System Tasks

For certain routine operations Verilog provides system tasks. all such tasks are in the form $keyword. In this section we will discuss writing to output, monitoring a simulation and ending a simulation.

Writing to Standard Output

Keyword: $display, $displayb, $displayh, $displayo, $write, $writeb, $writeh, $writeo.

The most useful of these is $display.This can be used for displaying strings, expression or values of variables. Here are some examples of usage.

        $display("Hello Dr Blair");
        --- output: Hello Dr Blair

        $display($time) // current simulation time.
        --- output: 460

        counter = 4'b10;
        $display(" The count is %b", counter);
        --- output:  The count is 0010

The formatting syntax is similar to that of printf in the C programming language. For $display and $display, they are:

        -------------------------------------------
        | Format Specifications                   |
        -------------------------------------------
        |  Format  |          Display             |
        | -------- | ---------------------------- |
        | %d or %D | Decimal                      |
        | %b or %B | Binary                       |
        | %h or %H | Hexadecimal                  |
        | %o or %O | Octal                        |
        | %m or %M | Hierarchical name            |
        | %t or %T | Time format                  |
        | %e or %E | Real in scientific format    |
        | %f or %F | Real in decimal formal       |
        | %g or %G | Real in shorter of above two |
        -------------------------------------------

The escape sequence for printing special characters are:

        --------------------
        | Escape Sequences |
        --------------------
        | \n |   newline   |
        | \t |   tabulate  |
        | \\ |   print \   |
        | \" |   print "   |
        | %% |   print %   |
        --------------------

$write is identical to $display except it does not automatically put a newline at the end of its output.

EXERCISE What does $display without any arguments output? What does that tell you about $write?

If the formatting character is omitted, the various commands default as below:

        ---------------------------
        | Default Format Specs    |
        ---------------------------
        |   Task    |   Default   |
        | --------- | ----------- |
        | $display  | decimal     |
        | $displayb | binary      |
        | $displayh | hexadecimal |
        | $displayo | octal       |
        | $write    | decimal     |
        | $writeb   | binary      |
        | $writeh   | hexadecimal |
        | $writeo   | octal       |
        ---------------------------

Thus

              $write(5'b01101);
              $writeb("  ", 5'b01101);
              $writeh("  ", 5'b01101);
              $writeo("  ", 5'b01101,"\n");

        13  01101  0d  15