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This commit is contained in:
Crystal
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
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<head>
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<!-- 2024-03-07 Thu 20:51 -->
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<!-- 2024-03-07 Thu 20:53 -->
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<meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
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<meta name="viewport" content="width=device-width, initial-scale=1" />
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<title>x86 Assembly from my understanding</title>
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@ -23,9 +23,9 @@
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<p>
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Soooo this article (or maybe even a series of articles, who knows ?) will be about x86 assembly, or rather, what I understood from it and my road from the bottom-up hopefully reaching a good level of understanding
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</p>
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<div id="outline-container-orgbee531b" class="outline-2">
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<h2 id="orgbee531b">Memory :</h2>
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<div class="outline-text-2" id="text-orgbee531b">
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<div id="outline-container-org2a24f51" class="outline-2">
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<h2 id="org2a24f51">Memory :</h2>
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<div class="outline-text-2" id="text-org2a24f51">
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<p>
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Memory is a sequence of octets (Aka 8bits) that each have a unique integer assigned to them called <b>The Effective Address (EA)</b>, in this particular CPU Architecture (the i8086), the octet is designated by a couple (A segment number, and the offset in the segment)
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</p>
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@ -40,9 +40,9 @@ Memory is a sequence of octets (Aka 8bits) that each have a unique integer assig
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The offset and segment are encoded in 16bits, so they take a value between 0 and 65535
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</p>
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</div>
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<div id="outline-container-org3db3907" class="outline-4">
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<h4 id="org3db3907">Important :</h4>
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<div class="outline-text-4" id="text-org3db3907">
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<div id="outline-container-org342f4bf" class="outline-4">
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<h4 id="org342f4bf">Important :</h4>
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<div class="outline-text-4" id="text-org342f4bf">
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<p>
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The relation between the Effective Address and the Segment & Offset is as follow :
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</p>
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@ -52,8 +52,8 @@ The relation between the Effective Address and the Segment & Offset is as fo
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</p>
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</div>
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<ul class="org-ul">
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<li><a id="org37b19f8"></a>Example :<br />
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<div class="outline-text-5" id="text-org37b19f8">
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<li><a id="org7dc9b55"></a>Example :<br />
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<div class="outline-text-5" id="text-org7dc9b55">
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<p>
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Let the Physical address (Or Effective Address, these two terms are enterchangeable) <b>12345h</b> (the h refers to Hexadecimal, which can also be written like this <b>0x12345</b>), the register <b>DS = 1230h</b> and the register <b>SI = 0045h</b>, the CPU calculates the physical address by multiplying the content of the segment register <b>DS</b> by 10h (or 16) and adding the content of the register <b>SI</b>. so we get : <b>1230h x 10h + 45h = 12345h</b>
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</p>
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@ -66,16 +66,16 @@ Now if you are a clever one ( I know you are, since you are reading this <3 )
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</li>
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</ul>
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</div>
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<div id="outline-container-org9bc1004" class="outline-3">
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<h3 id="org9bc1004">Registers</h3>
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<div class="outline-text-3" id="text-org9bc1004">
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<div id="outline-container-org6338772" class="outline-3">
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<h3 id="org6338772">Registers</h3>
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<div class="outline-text-3" id="text-org6338772">
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<p>
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The 8086 CPU has 14 registers of 16bits of size. From the POV of the user, the 8086 has 3 groups of 4 registers of 16bits. One state register of 9bits and a counting program of 16bits inaccessible to the user (whatever this means).
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</p>
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</div>
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<div id="outline-container-org9277a22" class="outline-4">
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<h4 id="org9277a22">General Registers</h4>
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<div class="outline-text-4" id="text-org9277a22">
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<div id="outline-container-org5583fa8" class="outline-4">
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<h4 id="org5583fa8">General Registers</h4>
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<div class="outline-text-4" id="text-org5583fa8">
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<p>
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General registers contribute to arithmetic’s and logic and addressing too.
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</p>
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@ -126,28 +126,28 @@ Now here are the Registers we can find in this section:
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</div>
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</div>
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</div>
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<div id="outline-container-org2702b3c" class="outline-3">
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<h3 id="org2702b3c">Addressing and registers…again</h3>
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<div class="outline-text-3" id="text-org2702b3c">
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<div id="outline-container-orga6303cd" class="outline-3">
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<h3 id="orga6303cd">Addressing and registers…again</h3>
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<div class="outline-text-3" id="text-orga6303cd">
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</div>
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<div id="outline-container-orgef839da" class="outline-4">
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<h4 id="orgef839da">I realized what I wrote here before was almost gibberish, sooo here we go again I guess ?</h4>
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<div class="outline-text-4" id="text-orgef839da">
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<div id="outline-container-org5b9f758" class="outline-4">
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<h4 id="org5b9f758">I realized what I wrote here before was almost gibberish, sooo here we go again I guess ?</h4>
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<div class="outline-text-4" id="text-org5b9f758">
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<p>
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Well lets take a step back to the notion of effective addresses VS relative ones.
