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<h1 id="memory--pointers-1">Memory & Pointers 1</h1>
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<p>CMPT-225, Fall 2021</p>
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<h2 id="computer-memory">Computer Memory</h2>
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<ul>
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<li>A sequence of locations</li>
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<li>Indexed by address: 0,1,2,…</li>
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<li>Each location stores a data <em>byte</em></li>
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<li>Processor can <em>read</em> or <em>write</em> the byte at each address.</li>
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<li>Regions of memory are allocated to processes <em>as needed</em>, according to some scheme.</li>
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</ul>
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<p>Diagram displaying “Code + Data for running part of OS” at the start of memory, “Free memory” in the middle, and “Code + Data for running user processes” and the end.</p>
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<h2 id="variables--memory">Variables & Memory</h2>
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<p>A variable is (roughly) and names & tagged collection of bytes:</p>
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<p>Diagram showing:</p>
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<ul>
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<li>“int x; x = 6” taking 4 bytes of memory,</li>
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<li>“char c; c = ‘x’;” taking 1 byte of memory</li>
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<li>
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<p>“bool b; b = ‘true’;” taking 1 byte of memory</p>
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</li>
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<li>So, <em>at run time</em>, each variable has an <em>address</em> in memory.</li>
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<li>In C, C++ we can:
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<ul>
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<li>access the address of a variable</li>
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<li>access a variable or memory location by its address</li>
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<li>declare variables by storing addresses (pointers).</li>
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</ul>
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</li>
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</ul>
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<h2 id="addresses--pointers---by-example">Addresses & Pointers - By Example</h2>
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<ul>
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<li>“int i = 5;”
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<ul>
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<li>allocate space for an <em>int</em>,</li>
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<li>name the space “i”,</li>
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<li>store 5 there</li>
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</ul>
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</li>
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<li>“int *p;”
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<ul>
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<li>allocate space for an address,</li>
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<li>name it p,</li>
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<li>record its type as “pointer to int”</li>
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</ul>
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</li>
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<li>“p = ∧i;”
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<ul>
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<li>“&i” is the address of “i”</li>
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<li>store ∧i in p</li>
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<li>so, p becomes a <em>pointer to i</em></li>
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</ul>
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</li>
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<li>“cout « i;”
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<ul>
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<li>outputs the value stored in i, <em>5</em></li>
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</ul>
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</li>
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<li>“cout « p;”
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<ul>
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<li>outputs the address of i: 0xbffffbbc</li>
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</ul>
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</li>
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<li>“cout « *p;”
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<ul>
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<li>”*” <em>dereferences</em> p. That is, *p is the value <em>pointed to by p</em>. In this case <em>5</em></li>
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</ul>
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</li>
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</ul>
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<p>Diagram of memory:</p>
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<table>
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<tbody>
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<tr>
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<td>Address</td>
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<td>Name</td>
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<td>Value</td>
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</tr>
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<tr>
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<td>…</td>
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<td>…</td>
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<td>…</td>
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</tr>
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<tr>
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<td><span id="point">254</span></td>
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<td>i</td>
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<td>5</td>
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</tr>
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<tr>
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<td>…</td>
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<td>…</td>
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<td>…</td>
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</tr>
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<tr>
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<td>923</td>
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<td>p</td>
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<td><a href="#point">254</a></td>
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</tr>
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</tbody>
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</table>
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<p>Second diagram of memory:</p>
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<table>
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<tbody>
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<tr>
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<td>Address</td>
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<td>Name</td>
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<td>Value</td>
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<td>Size</td>
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</tr>
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<tr>
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<td>0x6fffbe</td>
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<td>i</td>
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<td>5</td>
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<td>4 bytes</td>
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</tr>
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<tr>
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<td>…</td>
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<td>…</td>
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<td>…</td>
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<td>…</td>
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</tr>
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<tr>
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<td>???</td>
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<td>p</td>
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<td>0x6fffbe</td>
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<td>4 bytesa</td>
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</tr>
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</tbody>
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</table>
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<p>Slide showing program code: see pointers.c</p>
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<p>Slide showing output: see output.txt</p>
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<p>Slide showing same program with different highlights.</p>
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<p>Slide showing same output with different highlights.</p>
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<h2 id="end">End</h2>
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