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\begin{document}

Name: \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Directions: {\bf Work only on this sheet} (on both sides, if needed); do not
turn in any supplementary sheets of paper. There is actually plenty of room
for your answers, as long as you organize yourself BEFORE starting writing.

{\bf 1.} (20) The code below subtracts 1 from EAX if the contents of that
register are negative, but leaves EAX unchanged otherwise.  Fill in the
blanks:

\begin{Verbatim}[fontsize=\relsize{-2}]
   subl $0,%eax
   ____________________
   _____________ %eax
done: ...
\end{Verbatim}

{\bf 2.} This problem concerns the sorting example on pp.72ff.  

\begin{itemize}

\item [(a)] (20) Give the line number of the instruction executed after
the one in line 88.

\item [(b)] (20) The first time 35 is executed, what specific value will
be in EDI?

\end{itemize}

Parts (c) and (d) involve the following context:

Suppose in executing the program from within GDB, I set a breakpoint at
line 76 (a {\bf js} instruction), and run the program.  When the program
reaches line 76, I issue the command {\bf info registers}, and get the
following output:

\begin{Verbatim}[fontsize=\relsize{-2}]
eax            0x80490c4        
ecx            0x1      
edx            0x80490d0        
ebx            0x6      
ebp            0x4      
esi            0x80490cc        
edi            0x2      
eflags         0x287   
\end{Verbatim}

The columns here give name of the register, and its contents in hex.
(I've omitted some material.) 

\begin{itemize}

\item [(c)] (20) Let's refer to the loop in lines 31-42 as the ``main''
loop.  What iteration of that loop was the program in when the current
call to {\bf findmin()} was executed?  Answer first, second etc.

\item [(d)] (20) Based purely on the above information above and NOT on
the specific values in the array {\bf x} (1,5,2,...), state whether the
jump will be taken or not, i.e. whether we'll jump to line 82.  {\bf
Explain which specific quantity (not just which register) shows this.}

\end{itemize}

{\bf Solutions:}

{\bf 1.}

\begin{Verbatim}[fontsize=\relsize{-2}]
   subl $0,%eax
   js done
   decl %eax
done: ...
\end{Verbatim}

{\bf 2a.} 35

{\bf 2b.} 2

{\bf 2c.} second

{\bf 2d.} JS looks at Bit 7 of EFLAGS; the latter has value 0x287, which
is 11 1000 0111, so Bit 7 had a 1, so we do jump

\end{document}