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</p>
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</div>
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</div>
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<div id="outline-container-orgad7af48" class="outline-4">
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<h4 id="orgad7af48">Effective = 10h x Segment + Offset . Part1</h4>
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<div class="outline-text-4" id="text-orgad7af48">
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<div id="outline-container-org44d3398" class="outline-4">
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<h4 id="org44d3398">Effective = 10h x Segment + Offset . Part1</h4>
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<div class="outline-text-4" id="text-org44d3398">
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<p>
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When trying to access a specific memory space, we use this annotation <b>[Segment:Offset]</b>, so for example, and assuming <b>DS = 0100h</b>. We want to write the value <b>0x0005</b> to the memory space defined by the physical address <b>1234h</b>, what do we do ?
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</p>
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</div>
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<ul class="org-ul">
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<li><a id="org0b2c15e"></a>Answer :<br />
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<div class="outline-text-5" id="text-org0b2c15e">
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<li><a id="orga1b16b6"></a>Answer :<br />
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<div class="outline-text-5" id="text-orga1b16b6">
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<div class="org-src-container">
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<pre class="src src-asm"><span style="color: #89b4fa;">MOV</span> [DS:0234h], 0x0005
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</pre>
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@ -155,7 +155,7 @@ When trying to access a specific memory space, we use this annotation <b>[Segmen
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<p>
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Why ? Let’s break it down :
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<img src="../../gifs/lain-dance.gif" alt="lain-dance.gif" />
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<img src="file:///src/gifs/lain-dance.gif" alt="lain-dance.gif" />
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</p>
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@ -172,9 +172,9 @@ Simple, right ?, now for another example
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</li>
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</ul>
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</div>
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<div id="outline-container-org0b9e5e9" class="outline-4">
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<h4 id="org0b9e5e9">Another example :</h4>
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<div class="outline-text-4" id="text-org0b9e5e9">
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<div id="outline-container-org577af1c" class="outline-4">
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<h4 id="org577af1c">Another example :</h4>
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<div class="outline-text-4" id="text-org577af1c">
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<p>
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What if we now have this instruction ?
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</p>
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@ -187,9 +187,9 @@ What does it do ? You might or might not be surprised that it does the exact sam
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</p>
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</div>
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</div>
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<div id="outline-container-orge060532" class="outline-4">
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<h4 id="orge060532">Segment + Register <3</h4>
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<div class="outline-text-4" id="text-orge060532">
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<div id="outline-container-org99e946d" class="outline-4">
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<h4 id="org99e946d">Segment + Register <3</h4>
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<div class="outline-text-4" id="text-org99e946d">
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<p>
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Consider <b>DS = 0100h</b> and <b>BX = BP = 0234h</b> and this code snippet:
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</p>
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@ -225,8 +225,8 @@ The General rule of thumb is as follows :
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</ul>
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</div>
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<ul class="org-ul">
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<li><a id="orgc97f34f"></a>Note<br />
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<div class="outline-text-5" id="text-orgc97f34f">
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<li><a id="orgb46cb7c"></a>Note<br />
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<div class="outline-text-5" id="text-orgb46cb7c">
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<p>
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The values of the registers CS DS and SS are automatically initialized by the OS when launching the program. So these segments are implicit. AKA : If we want to access a specific data in memory, we just need to specify its offset. Also you can’t write directly into the DS or CS segment registers, so something like
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</p>
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@ -244,7 +244,7 @@ The values of the registers CS DS and SS are automatically initialized by the OS
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</div>
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<div id="postamble" class="status">
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<p class="author">Author: Crystal</p>
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<p class="date">Created: 2024-03-07 Thu 20:51</p>
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<p class="date">Created: 2024-03-07 Thu 20:53</p>
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</div>
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</body>
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</html>
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@ -80,7 +80,7 @@ MOV [DS:0234h], 0x0005
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#+END_SRC
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Why ? Let's break it down :
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[[../../gifs/lain-dance.gif]]
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[[/src/gifs/lain-dance.gif]]
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