VHDL Programming

98 Chapter Four

Fifo Element

fifo_ptr

temp_ptr

Figure 4-3
Multiple Access Type
References.

Incomplete Types

When implementing recursive structures such as linked lists, you need

another VHDL language feature to complete the declarations. This feature

is called the incomplete type. The incomplete type allows the declaration

of a type to be defined later.

Following is an example that demonstrates why this would be useful:

PACKAGE stack_types IS

TYPE data_type IS ARRAY(0 TO 7) OF std_logic; --line 1

TYPE element_rec; --incomplete type line 2

TYPE element_ptr IS ACCESS element_rec; --line 3

TYPE element_rec IS --line 4

RECORD --line 5

data : data_type; --line 6

nxt : element_ptr; --line 7

END RECORD; --line 8

END stack_types;

USE WORK.stack_types.ALL;

ENTITY stack IS

PORT(din : IN data_type;

clk : IN std_logic;

dout : OUT data_type;

r_wb : IN std_logic);

END stack;

ARCHITECTURE stack OF stack IS

BEGIN

PROCESS(clk)

VARIABLE list_head : element_ptr := NULL; --line 9

VARIABLE temp_elem : element_ptr := NULL; --line 10

VARIABLE last_clk : std_logic := U; --line 11

BEGIN

IF (clk = ‘ 1 ’) AND (last_clk = ‘ 0 ’) THEN --line 12

IF (r_wb = ‘ 0 ’) THEN --line 13

temp_elem := NEW element_rec; --line 14

temp_elem.data := din; --line 15