There are tasks and functions that are used to generate input and output during simulation. Their names begin with a dollar sign ($). The synthesis tools parse and ignore system functions, and hence can be included even in synthesizable models.

These commands have the same syntax, and display text on the screen during simulation. They are much less convenient than waveform display tools like GTKWave. or Undertow or Debussy. $display and $strobe display once every time they are executed, whereas $monitor displays every time one of its parameters changes. The difference between $display and $strobe is that $strobe displays the parameters at the very end of the current simulation time unit rather than exactly when it is executed. The format string is like that in C/C++, and may contain format characters. Format characters include %d (decimal), %h (hexadecimal), %b (binary), %c (character), %s (string) and %t (time), %m (hierarchy level). %5d, %5b etc. would give exactly 5 spaces for the number instead of the space needed. Append b, h, o to the task name to change default format to binary, octal or hexadecimal.

• $display ("format_string", par_1, par_2, ... );
• $strobe ("format_string", par_1, par_2, ... );
• $monitor ("format_string", par_1, par_2, ... );
• $displayb (as above but defaults to binary..);
• $strobeh (as above but defaults to hex..);
• $monitoro (as above but defaults to octal..);

These return the current simulation time as a 64-bit integer, a 32-bit integer, and a real number, respectively.

$reset resets the simulation back to time 0; $stop halts the simulator and puts it in interactive mode where the user can enter commands; $finish exits the simulator back to the operating system.

$scope(hierarchy_name) sets the current hierarchical scope to hierarchy_name. $showscopes(n) lists all modules, tasks and block names in (and below, if n is set to 1) the current scope.

$random generates a random integer every time it is called. If the sequence is to be repeatable, the first time one invokes random giving it a numerical argument (a seed). Otherwise the seed is derived from the computer clock.

These can dump variable changes to a simulation viewer like Debussy. The dump files are capable of dumping all the variables in a simulation. This is convenient for debugging, but can be very slow.

• $dumpfile("filename.vcd")
• $dumpvar dumps all variables in the design.
• $dumpvar(1, top) dumps all the variables in module top and below, but not modules instantiated in top.
• $dumpvar(2, top) dumps all the variables in module top and 1 level below.
• $dumpvar(n, top) dumps all the variables in module top and n-1 levels below.
• $dumpvar(0, top) dumps all the variables in module top and all level below.
• $dumpon initiates the dump.
• $dumpoff stop dumping.

These commands write more selectively to files.

• $fopen opens an output file and gives the open file a handle for use by the other commands.
• $fclose closes the file and lets other programs access it.
• $fdisplay and $fwrite write formatted data to a file whenever they are executed. They are the same except $fdisplay inserts a new line after every execution and $write does not.
• $strobe also writes to a file when executed, but it waits until all other operations in the time step are complete before writing. Thus initial #1 a=1; b=0; $fstrobe(hand1, a,b); b=1; will write write 1 1 for a and b.
• $monitor writes to a file whenever any of its arguments changes.

• handle1=$fopen("filenam1.suffix")
• handle2=$fopen("filenam2.suffix")
• $fstrobe(handle1, format, variable list) //strobe data into filenam1.suffix
• $fdisplay(handle2, format, variable list) //write data into filenam2.suffix
• $fwrite(handle2, format, variable list) //write data into filenam2.suffix all on one line. Put in the format string where a new line is desired.

Copyright © 1998-2014

Deepak Kumar Tala - All rights reserved

Do you have any Comment? mail me at:deepak@asic-world.com