IEEE 1164

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