IEEE 1164
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The IEEE Standard 1164 defines a package design unit that contains declarations that support a uniform representation of a logic value in a VHDL hardware description. The standardization effort was based on the donation of the Synopsys MVL-9 type declaration.
The primary data type std_ulogic (standard unresolved logic) consists of nine character literals in the following order:
'U' - uninitialized
'X' - strong drive logic unknown
'0' - strong drive logic zero
'1' - strong drive logic one
'Z' - strong high impedance
'W' - Weak drive unknown
'L' - weak drive logic zero
'H' - weak drive logic one
'-' - Don't care
This system promoted a useful set of logic values that typical CMOS logic design could utilize in the vast majority of modeling situations. The 'Z' literal makes tri-state logic easy. The 'W' and 'L' weak drives permit wire-and and wire-or logic. Additionally, the 'U' state is the default value for all object declarations so that during simulations unitialized values are easily detectable and thus easily corrected if necessary.
The hardware designer makes the declarations visible via the following library and use statements:
library ieee; use ieee.std_logic_1164.all;