VHDL Programming

104 Chapter Four

disk where the file is located. (In most implementations this is true, but it

is not necessarily true.)

FILE TYPE EXAMPLES To read the contents of a file, you can call the

READprocedure within a loop statement. The loop statement can perform

read operations until an end of file is reached, at which time the loop is

terminated. Following is an example of a file read operation:

LIBRARY IEEE;

USE IEEE.std_logic_1164.ALL;

ENTITY rom IS

PORT(addr : IN INTEGER;

PORT(cs : IN std_logic;

PORT(data : OUT INTEGER);

END rom;

ARCHITECTURE rom OF rom IS

BEGIN

PROCESS(addr, cs)

VARIABLE rom_init : BOOLEAN := FALSE; --line 1

TYPE rom_data_file_t IS FILE OF INTEGER; --line 2

FILE rom_data_file : rom_data_file_t IS IN

“/doug/dlp/test1.dat”; --line 3

TYPE dtype IS ARRAY(0 TO 63) OF INTEGER;

VARIABLE rom_data : dtype; --line 4

VARIABLE i : INTEGER := 0; --line 5

BEGIN

IF (rom_init = false) THEN --line 6

WHILE NOT ENDFILE(rom_data_file) --line 7

AND (i < 64) LOOP

READ(rom_data_file, rom_data(i)); --line 8

i := i  1; --line 9

END LOOP;

rom_init := true; --line 10

END IF;

IF (cs = ‘ 1 ’) THEN --line 11

data <= rom_data(addr); --line 12

ELSE

data <= -1; --line 13

END IF;

END PROCESS;

END rom;

This example shows how a romcan be initialized from a file the first time

the model is executed and never again. A variable called rom_initis used

to keep track of whether the romhas been initialized or not. If false, the rom

has not been initialized; if true, the romhas already been initialized.