VHDL Programming

96 Chapter Four

packet.data(10)(4) := ‘ 1 ’; --error line 4

packet.data(10)(0) := ‘ 1 ’; --Ok line 5

END PROCESS;

This example shows how complex record types are accessed. In line 1,

a record field of a record is accessed. Field keyis a record field of record

addr_type, which is a field of record data_packet. This line assigns the

value 5 to that field. Line 2 assigns an aggregate to the whole field called

addrin record data_packet.

In line 3, the data field is assigned an aggregate for the 0th element

of the array. Line 4 tries to assign to only one bit of the eleventh ele-

ment of the data array field in record data_packet, but the second index

value is out of range. Finally, line 5 shows how to assign to a single bit

of the array correctly.

Composite types are very powerful tools for modeling complex and

abstract data types. By using the right combination of records and arrays,

you can make models easy to understand and efficient.

ACCESS TYPES Most hardware design engineers using VHDL

probably never use access types directly (a hardware designer may use

the TextIO package, which uses access types, thereby an indirect use of

access types), but access types provide very powerful programming lan-

guage type operations. An access type in VHDL is very similar to a

pointer in a language like Pascal or C. It is an address, or a handle, to

a specific object.

Access types allow the designer to model objects of a dynamic nature. For

instance, dynamic queues, fifos, and so on can be modeled easily using

access types. Probably the most common operation using an access type

is creating and maintaining a linked list.

Only variables can be declared as access types. By the nature of access

types, they can only be used in sequential processing. Access types are

currently not synthesizable because they are usually used to model the

behavior of dynamically sized structures such as a linked list.

When an object is declared to be of an access type, two predefined functions

are automatically available to manipulate the object. These functions are

named NEWand DEALLOCATE. Function NEWallocates memory of the size of

the object in bytes and returns the access value. Function DEALLOCATEtakes

in the access value and returns the memory back to the system. Following

is an example that shows how this all works: