VHDL Programming

Data Types 97

PROCESS(X)

TYPE fifo_element_t IS ARRAY(0 TO 3)

OF std_logic; --line 1

TYPE fifo_el_access IS

ACCESS fifo_element_t; --line 2

VARIABLE fifo_ptr : fifo_el_access := NULL; --line 3

VARIABLE temp_ptr : fifo_el_access := NULL; --line 4

BEGIN

temp_ptr := new fifo_element_t; --Ok line 5

temp_ptr.ALL := (‘ 0 ’, ‘ 1 ’, ‘ 0 ’, ‘ 1 ’); --Ok line 6

temp_ptr.ALL := (‘ 0 ’, ‘ 0 ’, ‘ 0 ’, ‘ 0 ’); --Ok line 7

temp_ptr.ALL(0) := ‘ 0 ’; --Ok line 8

fifo_ptr := temp_ptr; --Ok line 9

fifo_ptr.ALL := temp_ptr.ALL; --Ok line 10

END PROCESS;

In line 2, an access type is declared using the type declared in line 1.

Lines 3 and 4 declare two access type variables of fifo_el_accesstype

from line 2. This process now has two access variable objects that can be

used to access objects of type fifo_element_t.

Line 5 calls the predefined function NEW, which allocates enough memory

for a variable of type fifo_element_tand returns an access value to

the memory allocated. The access value returned is then assigned to

variable temp_ptr. Variable temp_ptris now pointing to an object of type

fifo_element_t. This value can be read from or assigned to using variable

assignment statements.

In line 6, a value is assigned to the object pointed to by temp_ptr. Line

7 shows another way to assign a value using an access value. The key-

word .ALLspecifies that the entire object is being accessed. Subelements

of the object can be assigned by using a subelement name after the access

variable name. Line 8 shows how to reference a subelement of an array

pointed to by an access value. In this example, the first element of the

array will have a value assigned to it.

In the next few statements, we examine how access values can be

copied among different objects. In line 9, the access value of temp_ptris

assigned to fifo_ptr. Now both temp_ptrand fifo_ptrare pointing to

the same object. This is shown in Figure 4-3.

Both temp_ptrand fifo_ptrcan be used to read from and assign to

the object being accessed.

Line 10 shows how one object value can be assigned to another using

access types. The value of the object pointed to by temp_ptris assigned

to the value pointed to by fifo_ptr.