │ NOT B │ A AND B │ A OR B │ A XOR B │ A EQV B │ A IMP B │
├───────┼───────┼───────┼─────────┼────────┼─────────┼─────────┼─────────┤
│ true │ true │ false │ true │ true │ false │ true │ true │
├───────┼───────┼───────┼─────────┼────────┼─────────┼─────────┼─────────┤
│ true │ false │ true │ false │ true │ true │ false │ false │
├───────┼───────┼───────┼─────────┼────────┼─────────┼─────────┼─────────┤
│ false │ true │ false │ false │ true │ true │ false │ true │
├───────┼───────┼───────┼─────────┼────────┼─────────┼─────────┼─────────┤
│ false │ false │ true │ false │ false │ false │ true │ true │
└───────┴───────┴───────┴─────────┴────────┴─────────┴─────────┴─────────┘
Note: In most BASIC languages incl. QB64 these are bitwise operations,
hence the logic is performed for each corresponding bit in both
operators, where true or false indicates whether a bit is set or
not set. The outcome of each bit is then placed into the respective
position to build the bit pattern of the final result value.
As all Relational Operations return negative one (-1, all bits set) for
true and zero (0, no bits set) for false, this allows us to use these
bitwise logical operations to invert or combine any relational checks,
as the outcome is the same for each bit and so always results into a
true (-1) or false (0) again for further evaluations.
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Again, note that the NOT operator only has one operand. It is shown in the same table for convenience.
If one input has more bits than the other (say, an INTEGER vs a LONG) the shorter will be considered to have 0's in the missing bit positions if it is positive, or 1's if it is negative. This scheme comes about because of the Two's Complement system for representing negative numbers. As a general rule, there should not be any surprises.
Use as logical operators
QB64 does not have AND/OR/NOT operators dedicated to operating on the overall truth of values. A numeric value is defined to be false if it is equal to 0, and true for any other value, though -1 is the standard true value, returned by the <, <= etc. operators. One can use the bitwise operators mostly like regular logical operators, but with caution. For instance, 3 is a true value, so as a logical operator NOT 3 would be 0 (false). Because it is in fact a bitwise operator, it evaluates to -4.
Examples
- Example 1
- Use AND to mask certain bits in a value. In this example, the 1's in the mask (y&) specify which bits in (x&) we are interested in, forcing all others to 0.
x& = VAL("&B101010") 'Arbitrary collection of bits y& = VAL("&B001100") 'A bit mask PRINT "Input 1: "; BinStr$(x&, 6) '6 indicates we want 6 bits of output PRINT "Input 2: "; BinStr$(y&, 6) PRINT "Output: "; BinStr$(x& AND y&, 6) 'Converts the number n& to a string of binary digits, digits& long (padding or truncating as necessary). FUNCTION BinStr$ (n&, digits&) FOR i& = digits& - 1 TO 0 STEP -1 IF (n& AND 2 ^ i&) THEN B$ = B$ + "1" ELSE B$ = B$ + "0" NEXT BinStr$ = B$ END FUNCTION |
Input 1: 101010 Input 2: 001100 Output: 001000 |
- Example 2
- Use OR to combine bit flags into a single value. The presence of a flag can then be tested by using the flag as a mask with AND.
'The trick here is to give each flag a value corresponding to a different bit being 1 FLAG_A& = VAL("&B0001") FLAG_B& = VAL("&B0010") FLAG_C& = VAL("&B0100") FLAG_D& = VAL("&B1000") flags& = FLAG_A& OR FLAG_C& 'Set flags A, C 'Use each flag as a bitmask to test for its presence: IF flags& AND FLAG_A& THEN PRINT "Flag A is set" IF flags& AND FLAG_B& THEN PRINT "Flag B is set" IF flags& AND FLAG_C& THEN PRINT "Flag C is set" IF flags& AND FLAG_D& THEN PRINT "Flag D is set" |
Flag A is set Flag C is set |
- Example 3
- Use XOR to toggle a bit flag (that is, change its state to the opposite of what it was). This example is the same as the OR example above, but with one extra line added. This time we enable flags A & C, then toggle flags A & B. This will disable flag A and enable B.
'The trick here is to give each flag a value corresponding to a different bit being 1 FLAG_A& = VAL("&B0001") FLAG_B& = VAL("&B0010") FLAG_C& = VAL("&B0100") FLAG_D& = VAL("&B1000") flags& = FLAG_A& OR FLAG_C& 'Set flags A, C flags& = flags& XOR FLAG_A& XOR FLAG_B& 'Toggle flags A, B 'Use each flag as a bitmask to test for its presence: IF flags& AND FLAG_A& THEN PRINT "Flag A is set" IF flags& AND FLAG_B& THEN PRINT "Flag B is set" IF flags& AND FLAG_C& THEN PRINT "Flag C is set" IF flags& AND FLAG_D& THEN PRINT "Flag D is set" |
Flag B is set Flag C is set |