Compiler Directives and System Tasks

Internal variable monitoring system tasks

$display, $write, $strobe, $monitor
$display or $write displays the specified variables once when the command is executed in the code flow
$display automatically adds a newline character to the end of its output, whereas the $write task does not

// syntax
$display("mreg = %h hex, %d decimal", mreg, mreg);

$strobe display simulation data at a selected time (at the end of the current simulation time), when all the simulation events that have occurred for that simulation time
Each time a variable or an expression in the argument list changes value, the entire argument list is displayed at the end of the time step.

$time - Returns an unsigned integer that is a 64-bit time, scaled to the timescale unit of the module that invoked it

$fopen, $fclose, $readmemb
The function $fopen opens the file specified as the filename argument and returns either a 32 bit multichannel descriptor, or a 32 bit file descriptor.
The channel descriptor for each channel is a 32 bit value with only one bit set that corresponds to the channel number
$fmonitor used to write within the file
$stop causes simulation to be suspended
$finish makes the simulator exit and pass control back to the host operating system
$finish(2) will print the CPU usage time

$random

The function returns a new 32-bit random signed integer number each time it is called
Seed parameter controls the numbers that $random returns

reg [23:0] rand, seed num;
rand = Srandom (seed num);

Creates a macro for text substitution
Can be used both inside and outside module definitions
Can be used in the source description by using the ( ` ) character, followed by the macro name

`define dt_width 8
`define ad_depth 256

Used to insert the entire contents of a source file in another file during compilation
Used to include global or commonly used definitions and tasks

//File - state_definition.v

`define HOLD 0
`define RESET 1
`define SET 2
`define TOGGLE 3

//Main file

`include "state_definition.v"

Specifies the time unit and time precision of the modules that follow `timescale
time_unit specifies the unit of measurement for times and delay
time_precision specifies how delay values are rounded before being used in simulation
time_unit > time_precision
Error if some modules have a `timescale specified and others do not

`timescale time_unit / time_precision
`timescale 10ns / 10ps