VHDL Programming

102 Chapter Four

Access types are very powerful tools for modeling complex and abstract

types of systems. Access types bring programming language types of

operations to VHDL processes.

File Types

A file type allows declarations of objects that have a type FILE. A file object

type is actually a subset of the variable object type. A variable object can be

assigned with a variable assignment statement, while a file object cannot

be assigned. A file object can be read from, written to, and checked for end

of file only with special procedures and functions.

Files consist of sequential streams of a particular type. A file whose

base object type is INTEGERconsists of a sequential stream of integers.

This is shown in Figure 4-10.

A file whose object type is a complex record type consists of a sequential

stream of complex records. An example of how this might look is shown

in Figure 4-11.

At the end of the stream of data is an end-of-file mark. Two procedures

and one function allow operations on file objects:

READ(file, data)Procedure

Stack Element

Data

NXT

Stack Element

Data

NXT

Stack Element

Data

NXT

List_Head

Temp_Elem

Figure 4-8
Move Head Pointer
to Next Element.

Stack Element

Data

NXT

Stack Element

Data

NXT

List_Head

Temp_Elem

Figure 4-9
Deallocate Element.