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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 12 – Assembly language – Program Control – Conditional Statements</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>Demo
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<ul>
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<li>Observation: C compiler will figure out different instruction combinations to carry out the computations in our C code</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 (selected)</li>
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<li>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="completing-our-demo">Completing our Demo</h2>
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<ol>
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<li>(checked) <code class="language-plaintext highlighter-rouge">gcc</code> uses <code class="language-plaintext highlighter-rouge">leaq</code> for addition -> <code class="language-plaintext highlighter-rouge">sum_store.c</code> our own assembly code (arith.s) using arithmetic</li>
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<li>(checked) Writing instructions of x86-64 assembly language</li>
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<li>makefile
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<ul>
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<li>when we compile our own *.s files with *.c files</li>
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<li>when we compile only *.c files</li>
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</ul>
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</li>
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<li>How would <code class="language-plaintext highlighter-rouge">gcc</code> compile our <code class="language-plaintext highlighter-rouge">arith.c</code> into <code class="language-plaintext highlighter-rouge">arith.s</code>?</li>
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</ol>
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<h2 id="program-control-overview">Program Control Overview</h2>
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<ul>
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<li>We can change the execution flow of a program
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<ol>
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<li>Based on a condition</li>
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<li>Unconditionally</li>
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</ol>
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</li>
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<li>Control statements (in C)
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<ul>
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<li>if/else</li>
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<li>switch</li>
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<li>cmp* instruction (compare)</li>
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<li>for loop</li>
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</ul>
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</li>
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<li>Control statements (in x86-64 assemvly)
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<ul>
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<li>cmp* instruction (compare)</li>
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<li>jX instructions (jump)</li>
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</ul>
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</li>
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</ul>
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<p>Function calls -> <code class="language-plaintext highlighter-rouge">call</code> and <code class="language-plaintext highlighter-rouge">ret</code></p>
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<h2 id="conditional-statement-ifelse">Conditional statement: <code class="language-plaintext highlighter-rouge">if/else</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>void func(long x,long y){
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if ( x < y ) {
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// stmts true
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} else {
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// stmts false
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}
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return;
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}
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</code></pre></div></div>
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<p>A label is a memory address.</p>
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<p>in assembly:</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code># func is a label
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func:
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cmpq %rsi,%rdi # x – y
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jge else #
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... # stmts true
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jmp endif #
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# else is a label
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else: ... # stmts false
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# endif is a label
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endif: ret #
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</code></pre></div></div>
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<p>We branch (jump) when the condition is false -> This technique is called “coding the false condition first”</p>
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<h2 id="comparison-instructions">comparison instructions</h2>
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<ul>
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<li>* -> Size designator</li>
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<li>q -> long/64-bits</li>
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<li>l -> int/32-bits</li>
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<li>w -> short/16-bits</li>
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<li>b -> char/8-bits</li>
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</ul>
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<p>Remember in Lecture 9, we saw… (See header “Programming in x86-64 assembly” from lecture 9)</p>
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<table>
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<thead>
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<tr>
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<th>Syntax</th>
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<th>Meaning/Effect</th>
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<th>Example</th>
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<th>Notes</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>cmp* Src2, Src1</td>
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<td>Src1 – Src2 -><br />> 0? -> Src1 > Src2<br />= 0? -> Src1 == Src2<br />< 0? -> Src1 < Src2</td>
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<td><code class="language-plaintext highlighter-rouge">cmpq %rsi,%rdi</code></td>
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<td>without saving the result in the destination operand (no Dest). Sets <strong>condition codes</strong> based on value of <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mtext>Src1</mtext><mo>−</mo><mtext>Src2</mtext></mrow><annotation encoding="application/x-tex">\text{Src1} - \text{Src2}</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.76666em;vertical-align:-0.08333em;"></span><span class="mord text"><span class="mord">Src1</span></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:0.68333em;vertical-align:0em;"></span><span class="mord text"><span class="mord">Src2</span></span></span></span></span></td>
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</tr>
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<tr>
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<td>test* Src2, Src1</td>
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<td>Src1 & Src2 -></td>
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<td><code class="language-plaintext highlighter-rouge">testq %rax,%rax</code></td>
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<td>without saving the result in the destination operand (no <strong>Dest</strong>); sets <strong>condition code</strong> based on value of <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mtext>Src1</mtext><mo>∧</mo><mtext>Src2</mtext></mrow><annotation encoding="application/x-tex">\text{Src1} \land \text{Src2}</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.68333em;vertical-align:0em;"></span><span class="mord text"><span class="mord">Src1</span></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:0.68333em;vertical-align:0em;"></span><span class="mord text"><span class="mord">Src2</span></span></span></span></span>; useful when one of the operands is a bit mask</td>
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</tr>
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</tbody>
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</table>
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<h2 id="jx-jump-family-instructions-branching"><code class="language-plaintext highlighter-rouge">jX</code> jump family instructions (branching)</h2>
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<ul>
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<li>Jump to different part of the program depending on result of previous instructions (i.e., condition codes)</li>
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</ul>
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<table>
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<thead>
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<tr>
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<th><code class="language-plaintext highlighter-rouge">jX</code></th>
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<th>Description</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>jmp</td>
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<td>unconditional</td>
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</tr>
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<tr>
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<td>je</td>
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<td>Equal / Zero</td>
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</tr>
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<tr>
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<td>jne</td>
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<td>Not Equal / Not Zero</td>
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</tr>
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<tr>
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<td>js</td>
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<td>Negative</td>
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</tr>
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<tr>
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<td>jns</td>
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<td>Nonnegative</td>
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</tr>
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<tr>
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<td>jg</td>
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<td>Greater (Signed)</td>
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</tr>
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<tr>
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<td>jge</td>
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<td>Greater or Equal (Signed)</td>
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</tr>
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<tr>
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<td>jl</td>
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<td>Less (Signed)</td>
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</tr>
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<tr>
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<td>jle</td>
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<td>Less or Equal (Signed</td>
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</tr>
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<tr>
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<td>ja</td>
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<td>Above (unsigned)</td>
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</tr>
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<tr>
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<td>jb</td>
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<td>Below (unsigned)</td>
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</tr>
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</tbody>
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</table>
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<h2 id="example--int-absint-x">Example – <code class="language-plaintext highlighter-rouge">int abs(int x)</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 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:</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code># x in edi, result in eax
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abs:
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movl %edi,%eax # eax <- x
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____ #
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____ # ret if x >= 0
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____ # x = -x
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endif:
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ret
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</code></pre></div></div>
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<h2 id="int-maxint-x-int-y---homework"><code class="language-plaintext highlighter-rouge">int max(int x, int y</code> )- Homework</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>In assembly:</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code># x in edi, y in esi, result in eax
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max:
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movl %edi,%eax # result = x
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____
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____
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____
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endif:
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ret
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</code></pre></div></div>
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<h2 id="summary">Summary</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><code class="language-plaintext highlighter-rouge">cmp*</code> instruction (compare)</li>
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<li><code class="language-plaintext highlighter-rouge">jX</code> instructions (jump)</li>
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<li><code class="language-plaintext highlighter-rouge">call</code> and <code class="language-plaintext highlighter-rouge">ret</code> instructions</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>(selected) 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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</div>
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</main>
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<hr>
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<footer>
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</footer>
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</body>
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