callthru is a utility that allows the simulated system to use services of the
underlying host system.

Usage : callthru [exit|sink|source] ...\n");
        exit\n");
            Stop the simulation.\n");
        sink host-file-name\n");
            Copy stdin from simulated system to host file system.\n");
            Example : callthru sink /tmp/foobar<foobar\n");
            will copy foobar from the simulator's current\n");
            directory to /tmp/foobar on the host\n");
        source host-file-name\n");
            Copy file from host file system to stdout of simulated system.\n");
            Example : callthru source /tmp/foobar>foobar\n");
            will copy /tmp/foobar from the host\n");
            to foobar in the simulator's current directory\n");
        eval tcl-simulator-command\n");
            Execute the specified simulator Tcl command\n");
