VHDL Programming

Data Types 91

value := ((e1, e2,..., en), --E1

value := ((e1, e2,..., en), --E2

value := (((......

value := (((......

value := ((e1, e2,..., en) ) --En

In the statement part of the romexample, there is one conditional signal

assignment statement. The output port data is assigned a value based on

the value of the csinput. The data type of the value assigned to port data

must be of type data_outbecause port data has a type of data_out.By

addressing the rom_dataconstant with an integer value, a data type of

data_outis returned.

A single value can be returned from the array of arrays by using the

following syntax:

bit_value := rom_data(addr) (bit_index);

The first index (addr) returns a value with a data type of data_out. The

second index (bit_index) indexes the data_outtype and returns a single

element of the array.

MULTIDIMENSIONAL ARRAYS

The constant rom_datain the romexample was represented using an

array of arrays. Following is another method for representing the data

with a multidimensional array:

TYPE mem_data_md IS ARRAY(0 TO memsize, 0 TO width) OF

std_logic;

CONSTANT rom_data_md : mem_data :=

( ( ‘ 0 ’, ‘ 0 ’, ‘ 0 ’, ‘ 0 ’),

( ( ‘ 0 ’, ‘ 0 ’, ‘ 0 ’, ‘ 1 ’),

( ( ‘ 0 ’, ‘ 0 ’, ‘ 1 ’, ‘ 0 ’),

( ( ‘ 0 ’, ‘ 0 ’, ‘ 1 ’, ‘ 1 ’),

( ( ‘ 0 ’, ‘ 1 ’, ‘ 0 ’, ‘ 0 ’),

( ( ‘ 0 ’, ‘ 1 ’, ‘ 0 ’, ‘ 1 ’),

( ( ‘ 0 ’, ‘ 1 ’, ‘ 1 ’, ‘ 0 ’),

(( ‘ 0 ’, ‘ 1 ’, ‘ 1 ’, ‘ 1 ’) );

The declaration shown here declares a two-dimensional array type

mem_data_md. When constant rom_data_mdis declared using this type, the

initialization syntax remains the same, but the method of accessing an el-

ement of the array is different. In the following example, a single element

of the array is accessed:

X := rom_data_md(3, 3);