VHDL Programming

CPU: RTL Simulation 349

CPU Simulation

Simulating the CPU design is different from most other entities because

the CPU design doesn’t need much outside stimulus. The memory device

provides the input data for the CPU much as a stimulus file would for

other entities. The CPU reads its program from the memory device. The

CPU need only have the clksignal and resetsignal stimulated properly,

and the CPU reads and executes instructions from that point forward.

The only stimulus needed to start the operation of the CPU is a uniform

signal applied to the clkinput and a pulse applied to the resetinput for

at least 2 clock cycles. This starts the CPU into the reset sequence. After

the reset sequence has been started, the CPU is initialized and starts

executing the CPU instructions from the mementity.

The CPU is simulated as stimulus only initially to verify that the device

seems to be functioning. More complex testbenches need to be created that

include comparison against a known good result to verify correctness. The

simplest method for doing this is to manually verify the results the first

time, capture the output results, and then use them for comparison later.

The first step in simulating the CPU is to compile all the files that

make up the design into a format that the simulator can use. The com-

piled format is loaded into the simulator, and the simulation is executed.

The ModelSim simulator from Model Technology is used for the simu-

lation process.

The first step in compiling all of the files in the design is to create one

or more libraries to store the compiled data. The default library to store

the compiled data is a library called work. The name workis the logical

name of the library; the physical location of the library can be anywhere.

To create a library, the VLIBcommand is used as shown here:

vlib work

This creates the worklibrary in the current working directory of the

current disk. After the library has been created, the VHDL source files for

the design can be compiled into the target library. To compile each of the

files, the VCOMcommand must be run either from the GUI (Graphical User

Interface) or from the command line. Most of the operations of the simu-

lator have a GUI method of performing the command line command. This

allows casual users as well as expert users to effectively use the simula-

tor. Normally, casual users use the GUI and experts use the command line

and script interface.