What does the "pseudo-register number" argument to windbg StopOnException do? - windbg

Question

Here's the help text for the windbg command StopOnException (aka soe for short):
0:000> !soe
usage: StopOnException [-derived] [-create | -create2] <type name>
[<pseudo-register number for result>]
ex: StopOnException -create System.OutOfMemoryException 1
What is the argument "pseudo-register number for result" for? The example uses the value 1 but without any follow up.
The only other documentation I can finds says just this:
StopOnException [-derived] [-create | -create2] <Exception> <Pseudo-register number>
Causes the debugger to stop when the specified exception is thrown,
but to continue running when other exceptions are thrown.
The -derived option catches the specified exception and every
exception that derives from the specified exception.
...no mention of this argument at all in the text. Nor any other use of the word "pseudo" on that page, or "register" with the same meaning.

Answer 1

There is additional help text within windbg that does explain it. Took me a while to realize this, maybe it will be useful to others new to windbg:
0:000> !help stoponexception
produces:
!StopOnException [-derived]
[-create | -create2]
<Exception>
[<Pseudo-register number>]
!StopOnException helps when you want the Windows Debugger to stop on a
particular managed exception, say a System.OutOfMemoryException, but
continue running if other exceptions are thrown. The command can be
used in two ways:
When you just want to stop on one particular CLR exception
At the debugger prompt, anytime after loading SOS, type:
!StopOnException -create System.OutOfMemoryException 1
The pseudo-register number (1) indicates that SOS can use register
$t1 for maintaining the breakpoint. The -create parameter allows
SOS to go ahead and set up the breakpoint as a first-chance
exception. -create2 would set it up as a 2nd-chance exception.
When you need more complex logic for stopping on a CLR exception
!StopOnException can be used purely as a predicate in a larger
expression. If you type:
!StopOnException System.OutOfMemoryException 3
then register $t3 will be set to 1 if the last thrown exception on
the current thread is a System.OutOfMemoryException. Otherwise,
$t3 will be set to 0. Using the Windows Debugger scripting
language, you could chain such calls together to stop on various
exception types. You'll have to manually create such predicates,
for example:
sxe -c "!soe System.OutOfMemoryException 3;
!soe -derived System.IOException 4;
.if(#$t3==1 || #$t4==1) { .echo 'stop' } .else {g}"
The -derived option will cause StopOnException to set the
pseudo-register to 1 even if the thrown exception type doesn't exactly
match the exception type given, but merely derives from it. So,
"-derived System.Exception" would catch every exception in the
System.Exception heirarchy.
The pseudo-register number is optional. If you don't pass a number,
SOS will use pseudo-register $t1.
I highlighted the relevant portions.
Essentially, this option allows you to specify which pseudo-register should be used to keep track of the breakpoint. Perhaps in some cases if you don't want to overwrite an existing important value, specifying your own register selection can avoid that.
More info on pseuo-register values: https://learn.microsoft.com/en-us/windows-hardware/drivers/debugger/pseudo-register-syntax