VHDL Programming

100 Chapter Four

data. It always points to the first element of the list of items in the stack.

Variable temp_elemis used to hold a newly allocated element until it is

connected into the stack list. Variable last_clkis used to hold the previ-

ous value of clkto enable transitions on the clock to be detected. (This

behavior can be duplicated with attributes, which are discussed in Chapter

7,“Configurations.”)

Line 12 checks to see if a 0 to 1 transition has occurred on the clk

input. If so, then the stack needs to do a read or write depending on the

r_wbinput. Line 13 checks to see if r_wbis set up for a write to the stack.

If so, lines 14 through 17 create a new data storage element and connect

this element to the list.

Line 14 uses the predefined function NEWto allocate a record of type

element_rec and return an access value to be assigned to variable

temp_elem. This creates a structure that is shown in Figure 4-4.

Lines 15 and 16 fill in the newly allocated object with the data from

input dinand the access value to the head of the list. After line 16, the

data structures look like Figure 4-5.

Finally, in line 17, the new element is added to the head of the list. This

is shown in Figure 4-6.

Lines 18 through 22 of the model provide the behavior of the stack

when an element is read from the stack. Line 19 copies the data from the

stack element to the output port. Lines 20 through 22 disconnect the

element from the stack list and return the memory to the system.

Line 20 assigns the temp_elemaccess variable to point to the head of

the list. This is shown in Figure 4-7.

Line 21 moves the head of the list to the next element in the list. This

is shown in Figure 4-8.

Stack Element

Data

NXT

Stack Element

Data

NXT

Stack Element

Data

NXT

List_Head

Temp_Elem

Figure 4-4
Allocate New Stack
Element.