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<h1 class="title">x86 Assembly from my understanding</h1>
<p>
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
</p>
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<h2 id="org8392211">Memory :</h2>
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<p>
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)
</p>
<ul class="org-ul">
<li>The Segment is a set of 64 consecutive Koctets (1 Koctet = 1024 octets).</li>
<li>And the offset is to specify the particular octet in that segment.</li>
</ul>
<p>
The offset and segment are encoded in 16bits, so they take a value between 0 and 65535
</p>
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<h4 id="orgb021039">Important :</h4>
<div class="outline-text-4" id="text-orgb021039">
<p>
The relation between the Effective Address and the Segment & Offset is as follow :
</p>
<p>
<b><b>Effective address = 16 x segment + offset</b></b> keep in mind that this equation is encoded in decimal, which will change soon as we use Hexadecimal for convention reasons.
</p>
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<ul class="org-ul">
<li><a id="orgda02cd3"></a>Example :<br />
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<p>
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>
</p>
<p>
Now if you are a clever one ( I know you are, since you are reading this <3 ) you may say that the physical address <b>12345h</b> can be written in more than one way….and you are right, more precisely : <b>2<sup>12</sup> = 4096</b> different ways !!!
</p>
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</li>
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<h3 id="orgd56c9fa">Registers</h3>
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<p>
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).
</p>
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<h4 id="orgd10cc4b">General Registers</h4>
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<p>
General registers contribute to arithmetic’s and logic and addressing too.
</p>
<p>
Each half-register is accessible as a register of 8bits, therefor making the 8086 backwards compatible with the 8080 (which had 8bit registers)
</p>
<p>
Now here are the Registers we can find in this section:
</p>
<p>
<b>AX</b>: This is the accumulator. It is of 16 bits and is divided into two 8-bit registers AH and AL to also perform 8-bit instructions. It is generally used for arithmetical and logical instructions but in 8086 microprocessor it is not mandatory to have an accumulator as the destination operand. Example:
</p>
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #89b4fa;">ADD</span> <span style="color: #cba6f7;">AX</span>, AX <span style="color: #6c7086;">;</span><span style="color: #6c7086;">(AX = AX + AX)</span>
</pre>
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<p>
<b>BX</b>: This is the base register. It is of 16 bits and is divided into two 8-bit registers BH and BL to also perform 8-bit instructions. It is used to store the value of the offset. Example:
</p>
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">BL</span>, [<span style="color: #fab387;">500</span>] <span style="color: #6c7086;">;</span><span style="color: #6c7086;">(BL = 500H)</span>
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<div id="postamble" class="status">
<p class="author">Author: Crystal</p>
<p class="date">Created: 2024-02-24 Sat 17:36</p>
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