c_when.htm

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<title>The when Command</title>
<h1>when</h1>

<!--INDEX "when command" "complex breakpoints" "pseudo registers" -->


The when command sets complex breakpoints <p>

</dl><h2>Format</h2><dl><dd>

The format for the when command is: <p>

<pre>
when <var>condition</var> <var>action</var>
</pre> <p>

where: <p>

<table><tr>
<td valign=top><var>condition</var>  </td><td>Specifies the conditions under which the 
		     breakpoint will occur.

</td></tr><tr>
<td valign=top>	<var>action</var>    </td><td> Specifies the action to be taken when the 
		     breakpoint occurs.
</td></tr>
</table><p>

</dl><h2>Functional Description</h2><dl><dd>


The <samp>when</samp> command provides a very flexible way to set <a
href="brkpts.htm">breakpoints</a> and to specify the action that
should be taken when they are encountered.<p>

Each <samp>when</samp> command takes two arguments, a condition, and an
action.  The condition can be something as simple as @pc==main (when
the program counter is equal to value of the symbol main), or it can be
a complex expression that might include specifying the contents of
memory locations.<p>

Actions can be any <a href="mondef.htm">Monitor</a> command string.
But it can also include the pseudo command stop. For example, the
command, "<samp>when @pc==main stop</samp>" specifies that when the
program counter has the value of the symbol main, execution should
stop. <p>

Conditional expressions can be combined in any arbitrary
manner to provide extremely complex breakpoint conditions. For
example,<p>
<pre>
	PMON> when @pc==main2&&(^tcbchn==task1||^tcbchn==task2) stop
</pre><p>

which will stop execution when the pc is equal to main2, and
either tcbchn is equal to task1 or task2. <p>

In the preceeding examples we have chosen to stop execution when
the condition is met. However, it is possible to specify a list
of commands. For example,<p>

<pre>
	PMON> when @pc==sort "r;d -w dat3 1"
</pre><p>

	specifes that when the pc is equal to 'sort', the commands
	'r' and 'd -w dat3 1' will be executed. The ';' is used to
	separate commands, and the double quotes are necessary because
	the argument contains spaces.  <p>

The <a href="mondef.htm">Monitor</a> is responsible for deciding when
to use hardware breakpoint registers (if present). In general, the <a
href="mondef.htm">Monitor</a> uses software breakpoints unless there
is a specific reason that a hardware breakpoint is required.<p>

If you issue a command that requires a hardware breakpoint register.
But that the <a href="mondef.htm">Monitor</a> finds that there is not
one available. The <a href="mondef.htm">Monitor</a> will automatically
use trace mode when you issue the c command to continue execution.
Because trace mode is not real-time, the <a
href="mondef.htm">Monitor</a> will warn you at the time that you set
the breakpoint that this will require non real-time execution.<p>

</dl><h2>Examples</h2><dl><dd>

<ul>
<li>Stop when a memory location changes.<p>

	You can specify data locations by using the dereferencing
	operator '^'. For example,<p>
<pre>
		PMON> when '^tcbchn!=0' stop
</pre><p>

	means that execution should stop when the contents
	of the memory location specified by the symbol tcbchn is not
	equal to zero. Note that expressions that include the character
	'!' must be enclosed within single- or double-quotes to 
	supress the normal history substitution mechanism. Single- or
	double-quotes must also be used if the expression contains
	spaces.  <p>

<li>Break on the 20th time around a loop.<p>

<pre>
	when @pc==0x80021234 "r a @a+1"
	when @a==0t20 stop
</pre>

These commands specify that execution should pause each time the pc has
the value 0x80021234. Each execution pauses, the command "<samp>r a
@a+1</samp>" will be executed. This command increments the value of
pseudo register 'a'.  See the <a href="c_r.htm"><samp>r</samp></a>
command for more information on the pseudo registers.<p>

If during one of these pauses in execution pseudo register 'a' has
the value 20 (decimal), execution will stop.<p>


<li>Break when fred() is executed after jim(), but not after mike().

<pre>
	when @pc==jim "r a 1"
	when @pc==mike "r a 0"
	when "@pc==fred && @a==1" stop
</pre>

<li>Stop if dat1 ever gets set back to zero.
<pre>
	when "^dat1 != 0" "r a 1"
	when "^dat1 == 0 && @a == 1" stop
</pre>

This requires a hardware data breakpoint register in order to execute
this in real-time. In this mode a brief pause will occur each time
a write to dat1 occurs.<p>

If you don't have a hardware data breakpoint register, you can
maintain real-time performance by only checking the value at specific
points in the program.<p>

<pre>
	when "@pc == fred && ^jim != 0" "r a 1"
	when "@pc == fred && ^jim == 0 && @a == 1" stop
</pre>

<li>Find the maximum value written to a specified memory location.

<pre>
	when "^dat1 > @a" "r a ^dat1"
</pre>

Use the command "<samp>r a</samp>" to examine the value when the
program stops.<p>

<li>Stop if jim() is ever called with a 2nd argument of zero.

<pre>
	when "@pc == jim && @a1 == 0" stop
</pre>

<li>Count how many times jim() is executed.

<pre>
	when "@pc == jim" "r a @a+1"
</pre>

Use the command "<samp>r a</samp>" to examine the value when the
program stops.<p>
</ul>


</dl><h2>See Also </h2><dl><dd>
<a href="c_b.htm">b</a>, 
<a href="c_db.htm">db</a>,
and <a href="c_load.htm">load</a> commands.


</dl><p><hr>
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