Bit
Fields
·
We have been using integer
words of size 16 bits to store data.
·
There are occasions where data
items require much less than 16 bits space, in such cases we waste memory space
·
C language permits us to use
small bit fields to hold data items and thereby to pack several data items in a
word of memory.
·
Bit fields allow direct
manipulation of strings of preselected bits as if it is represented as an
integral quantity.
·
A bit field is a set of
adjacent bits whose size can be from 1 to 16 bits in length.
·
A word can therefore be divided
into a number of bit fields.
·
The name and size of bit fields
are defined using a structure.
·
Bit field definition:
struct
tag-name
{
data-type
name 1 :bit-length;
data-type
name 2 :bit-length;
data type
name n:bit-length;
}
·
The data-type is either int or
unsigned int or signed int and the bit-length is the number of bits used for
the specified name.
·
Field name should be followed
by a colon.
·
Signed bit field should have at
least 2 bits (one bit for sign).
·
The bit-length is decided by
the range of values to be stored.
·
The largest value that can be
stored is 2n-1, where 'n' is the bit-length.
·
The internal representation of
a bit field is machine dependent i.e. it depends on the size of int and the
ordering of bits.
·
Some machines store bits from
left to right and others from right to left.
·
Here, the diagram below
illustrates the layout of bit fields assuming a 16-bit word that is ordered
from right to left( most significant bit in left most).
 |
i)
The first field always starts
with the first bit of the word.
ii)
A bit field can't overlap
integer boundaries. i.e. the sum of lengths of all the fields in a structure
should not be more than the size of a word. In case it is more, the overlapping
field is automatically forced to the beginning of the next word.
iii)
There can be unnamed fields
declared with size. Example: unsigned : bit - length Such fields provide
padding within the words.
iv)
There can be unused bits in a
word.
v)
We can't take the address of a
bit field variable it means that, Q we cannot use scanf to read values into bit
fields. 9 we can neither use pointers to access the bit fields.
vi) Bit fields cannot be arranged.
vii) Bit
fields should be assigned values within the range of their size. If we try to
assign larger values, behaviour would be unpredictable.
