VHDL Programming

52 Chapter Three

another variable of the same name exists in the process, function, or

procedure, then these two variables are treated as separate variables

and are accessed by context. Let’s look at an example to illustrate this

point:

PROCESS(i)

BEGIN

x <= i + 1; -- x is a signal

FOR i IN 1 to a/2 LOOP

q(i) := a; -- q is a variable

END LOOP;

END PROCESS;

Whenever the value of the signal i in the process sensitivity list

changes value, the process will be invoked. The first statement schedules

the value i + 1on the signal x. Next, the FORloop is executed. The index

value iis not the same object as the signal ithat was used to calculate

the new value for signal x. These are separate objects that are each

accessed by context. Inside the FORloop, when a reference is made to i,

the local index is retrieved. But outside the FORloop, when a reference is

made to i, the value of the signal iin the sensitivity list of the process

is retrieved.

The values used to specify the range in the FORloop need not be specific

integer values, as has been shown in the examples. The range can

be any discrete range. A discrete_range can be expressed as a

subtype_indicationor a range statement. Let’s look at a few more exam-

ples of how FORloops can be constructed with ranges:

PROCESS(clk)

TYPE day_of_week IS (sun, mon, tue, wed, thur, fri,

sat);

BEGIN

FOR i IN day_of_week LOOP

IF i = sat THEN

son <= mow_lawn;

ELSIF i = sun THEN

church <= family;

ELSE

dad <= go_to_work;

END IF;

END LOOP;

END PROCESS;