Trace of a different process

Use the -p feature to produce a stack trace of a different process. The argument is either a process id or the address of a process's proc structure. Note that the stack frame addresses in such a trace do not refer to the current system stack. Normally, only user processes that are not swapped out can have their stacks traced. However, if the saveu(N) function has been called in advance to reserve memory for as many as 0xN u-blocks, the u-block of a process is copied when it is swapped out, to allow stack trace of that process. In such a case, ps() no longer shows the process as <swapped out>, but that can be deduced from the process flags. At present, only two pages of a u-block are saved; consequently, stack trace of a swapped out process does not work with a large number of open files, when the tss falls in the third page or beyond.