_BIT

The _BIT datatype can return only values of 0 (bit off) and -1 (bit on).

Syntax

DIM variable AS [[[:Template:KW]]] _BIT [* numberofbits]

_DEFINE Letter[-Range|,...] AS [[[:Template:KW]]] _BIT [* numberofbits]

Description

• 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.
• 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.
• 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.
• 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).
• When a variable has not been assigned or has no type suffix, the value defaults to SINGLE.
• _BIT is not supported in User Defined TYPES. Use a _BYTE and assign up to 8 bit values as shown below.

• 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.

You can define a bit on-the-fly by adding a ` after the variable, like this: variable` = -1

If you want an unsigned bit you can define it on-the-fly by adding ~` instead, like this: variable~` = 1

You can set the number of bits on the fly by just adding that number - this defines it as being two bits: variable`2 = -1

BITS

• 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:

• "Big-endian": MSB is the first bit encountered, decreasing to the LSB as the last bit by position, memory address or time.
• "Little-endian": LSB is the first bit encountered, increasing to the MSB as the last bit by position, memory address or time.

        Offset or Position:    0    1   2   3   4   5   6   7      Example: 11110000
                             ----------------------------------             --------
   Big-Endian Bit On Value:   128  64  32  16   8   4   2   1                 240
Little-Endian Bit On Value:    1    2   4   8  16  32  64  128                 15

The big-endian method compares exponents of 2⁷ down to 2⁰ while the little-endian method does the opposite.

BYTES

• 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!

Template:WhiteStart 16 BIT INTEGER OR REGISTER

             AH (High Byte Bits)                         AL (Low Byte Bits)
  BIT:    15    14   13   12   11   10   9   8  |   7   6    5   4    3    2   1    0
         ---------------------------------------|--------------------------------------
  HEX:   8000  4000 2000 1000  800 400  200 100 |  80   40  20   10   8    4   2    1
                                                |
  DEC: -32768 16384 8192 4096 2048 1024 512 256 | 128   64  32   16   8    4   2    1

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.

Examples

Example: Shifting bits in a value in QB64 versions prior to 1.3 (you can use _SHL and _SHR starting with version 1.3).

n = 24 Shift = 3 PRINT LShift(n, Shift) PRINT RShift(n, Shift) END FUNCTION LShift&(n AS LONG, LS AS LONG) IF LS < 0 THEN EXIT FUNCTION LShift = INT(n * (2 ^ LS)) END FUNCTION FUNCTION RShift&(n AS LONG, RS AS LONG) IF RS < 0 THEN EXIT FUNCTION RShift = INT(n / (2 ^ RS)) END FUNCTION

Adapted from code by RThorpe

192 3

See also

• &B (binary), _BYTE
• _SHL, _SHR
• _DEFINE, _UNSIGNED
• DIM
• Binary, Boolean
• Variable Types
• Converting Bytes to Bits

---

Navigation: Main Page with Articles and Tutorials Keyword Reference - Alphabetical Keyword Reference - By usage Report a broken link