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91 lines
5.3 KiB
Text
91 lines
5.3 KiB
Text
{{DISPLAYTITLE:_BIT}}
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The [[_BIT]] datatype can return only values of 0 (bit off) and -1 (bit on).
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{{PageSyntax}}
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: [[DIM]] {{Parameter|variable}} [[AS]] [{{KW|_UNSIGNED}}] [[_BIT]] [* {{Parameter|numberofbits}}]
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: [[_DEFINE]] {{Parameter|Letter}}[{{Parameter|-Range}}|,...] [[AS]] [{{KW|_UNSIGNED}}] [[_BIT]] [* {{Parameter|numberofbits}}]
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{{PageDescription}}
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* An [[_UNSIGNED]] _BIT can hold 0 or 1 instead of 0 and -1, if you set the numberofbits you can hold larger values depending on the number of bits you have set (_BIT * 8 can hold the same values as [[_BYTE]] for example) and the information below is compromised if setting any number of bits other than 1.
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* If you set the variable to any other number then the least significant bit of that number will be set as the variables number, if the bit is 1 (on) then the variable will be -1 and if the bit is 0 (off) then the variable will be 0.
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*The least significant bit is the last bit on a string of bits (11111) since that bit will only add 1 to the value if set. The most significant bit is the first bit on a string of bits and changes the value more dramatically (significantly) if set on or off.
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*The _BIT datatype can be succesfully used as a [[Boolean]] (TRUE or FALSE) and it requires minimal amount of memory (the lowest amount possible actually, one byte can hold 8 bits, if you want to use bits in order to decrease memory usage, use them as arrays as a _BIT variable by itself allocates 4 bytes - DIM bitarray(800) AS _BIT uses 100 bytes).
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* '''When a variable has not been assigned or has no type suffix, the value defaults to [[SINGLE]].'''
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* '''[[Keywords_currently_not_supported_by_QB64|_BIT is not supported in User Defined TYPES.]]''' Use a [[_BYTE]] and assign up to 8 bit values as shown below.
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*'''Suffix Symbols''' The [[_BIT]] type suffix used is below the grave accent (`), usually located under the tilde (~) key (not an apostrophe). Foreign keyboards may not have the ` key. Try Alt+96 in the IDE.
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:You can define a bit on-the-fly by adding a ` after the variable, like this: {{InlineCode}}variable` = -1{{InlineCodeEnd}}
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:If you want an unsigned bit you can define it on-the-fly by adding ~` instead, like this: {{InlineCode}}variable~` = 1{{InlineCodeEnd}}
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:You can set the number of bits on the fly by just adding that number - this defines it as being two bits: {{InlineCode}}variable`2 = -1{{InlineCodeEnd}}
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<center>'''[[_BIT|BITS]]'''</center>
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* The '''MSB''' is the most significant(largest) bit value and '''LSB''' is the least significant bit of a binary or register memory address value. The order in which the bits are read determines the binary or decimal byte value. There are two common ways to read a byte:
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:* '''"Big-endian"''': MSB is the first bit encountered, decreasing to the LSB as the last bit by position, memory address or time.
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:* '''"Little-endian"''': LSB is the first bit encountered, increasing to the MSB as the last bit by position, memory address or time.
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{{WhiteStart}}
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'''Offset or Position: 0 1 2 3 4 5 6 7 Example: 11110000'''
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---------------------------------- --------
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'''Big-Endian Bit On Value:''' 128 64 32 16 8 4 2 1 240
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'''Little-Endian Bit On Value:''' 1 2 4 8 16 32 64 128 15
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{{WhiteEnd}}
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::The big-endian method compares exponents of 2 <sup>7</sup> down to 2 <sup>0</sup> while the little-endian method does the opposite.
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<center>'''[[_BYTE|BYTES]]'''</center>
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* [[INTEGER]] values consist of 2 bytes called the '''HI''' and '''LO''' bytes. Anytime that the number of binary digits is a multiple of 16 (2bytes, 4 bytes, etc.) and the HI byte's MSB is on(1), the value returned will be negative. Even with [[SINGLE]] or [[DOUBLE]] values!
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{{WhiteStart}} '''16 BIT INTEGER OR REGISTER'''
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'''AH (High Byte Bits) AL (Low Byte Bits)'''
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BIT: 15 14 13 12 11 10 9 8 | 7 6 5 4 3 2 1 0
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---------------------------------------|--------------------------------------
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HEX: 8000 4000 2000 1000 800 400 200 100 | 80 40 20 10 8 4 2 1
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DEC: -32768 16384 8192 4096 2048 1024 512 256 | 128 64 32 16 8 4 2 1
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{{WhiteEnd}}
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::The HI byte's '''MSB''' is often called the '''sign''' bit! When all 16 of the integer binary bits are on, the decimal return is -1.
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{{PageExamples}}
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''Example:'' Shifting bits in a value in QB64 versions prior to 1.3 (you can use [[_SHL]] and [[_SHR]] starting with version 1.3).
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{{CodeStart}} '' ''
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n = 24
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Shift = 3
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{{Cl|PRINT}} LShift(n, Shift)
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{{Cl|PRINT}} RShift(n, Shift)
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{{Cl|END}}
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{{Cl|FUNCTION}} LShift&(n AS {{Cl|LONG}}, LS AS {{Cl|LONG}})
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IF LS < 0 THEN {{Cl|EXIT FUNCTION}}
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LShift = {{Cl|INT}}(n * (2 ^ LS))
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{{Cl|END FUNCTION}}
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{{Cl|FUNCTION}} RShift&(n AS {{Cl|LONG}}, RS AS {{Cl|LONG}})
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IF RS < 0 THEN {{Cl|EXIT FUNCTION}}
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RShift = {{Cl|INT}}(n / (2 ^ RS))
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{{Cl|END FUNCTION}} '' ''
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{{CodeEnd}}
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{{small|Adapted from code by RThorpe}}
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{{OutputStart}} 192
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3
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{{OutputEnd}}
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{{PageSeeAlso}}
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* [[&B]] (binary), [[_BYTE]]
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* [[_SHL]], [[_SHR]]
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* [[_DEFINE]], [[_UNSIGNED]]
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* [[DIM]]
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* [[Binary]], [[Boolean]]
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* [[Variable Types]]
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* [[Converting Bytes to Bits]]
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{{PageNavigation}}
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