The programmer can create a new thread of control upon any procedure call and a thread creation usually adds only a few instructions to the procedure call. The net result is that, using fine-grain threads, you can reduce the cost of building a parallel program, either from scratch or from an existing sequential source, and the cost of maintenance. It is also useful as a compilation target of higher-level parallel programming languages.
HEML document is available here .
Not sure about 5.x. For Digitax UNIX, I had some troubles in compiling the library on version 3.2. The problem appeared to come from obsolete pthread.h in Digital UNIX 3.2. I believe modifications, if any, will beminor on UNIXes. I have no experiences on NT multiprocessors.