Stack has a built-in list of associations between dis directories and their associated source directories (e.g. it can automatically map from /dis/ls.dis to /appl/cmd/ls.sbl). Giving the -p option adds a new association to the head of this list: if a module path prefix matches dispath, stack will search for a symbol file in sblpath. If the environment variable $sblpath is set, pairs of items from it are added to the association list, as given as -p options. The -p options take precedence over $sblpath.

$ stack 1
unknown fn() Module $Sys PC 742103
waitfor() shnew.b:105.7, 38
runpipeline() shnew.b:483.2, 14
runit() shnew.b:552.3, 29
init() shnew.b:83.3, 28

The process is executing in the Sys module, a call to sys->read that originated at line 105 (characters 7 to 38) of the waitfor function in shnew.b.

Once again, with the -v option to reveal more:

$ stack -v 1
unknown fn() Module $Sys PC 742103
waitfor(pid=18) shnew.b:105.7, 38
        status=[0] ""
        buf=[64] @b419a4
        n=-1
        who=-1
runpipeline(ctx=nil, pipeline=@b41454) shnew.b:483.2, 14
        pid=18
runit(ctx=nil, pipes=nil) shnew.b:552.3, 29
        pipeline=@b41454
init(ctxt=nil, argv=nil) shnew.b:83.3, 28
        buf=[1024] @b40f04
        n=4
        arg=@b41634
        prompt=[21] "$ "
$