> In any case, that's not the real problem. Linux user threads do not > have true separate stacks. They share their _entire_ address space; > the stacks are all bounded (default is 2MB) and grouped together at > the top of the available memory region. Quite. A comment by Kevin reminded me of the real constraint (which the experts probably take for granted): this system is supposed to work on shared-memory multiprocessors and multithreaded CPUs. In both cases two or more threads within an address space can be active simultaneously. On a multithreaded CPU (in particular) there's only one TLB, so memory (including any memory specially handled by the kernel) is all held in common. The *only* thing available to a user privilege program which distinguishes the threads is the CPU register set. (Well, and the stack, which is a difference inherited from the value in the stack pointer register. But the stack pointer is not really going to help much to return a thread-characteristic pointer or ID.) So MIPS really do need to figure out which register can be freed up. Well, at least I know why now. Hope the rest of you aren't too bored! Dominic Sweetman Algorithmics Ltd http://www.algor.co.uk
