Context Separation

MT-Eus allocates private stacks and contexts to each threads so that they can run independently of each other. Objects such as symbols and conses are allocated in the shared heap memory as in sequential EusLisp. Therefore, thread-private data such as block labels, catch tags, and local variables are protected from other threads, whereas values (objects) pointed by global variables are visible to all threads allowing information exchange among threads.

Figure 5: Solaris operating system's thread model

A context consists of a C-stack, a binding-stack and frame pointers that chain lexical blocks such as lambda, block, catch, let, flet, and so on, and is established when a new thread is created. Since more than one context can be active at the same time on a real multi-processor machine, we cannot hold a single pointer to the current context in a global variable. Rather we have to add one more argument to every internal function to transfer the context pointer from the topmost eval to the memory manager at the bottom.