A process can create any reasonable number (e.g., hundreds) of threads of control to deal with asynchronism or to perform independent computations in parallel. The Taos implementation of Topaz can execute as many threads concurrently as it has processors (currently five). The Taos scheduler chooses the highest priority ready threads without regard to the address spaces in which those threads are running. The Ultrix implementation of Topaz simulates concurrent threads within a single-threaded Ultrix process.
The Thread interface contains procedures for forking new threads and rejoining terminated threads, for allowing threads to synchronize via mutexes and condition variables, and for associating private data with a thread. For convenience, Modula-2+ also provides a LOCK statement.
The implementation of OS avoids serializing concurrent calls from different threads (in the same or different processes) except where dictated by the semantics of the individual procedures. For example, two threads concurrently writing the same byte of the same file are serialized. But a thread that blocks within the operating system (e.g. waiting for a character to be typed on a tty device) does not prevent other threads in the same process from calling OS procedures.
Sometimes it is necessary to cancel a computation. Topaz provides the thread alert mechanism for this purpose. Whenever an alert is sent to a thread that is blocked for an indefinite period within a call to a procedure of the OS interface (and most other Topaz interfaces), that procedure will raise the Alerted exception.