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Robert Morris 2019-08-02 16:22:56 -04:00
parent 3bee885ed7
commit c5163e4a42
1 changed files with 15 additions and 17 deletions
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@ -287,7 +287,7 @@ use only one CPU, which you can do by running
are passing test0(), and should proceed to the next section. are passing test0(), and should proceed to the next section.
do they need to make sure at this point that they see multiple do they need to make sure at this point that they see multiple
alarm! printouts? or is it OK if they see one alarm! and alarm! printouts? or is it OK if they see one alarm! and
then a crash?</b> then a crash? may need to fix the sample ...alarm! output shown above</b>
</ul> </ul>
@ -301,35 +301,33 @@ about it, and hint or say why they are not done even though
test0() works.</b> test0() works.</b>
<p>Test0 doesn't test whether the handler returns correctly to <p>Test0 doesn't test whether the handler returns correctly to
the interrupted instruction. If you didn't get this right, it the user instruction that was interrupted by the timer.
is likely that test1 will fail (the program crashes or the program The previous section didn't require you to get this right.
If you didn't, test0 will probably succeed anyway, but
test1 will likely fail (the program crashes or the program
goes into an infinite loop). goes into an infinite loop).
Another challenge is that the register contents need to be
<p>A main challenge is to arrange that when the handler returns, it correct when control returns to the interrupted user instruction.
returns to the instruction where the program was interrupted. Which
register contains the return address of a function? When the kernel
receives an interrupt, which register contains the address of the
interrupted instruction?
<p>Your solution is likely to require you to save and restore <p>Your solution is likely to require you to save and restore
registers---what registers do you need to save and restore to resume registers---what registers do you need to save and restore to resume
the interrupted code correctly? (Hint: it will be many). There are the interrupted code correctly? (Hint: it will be many). There are
several ways to do this, but one convenient way is to add another several ways to restore the registers; one convenient plan is to add another
system call <tt>sigreturn</tt> that the handler calls when it is system call <tt>sigreturn</tt> that the handler calls when it is
done. Your job is to arrange that <tt>sigreturn</tt> returns to the done, and which restores registers and returns to the original
interrupted code. interrupted user instruction.
Some hints: Some hints:
<ul> <ul>
<li>Add the <tt>sigreturn</tt> system call, following the changes <li>Add a new <tt>sigreturn</tt> system call.
you made to support <tt>sigalarm</tt>.
<li>Save enough state when the timer goes in the <tt>struct <li>Have <tt>usertrap</tt> save enough state in
proc</tt> so that <tt>sigreturn</tt> can return to the <tt>struct proc</tt> when the timer goes off
that <tt>sigreturn</tt> can correctly return to the
interrupted code. interrupted code.
<li>Prevent re-entrant calls to the handler----if a handler hasn't <li>Prevent re-entrant calls to the handler----if a handler hasn't
returned yet, don't call it again. returned yet, the kernel shouldn't call it again.
</ul> </ul>
<p>Once you pass <tt>test0</tt> and <tt>test1</tt>, run usertests to <p>Once you pass <tt>test0</tt> and <tt>test1</tt>, run usertests to