Taos provides a different method than Ultrix for a debugger process to control a debuggee process. The Ultrix method uses the ptrace kernel call; it requires the debuggee to be a child of the debugger, and therefore to be resident on the same machine. The Taos method uses TeleDebug servers built into the Nub; the debuggee need not be a child of, nor even on the same machine as, the debugger. Even the Taos address space and the Nub itself may be debugged (this requires another machine to run the debugger). On Taos, the ptrace kernel call always raises SIGSYS.
Ultrix provides the profil kernel call for program-counter sampling. On Taos, the profil kernel call always raises SIGSYS; the Nub provides an alternate sampling facility. See the gprof package for implementation details. The -pg option works only with Modula-2+ programs, not for other languages such as C.
Taos interprets a range of negative operands to the chmk instruction as Nub calls rather than as errors reported via the SIGSYS signal.