Std_Logic
Definition:
A ninevalue resolved logic type
Std_logic is not a part of the VHDL Standard. It is defined in IEEE
Std 1164.
Syntax:
type std_ulogic is
( 'U',  Uninitialized
'X',
 Forcing Unknown
'0',
 Forcing 0
'1',
 Forcing 1
'Z',
 High Impedance
'W',
 Weak Unknown
'L',
 Weak 0
'H',
 Weak 1
''
 Don't Care
);
type std_ulogic_vector is
array (natural range
<> ) of std_ulogic;
function resolved (s :
std_ulogic_vector ) return std_ulogic;
subtype std_logic is
resolved std_ulogic;
Description
The Std_ulogic type is an extension of the standard Bit type. It
defines nine values, which allow specifying logical systems. Like
Bit, this type is not resolved, i.e. it is not allowed to specify two
value assignments to a signal of the Std_ulogic type.
In order to facilitate specification of multipledriven signals (like
data buses) the Std_Logic_1164 package defines resolution function
for Std_ulogic, which in turn serves as a basis for declaration of
Std_Logic type.
The Std_Logic_1164 package defines overloaded logical operators
("and", "nand", "or", "nor",
"xor", and "not") for operands of the Std_ulogic
type. Moreover, two conversion functions are defined as well:
Std_ulogic to Bit (function To_Bit),
and Bit to Std_ulogic (function To_StdULogic).
Examples
Example 1
Signal FlagC : Std_Logic := 'Z';
ALU : process
begin
. . .
if Carry then
FlagC <= '1'; end if;
end process ALU;
Comm : process
begin
. . .
FlagC <= '0';
end process Comm;
Std_Logic is a resolved type, which means that multiple assignments
to the same object are legal. If FlagC was of the Std_Ulogic type,
such a code would not be acceptable.
Important Notes

Std_Logic is defined as a subtype of Std_ULogic, therefore all
operators and functions defined for Std_Ulogic can be applied to Std_Logic.

Std_Logic is the industry standard logic type and in practice
majority of signals are of this type (or its vector derivative,
Std_Logic_Vector type).
