+#### struct thread *switch_threads (struct thread *cur, struct thread *next);
+####
+#### Switches from CUR, which must be the running thread, to NEXT,
+#### which must also be running switch_threads(), returning CUR in
+#### NEXT's context.
+####
+#### This function works by assuming that the thread we're switching
+#### into is also running switch_threads(). Thus, all it has to do is
+#### preserve a few registers on the stack, then switch stacks and
+#### restore the registers. As part of switching stacks we record the
+#### current stack pointer in CUR's thread structure.
+
+.globl switch_threads
+.func switch_threads
+switch_threads: