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<main>
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<div id="wrapper">
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<h1 id="cmpt-295">CMPT 295</h1>
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<ul>
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<li>Unit - Machine-Level Programming</li>
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<li>Lecture 13 – Assembly language – Program Control – cmovX</li>
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<li>Iterative Statements – Loops</li>
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</ul>
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<h2 id="last-lecture">Last Lecture</h2>
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<ul>
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<li>In C, we can change the execution flow of a program
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<ol>
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<li>Conditionaly
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<ul>
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<li>Conditional statements: if/else, switch</li>
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<li>Iterative statements: loops</li>
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</ul>
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</li>
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<li>Unconditionally
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<ul>
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<li>Functions calls</li>
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</ul>
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</li>
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</ol>
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</li>
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<li>In x86-64 assembly, we can also change the execution flow of a program
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<ul>
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<li>cmp* instruction (compare)</li>
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<li>jX insturction (jump)</li>
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<li>call and ret instructions</li>
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</ul>
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</li>
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</ul>
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<h2 id="todays-menu">Today’s Menu</h2>
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<ul>
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<li>Introduction
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<ul>
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<li>C program -> assembly code -> machine level code</li>
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</ul>
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</li>
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<li>Assembly language basics: data, move operation
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<ul>
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<li>Memory addressing modes</li>
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</ul>
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</li>
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<li>Operation leaq and Arithmetic & logical operations</li>
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<li>Conditional Statement – Condition Code + cmovX</li>
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<li>(highlighted) Loops</li>
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<li>Function call – Stack</li>
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<li>Array</li>
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<li>Buffer Overflow</li>
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<li>Floating-point operations</li>
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</ul>
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<h2 id="homework-int-maxint-x-int-y">Homework: <code class="language-plaintext highlighter-rouge">int max(int x, int y)</code></h2>
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<p>In C:</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>int max(int x,int y){
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int result = x;
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if(y>x)
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result=y;
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return result;
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}
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</code></pre></div></div>
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<p>version 1 – with jX instruction</p>
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<p>In Assembly: # x in %edi, y in %esi, result in %eax</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>max:
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movl %edi,%eax #result=x
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cmpl %edi,%esi #if y<=x then
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jle endif #return
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movl %esi,%eax #result=y
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endif:
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ret
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</code></pre></div></div>
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<p>We branch (jump) when the condition (y > x) is false, i.e., when (y <= x)
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-> This technique is called “coding the false condition first”
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or ”taking care of …”</p>
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<h2 id="conditional-move-instruction-cmovx">Conditional move instruction cmovX</h2>
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<p>What C code looks like when using conditional operator:</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>result=test?val2:val1;
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return result;
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</code></pre></div></div>
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<p>What logic of assembly
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code looks like when using
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cmovX (expressed in C):</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>result=val1;
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if (test) result = val2;
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return result;
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</code></pre></div></div>
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<p>Example: <code class="language-plaintext highlighter-rouge">cmovle Src,Dest</code></p>
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<p>Alternative: <code class="language-plaintext highlighter-rouge">int abs(int x)</code></p>
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<p>In C:</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>int abs(int x){
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if(x<0)
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x=-x;
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return x;
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}
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</code></pre></div></div>
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<p>in assembly: # <code class="language-plaintext highlighter-rouge">x</code> in <code class="language-plaintext highlighter-rouge">%edi</code>, <code class="language-plaintext highlighter-rouge">result</code> in <code class="language-plaintext highlighter-rouge">%eax</code></p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>abs:
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movl %edi,%eax # result = x
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negl %edi # x = -x
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cmpl $0,%eax # if x < 0 then
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cmovl %edi,%eax # result = -x
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ret
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</code></pre></div></div>
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<h2 id="advantage-of-conditional-move-cmovx">Advantage of conditional move cmovX</h2>
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<p>Note about branching:</p>
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<ul>
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<li>Branches are very disruptive to instruction flow through
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microprocessor CPU pipelines</li>
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<li>However, since conditional moves (cmovX) do not
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require control transfer (no branching/jumping required),
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they are less disruptive</li>
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<li>So, <code class="language-plaintext highlighter-rouge">gcc</code> tries to use them, but only when safe</li>
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</ul>
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<h2 id="what-do-we-mean-by-safe">What do we mean by “safe”?</h2>
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<ul>
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<li>In <code class="language-plaintext highlighter-rouge">result = test ? aVal : anotherVal;</code> both values
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(<code class="language-plaintext highlighter-rouge">aVal</code> and <code class="language-plaintext highlighter-rouge">anotherVal</code>) are computed so their
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computation must be “safe”</li>
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<li>Example of unsafe computations:
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<ol>
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<li>Expensive computations <code class="language-plaintext highlighter-rouge">val = Test(x) ? Hard1(x) : Hard2(x);</code>
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<ul>
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<li>Only makes sense when computations are very simple</li>
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</ul>
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</li>
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<li>Risky computations <code class="language-plaintext highlighter-rouge">val = p ? *p : 0;</code>
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<ul>
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<li>Only makes sense when computations do not crash the application</li>
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</ul>
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</li>
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<li>Computations with side effects <code class="language-plaintext highlighter-rouge">val = x > 0 ? x*=7 : x+=3;</code>
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<ul>
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<li>Only makes sense when computations do not have side effects</li>
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</ul>
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</li>
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</ol>
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</li>
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</ul>
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<h2 id="homework-example-alternate-int-maxint-x-int-y">Homework: Example: alternate <code class="language-plaintext highlighter-rouge">int max(int x, int y)</code></h2>
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<p>version 2 – with cmovX instruction</p>
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<p>In C:</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>int max(int x, int y){
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int result = x;
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if(y>x)
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result = y;
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return result;
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}
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</code></pre></div></div>
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<p>In Assembly: # x in %edi, y in %esi, result in %eax</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>max:
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movl %edi,%eax #result=x
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cmpl %edi,%esi #if y>x then
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cmovg %esi,%eax #result=y
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ret
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</code></pre></div></div>
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<h2 id="while-loop--coding-the-false-condition-first-updated">While loop – “coding the false condition first” (updated)</h2>
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<p><code class="language-plaintext highlighter-rouge">int x</code> and <code class="language-plaintext highlighter-rouge">int y</code> are arguments to function</p>
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<p>in C:</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>while(x<y){
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//stmts
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}
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return;
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</code></pre></div></div>
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<p>Steps:</p>
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<ol>
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<li>Analyze the condition: <code class="language-plaintext highlighter-rouge">x<y</code></li>
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<li>write <code class="language-plaintext highlighter-rouge">cmpl %esi,%edi</code></li>
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<li>List all outcomes:
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<ul>
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<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>x</mi><mo>−</mo><mi>y</mi><mo>></mo><mn>0</mn><mtext> </mtext><mo>⟹</mo><mtext> </mtext><mi>x</mi><mo>></mo><mi>y</mi></mrow><annotation encoding="application/x-tex">x-y > 0 \implies x>y</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.66666em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.7335400000000001em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.66844em;vertical-align:-0.024em;"></span><span class="mord">0</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">⟹</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.5782em;vertical-align:-0.0391em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.625em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span></span></span></span>: exits loop</li>
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<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>x</mi><mo>−</mo><mi>y</mi><mo><</mo><mn>0</mn><mtext> </mtext><mo>⟹</mo><mtext> </mtext><mi>x</mi><mo><</mo><mi>y</mi></mrow><annotation encoding="application/x-tex">x-y < 0 \implies x< y</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.66666em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.7335400000000001em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel"><</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.66844em;vertical-align:-0.024em;"></span><span class="mord">0</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">⟹</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.5782em;vertical-align:-0.0391em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel"><</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.625em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span></span></span></span>: loops</li>
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<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>x</mi><mo>−</mo><mi>y</mi><mo>=</mo><mn>0</mn><mtext> </mtext><mo>⟹</mo><mtext> </mtext><mi>x</mi><mo>=</mo><mi>y</mi></mrow><annotation encoding="application/x-tex">x-y = 0 \implies x=y</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.66666em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.625em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.66844em;vertical-align:-0.024em;"></span><span class="mord">0</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">⟹</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.43056em;vertical-align:0em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.625em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span></span></span></span>: exits loop</li>
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|
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|
|
</ul>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
<li>Which outcomes “exit loop”?</li>
|
|
|
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|
|
<li>Write for false condition i.e. g & e (<code class="language-plaintext highlighter-rouge">jge</code>)</li>
|
|
|
|
|
|
</ol>
|
|
|
|
|
|
|
|
|
|
|
|
<p>in assembly:</p>
|
|
|
|
|
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|
|
|
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|
|
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>loop: # x in %edi, y in %esi
|
|
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|
|
|
cmpl %esi,%edi # cmpl y,x
|
|
|
|
|
|
jge endloop
|
|
|
|
|
|
#stmts in loop
|
|
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|
|
|
jmp loop
|
|
|
|
|
|
endloop:
|
|
|
|
|
|
ret
|
|
|
|
|
|
</code></pre></div></div>
|
|
|
|
|
|
|
|
|
|
|
|
<p>Loop Pattern 1</p>
|
|
|
|
|
|
|
|
|
|
|
|
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>loop:
|
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|
|
if cond false
|
|
|
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|
|
goto done:
|
|
|
|
|
|
stmts
|
|
|
|
|
|
goto loop:
|
|
|
|
|
|
done:
|
|
|
|
|
|
</code></pre></div></div>
|
|
|
|
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|
|
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|
|
|
<h2 id="while-loop--jump-to-middle-updated">While loop – “jump-to-middle” (updated)</h2>
|
|
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|
|
|
|
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|
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<p><code class="language-plaintext highlighter-rouge">int x</code> and <code class="language-plaintext highlighter-rouge">int y</code> are arguments to function</p>
|
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|
|
|
|
|
|
|
|
|
|
<p>in C:</p>
|
|
|
|
|
|
|
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|
|
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>while(x<y){ //step 1
|
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|
|
//stmts
|
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|
}
|
|
|
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|
|
return;
|
|
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|
|
|
</code></pre></div></div>
|
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|
|
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|
|
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<p>Steps:</p>
|
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|
|
|
|
|
|
|
|
|
<ol>
|
|
|
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|
|
<li>(shown in C code)</li>
|
|
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|
|
|
<li>(shwon in assembly)</li>
|
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|
|
|
|
<li>All possible values
|
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<ul>
|
|
|
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|
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>x</mi><mo>−</mo><mi>y</mi><mo>></mo><mn>0</mn><mtext> </mtext><mo>⟹</mo><mtext> </mtext><mi>x</mi><mo>></mo><mi>y</mi></mrow><annotation encoding="application/x-tex">x-y >0 \implies x>y</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.66666em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.7335400000000001em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.66844em;vertical-align:-0.024em;"></span><span class="mord">0</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">⟹</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.5782em;vertical-align:-0.0391em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.625em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span></span></span></span>: exits loop, g</li>
|
|
|
|
|
|
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>x</mi><mo>−</mo><mi>y</mi><mo><</mo><mn>0</mn><mtext> </mtext><mo>⟹</mo><mtext> </mtext><mi>x</mi><mo><</mo><mi>y</mi></mrow><annotation encoding="application/x-tex">x-y < 0 \implies x< y</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.66666em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.7335400000000001em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel"><</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.66844em;vertical-align:-0.024em;"></span><span class="mord">0</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">⟹</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.5782em;vertical-align:-0.0391em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel"><</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.625em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span></span></span></span>: loops, l</li>
|
|
|
|
|
|
<li><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>x</mi><mo>−</mo><mi>y</mi><mo>=</mo><mn>0</mn><mtext> </mtext><mo>⟹</mo><mtext> </mtext><mi>x</mi><mo>=</mo><mi>y</mi></mrow><annotation encoding="application/x-tex">x-y = 0 \implies x=y</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.66666em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.625em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.66844em;vertical-align:-0.024em;"></span><span class="mord">0</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">⟹</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.43056em;vertical-align:0em;"></span><span class="mord mathnormal">x</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.625em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">y</span></span></span></span>: exits loops, e</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
<li>Consider outcome ???(can’t read) “loops”</li>
|
|
|
|
|
|
</ol>
|
|
|
|
|
|
|
|
|
|
|
|
<p>in assembly:</p>
|
|
|
|
|
|
|
|
|
|
|
|
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> jmp test
|
|
|
|
|
|
loop:
|
|
|
|
|
|
# stmts
|
|
|
|
|
|
test:
|
|
|
|
|
|
cmpl %esi,%edi #cmpl y,x (step 2)
|
|
|
|
|
|
jl loop (step 5)
|
|
|
|
|
|
ret
|
|
|
|
|
|
</code></pre></div></div>
|
|
|
|
|
|
|
|
|
|
|
|
<p>Loop Pattern 2</p>
|
|
|
|
|
|
|
|
|
|
|
|
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> goto test:
|
|
|
|
|
|
loop:
|
|
|
|
|
|
# stmts
|
|
|
|
|
|
test:
|
|
|
|
|
|
if cond true
|
|
|
|
|
|
goto loop:
|
|
|
|
|
|
done:
|
|
|
|
|
|
</code></pre></div></div>
|
|
|
|
|
|
|
|
|
|
|
|
<h2 id="do-while-loop--jump-to-middle-updated">Do While loop – “jump-to-middle” (updated)</h2>
|
|
|
|
|
|
|
|
|
|
|
|
<p><code class="language-plaintext highlighter-rouge">int x</code> and <code class="language-plaintext highlighter-rouge">int y</code> are arguments to function</p>
|
|
|
|
|
|
|
|
|
|
|
|
<p>in C:</p>
|
|
|
|
|
|
|
|
|
|
|
|
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>do{
|
|
|
|
|
|
stmts
|
|
|
|
|
|
} while(x<y);
|
|
|
|
|
|
return;
|
|
|
|
|
|
</code></pre></div></div>
|
|
|
|
|
|
|
|
|
|
|
|
<p>in assembly:</p>
|
|
|
|
|
|
|
|
|
|
|
|
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>loop:
|
|
|
|
|
|
#stmts
|
|
|
|
|
|
test:
|
|
|
|
|
|
jmpl %esi,%edi
|
|
|
|
|
|
jl loop
|
|
|
|
|
|
ret
|
|
|
|
|
|
</code></pre></div></div>
|
|
|
|
|
|
|
|
|
|
|
|
<p>Loop Pattern 2:</p>
|
|
|
|
|
|
|
|
|
|
|
|
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>(deleted) goto test:
|
|
|
|
|
|
loop:
|
|
|
|
|
|
stmts
|
|
|
|
|
|
test:
|
|
|
|
|
|
if cond true
|
|
|
|
|
|
goto loop:
|
|
|
|
|
|
done:
|
|
|
|
|
|
</code></pre></div></div>
|
|
|
|
|
|
|
|
|
|
|
|
<h2 id="for-loop-updated">For loop (updated)</h2>
|
|
|
|
|
|
|
|
|
|
|
|
<p>In C:</p>
|
|
|
|
|
|
|
|
|
|
|
|
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>//format: for(initialization; condition testing; increment)
|
|
|
|
|
|
for(int i=0;i<n;++i){
|
|
|
|
|
|
//stmts
|
|
|
|
|
|
}
|
|
|
|
|
|
return;
|
|
|
|
|
|
</code></pre></div></div>
|
|
|
|
|
|
|
|
|
|
|
|
<p>Becomes:</p>
|
|
|
|
|
|
|
|
|
|
|
|
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>i=0; // initialization
|
|
|
|
|
|
while(i<n){// condition testing
|
|
|
|
|
|
//stmts
|
|
|
|
|
|
i++; //increment
|
|
|
|
|
|
}
|
|
|
|
|
|
return;
|
|
|
|
|
|
</code></pre></div></div>
|
|
|
|
|
|
|
|
|
|
|
|
<p>Questions:</p>
|
|
|
|
|
|
|
|
|
|
|
|
<ol>
|
|
|
|
|
|
<li>Here are we coding the false condition first?</li>
|
|
|
|
|
|
<li>Which loop patterns are we using?</li>
|
|
|
|
|
|
</ol>
|
|
|
|
|
|
|
|
|
|
|
|
<p>Which becomes, in Assembly:</p>
|
|
|
|
|
|
|
|
|
|
|
|
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> xorl %ecx, %ecx # initialization
|
|
|
|
|
|
loop: # %ecx (i) <- 0
|
|
|
|
|
|
cmpl %edi, %ecx # while i < n true (testing)
|
|
|
|
|
|
jge endloop # jump when i>=n (false condition); In this situation "je" would also work. Do you see why?
|
|
|
|
|
|
#stmts
|
|
|
|
|
|
incl %ecx # i++ increment
|
|
|
|
|
|
jmp loop # loop again
|
|
|
|
|
|
endloop:
|
|
|
|
|
|
ret
|
|
|
|
|
|
</code></pre></div></div>
|
|
|
|
|
|
|
|
|
|
|
|
<h2 id="summary">Summary</h2>
|
|
|
|
|
|
|
|
|
|
|
|
<p>Compiler can produce different instruction combinations when assembling the same C code.</p>
|
|
|
|
|
|
|
|
|
|
|
|
<p><code class="language-plaintext highlighter-rouge">cmp*</code> and <code class="language-plaintext highlighter-rouge">test*</code> instructions set condition codes</p>
|
|
|
|
|
|
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>In x86-64 assembly, there are no conditional statements, however,
|
|
|
|
|
|
we can alter the execution flow of a program by using …
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li><code class="language-plaintext highlighter-rouge">cmp*</code> instruction (compare)</li>
|
|
|
|
|
|
<li><code class="language-plaintext highlighter-rouge">jX</code> instructions (jump)</li>
|
|
|
|
|
|
<li><code class="language-plaintext highlighter-rouge">call</code> and ret instructions</li>
|
|
|
|
|
|
<li><code class="language-plaintext highlighter-rouge">cmovX</code> instructions -> conditional move</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
<li>In x86-64 assembly, there are no iterative statements, however, we
|
|
|
|
|
|
can alter the execution flow of a program by using …
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li><code class="language-plaintext highlighter-rouge">cmp*</code> instruction</li>
|
|
|
|
|
|
<li><code class="language-plaintext highlighter-rouge">jX</code> instructions (jump)</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
<li>CPU uses these condition codes to decide whether a …
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li><code class="language-plaintext highlighter-rouge">jX</code> instruction (conditional jump) is to be exectued or a</li>
|
|
|
|
|
|
<li><code class="language-plaintext highlighter-rouge">cmovX</code> instruction (conditional move) is to be exectued</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
<li>2 loop patterns:
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>“coding the false condition first” -> <code class="language-plaintext highlighter-rouge">while</code> loops (hence <code class="language-plaintext highlighter-rouge">for</code> loops)</li>
|
|
|
|
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<li>“jump-in-middle” -> <code class="language-plaintext highlighter-rouge">while</code>, <code class="language-plaintext highlighter-rouge">do-while</code> (hence for loops)</li>
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</ul>
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</li>
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</ul>
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<h2 id="next-lecture">Next Lecture</h2>
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<ul>
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<li>Introduction
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<ul>
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<li>C program -> assembly code -> machine level code</li>
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</ul>
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</li>
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<li>Assembly language basics: data, move operation
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<ul>
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<li>Memory addressing modes</li>
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</ul>
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</li>
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<li>Operation leaq and Arithmetic & logical operations</li>
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<li>Conditional Statement – Condition Code + cmovX</li>
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<li>Loops</li>
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<li>(highlighted) Function call – Stack
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<ul>
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<li>(highlighted) Overview of Function Call</li>
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<li>(highlighted) Memory Layout and Stack - x86-64 instructions and registers</li>
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<li>(highlighted) Passing control</li>
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<li>Passing data – Calling Conventions</li>
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<li>Managing local data</li>
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</ul>
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</li>
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<li>Array</li>
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<li>Buffer Overflow</li>
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<li>Floating-point operations</li>
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</ul>
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</div>
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</main>
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<hr>
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<footer>
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