VHDL Programming

Subprograms and Packages 127

a stronger strength value than variable resultcauses variable resultto

be updated with the stronger value,R0.

Another example shows how the constant x_tabis used to predict the

correct value for conflicting inputs. The driver values are shown in the

array in Figure 5-5.

In this example, variable resultis initialized to F0. The first iteration

of the loop does nothing because the first driver and the result-

initialization value are the same value. The next iteration starts with

variable resultcontaining the value F0, and the next driver value as R0.

Because the value in variable resultis greater in strength than the value

of the new driver, no action is implemented, except to advance the loop to

the next driver.

The last driver contains the value F1. The strength of the value contained

in variable resultand the new driver value are the same. Therefore, the

IFstatement checking this condition is executed and succeeds. The next

IFstatement checks to see if the logical values are the same for both vari-

able resultand the new driver. Variable resultcontains an F0, and the

new driver value contains an F1. The values are not the same, and the

x_tabtable is used to return the correct unknown value for the strength

of the driver values. The x_tabtable returns the value FX, which is returned

as the resolved value.

A more efficient method to implement the loop would be to skip the

first iteration where the first driver is compared to itself, because the

value in variable resultis initialized to the first driver value. It is left

as an exercise to the reader to write this new loop iteration mechanism.

Driver Values

Initial

Value

F0 F0 R0 F1

F0

F0

FX Resultant Value

Figure 5-5
Nine State Resolution
with Three Values.