Std logic

What is the difference betweenand X in the std logic type Synthesis results seem to be the same regardless of which one is used. 

IN stdjogic SIGNAL co OUT std logic ) is - pragma map Jo opcrator ADDl OP begin... 

There are many multivalued logic systems available in the VHDL environment. BIT is the simplest, but the IEEE library and tool vendor libraries contain others that are in common use and so can be considered portable. The IEEE STD LOGIC l 164 package contains the standard multivalued... 

The three-value logic system is dedaied as a subt) of ihe stdjogc type and called XOI. Its declaration is located in the IEEE package STD LOGIC l I64. 

The STD LOGIC l 164 package contains the industry-standard set of multivalued logic systems. The standard logic system comes in two forms ... 

The difference between resolved and unresolved types is discussed in Box 7.7. Of the two types declared in the STD LOGIC package, stdjogic is the one from which a variety of subt)q es are declared and sets the main standard. As already mentioned, the XOlZ is a subtype of this type, but others such as UXOlZ also exist. These can exploit the wide range of functions available for the type and at the same time maintain their portability. 

The std Jogic type is the resolved form of the std ulogic type and is well supported by the functions and operators declared in the IEEE library package STD LOGIC l 164. The stdJoglc vector is the array form of the type. This package must therefore be visible to each design file in the sequence follower. 

If a more comprehensive simulation is to be performed then the t5 bit is inadequate and XOIZ or std logic types must be used. The consequences of this are twofold. 

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