fixes #13

Author: Max-Hailperin

threads.tex

@@ -699,7 +699,9 @@ \section{Preemptive Multitasking}\label{threads-preemptive-section}
 currently executing instruction and the next one.  Thus, rather than
 moving on to the program's next instruction, the processor jumps off
 to the special procedure called the \vocab{interrupt
-handler}.  The interrupt handler, which is part of the operating
+handler}.
+
+The interrupt handler, which is part of the operating
 system, deals with the hardware device and then executes a \vocab{return
 from interrupt} instruction, which jumps back to the instruction that
 had been about to execute when the interrupt occurred.  Of course, in
@@ -709,21 +711,31 @@ \section{Preemptive Multitasking}\label{threads-preemptive-section}
 
 Using this interrupt mechanism, an operating system can provide
 preemptive multitasking.  When an interrupt occurs, the interrupt
-handler first takes care of the immediate needs, such as
+handler first
+saves the registers to the current thread's stack and
+takes care of the immediate needs, such as
 accepting data from a network interface controller or updating the
 system's idea of the current time by one millisecond.  Then,
 rather than simply restoring the registers and executing a return from
 interrupt instruction, the interrupt handler checks whether it would
 be a good time to preempt the current thread and switch to another.
+
 For example, if the interrupt signaled the arrival of data for which a
 thread had long been waiting, it might make sense to switch to
 that thread.  Or, if the interrupt was from the timer and the current
 thread had been executing for a long time, it may make sense to give
 another thread a chance.  These policy decisions are related to
-scheduling, the topic of Chapter~\ref{scheduling-chapter}.  In any case, if the
+scheduling, the topic of Chapter~\ref{scheduling-chapter}.
+
+In any case, if the
 operating system decides to preempt the current thread, the interrupt
 handler switches threads using a
 mechanism such as the \verb|switchFromTo| procedure.
+This switching of threads includes switching to the new thread's stack,
+so when the interrupt handler restores registers before returning,
+it will be restoring the new thread's registers. The previously running
+thread's register values will remain safely on its own stack until that
+thread is resumed.
 
 \section{Security and Threads}