Enumeration type declaration

The enumeration literal can be an identifier (e.g. RED, Blue, green) or a character literal (e.g. V, X, ). Eadi must be unique within the type declaration. Identifiers (simple names) are discussed in detail in Chapter 4. 

The same enumeration literal can appear in more than one type declaration. In such cases it may be necessary to clarify the e being used in a particular context. A qualified expression can be used for this purpose. 

An object mtist be declared to be certain type. Integer and enumeration types are available. These can be formed from a single element - a scalar type - or multiple elements - a composite type. A composite type can be organized as an array or a record. 

The synthesizer sees 1001 and 1100 as arrays of flie enumerate type BIT, not as a direct and interchangeable representation of the discrete integer values 9 and 12. Note however that if S was declared as enumerate type CODES, which itself was defined as ... 

As the above example demonstrates, ambiguous expressions that need qualifying usually occur because the same enumeration literal(s) occurs in more than one type declaration. 

Thus, the Decoder has been implemented as a component. Figure 7.4 shows the VHDL description of this circuit, which demonstrates how to construct a state machine. It is always advisable to use enumerate type encoding for states as shown in the architecture s declarative part. This example has eight states, inferring three storage elements and using all possible permutations of 0 and I. The present state is stored on a signal PRESENTSTATE that is visible only to this architecture. 

SUB" is an enumeration literal defined in type "CHARACTER" in the package "STANDARD" which is implicitly visible in all VHDL designs. Since this definition as well as the component declaration for "SUB" are both visible in the code, a homograph is reported. You can explicitly avoid this ambiguity as follows... 

A hardware implementation can only represent values as groups of Is and Os. This is irrespective of the type - numeric, enumeration, etc. An integer value of 23, for example, will be represented by the binary pattern 10111. This does not mean that the designer can access individual bits of this string, as its VHDL type is INTEGER. To access these bits an object must be declared an appropriate type, such as BIT VECTOR. 

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