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Bits
Since
VHDL
is
used
to
model
digital
systems,
it
is
useful
to
have
a
data
type
to
repre- sent
bit
values.
The
predefined
enumeration
type
bit
serves
this
purpose.
It
is defined
as
type
bit
is ('0',
'1');
The
logical
operators
that
we
mentioned
for Boolean
values
can also
be applied
to
values
of
type
bit,
and
they
produce
results
of
type
bit.
The
value
‘0’
corresponds
to
false,
and
‘1’
corresponds
to
true.
So,
for example:
'0'
and '1' = '0', '1'
xor
'1' = '0'
The
difference
between
the types
boolean
and
bit
is
that
boolean
values
are
used
to model
abstract
conditions,
whereas
bit
values
are
used
to
model
hardware
logic levels.
Thus,
‘0’ represents
a low
logic
level
and ‘1’
represents
a high
logic
level.
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