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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>
<div id="outline-container-orgb0eec26" class="outline-2">
<h2 id="orgb0eec26">Memory :</h2>
<div class="outline-text-2" id="text-orgb0eec26">
<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>
</div>
<div id="outline-container-org57cd217" class="outline-4">
<h4 id="org57cd217">Important :</h4>
<div class="outline-text-4" id="text-org57cd217">
<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>
</div>
<ul class="org-ul">
<li><a id="orgcbdf7c0"></a>Example :<br />
<div class="outline-text-5" id="text-orgcbdf7c0">
<p>
Let the Physical address (Or Effective Address, these two terms are interchangeable) <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>
</div>
</li>
</ul>
</div>
<div id="outline-container-org758f05f" class="outline-3">
<h3 id="org758f05f">Registers</h3>
<div class="outline-text-3" id="text-org758f05f">
<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>
</div>
<div id="outline-container-orgbb2bc8f" class="outline-4">
<h4 id="orgbb2bc8f">General Registers</h4>
<div class="outline-text-4" id="text-orgbb2bc8f">
<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>
</div>
<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>
</pre>
</div>
<p>
<b>CX</b>: This is the counter register. It is of 16 bits and is divided into two 8-bit registers CH and CL to also perform 8-bit instructions. It is used in looping and rotation. Example:
</p>
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">CX</span>, <span style="color: #fab387;">0005</span>
<span style="color: #89b4fa;">LOOP</span>
</pre>
</div>
<p>
<b>DX</b>: This is the data register. It is of 16 bits and is divided into two 8-bit registers DH and DL to also perform 8-bit instructions. It is used in the multiplication and input/output port addressing. Example:
</p>
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #89b4fa;">MUL</span> <span style="color: #cba6f7;">BX</span> (DX, AX = AX * BX)
</pre>
</div>
</div>
</div>
</div>
<div id="outline-container-org6de3be7" class="outline-3">
<h3 id="org6de3be7">Addressing and registers…again</h3>
<div class="outline-text-3" id="text-org6de3be7">
</div>
<div id="outline-container-orgfe32dc7" class="outline-4">
<h4 id="orgfe32dc7">I realized what I wrote here before was almost gibberish, sooo here we go again I guess ?</h4>
<div class="outline-text-4" id="text-orgfe32dc7">
<p>
Well lets take a step back to the notion of effective addresses VS relative ones.
</p>
</div>
</div>
<div id="outline-container-org471cf7b" class="outline-4">
<h4 id="org471cf7b">Effective = 10h x Segment + Offset . Part1</h4>
<div class="outline-text-4" id="text-org471cf7b">
<p>
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 ?
</p>
</div>
<ul class="org-ul">
<li><a id="orgf021c83"></a>Answer :<br />
<div class="outline-text-5" id="text-orgf021c83">
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #89b4fa;">MOV</span> [DS:0234h], 0x0005
</pre>
</div>
<p>
Why ? Let’s break it down :
</p>
<div id="orgf6af9f9" class="figure">
<p><img src="../../src/gifs/lain-dance.gif" alt="lain-dance.gif" />
</p>
</div>
<p>
We Already know that <b>Effective = 10h x Segment + Offset</b>, So here we have : <b>1234h = 10h x DS + Offset</b>, we already know that <b>DS = 0100h</b>, we end up with this simple equation <b>1234h = 1000h + Offset</b>, therefor the Offset is <b>0234h</b>
</p>
<p>
Simple, right ?, now for another example
</p>
</div>
</li>
</ul>
</div>
<div id="outline-container-org87609ad" class="outline-4">
<h4 id="org87609ad">Another example :</h4>
<div class="outline-text-4" id="text-org87609ad">
<p>
What if we now have this instruction ?
</p>
<div class="org-src-container">
<pre class="src src-asm"> <span style="color: #cba6f7;">MOV</span> [0234h], 0x0005
</pre>
</div>
<p>
What does it do ? You might or might not be surprised that it does the exact same thing as the other snipped of code, why though ? Because apparently and for some odd reason I don’t know, the compiler Implicitly assumes that the segment used is the <b>DS</b> one. So if you don’t specify a register( we will get to this later ), or a segment. Then the offset is considered an offset with a DS segment.
</p>
</div>
</div>
<div id="outline-container-org6254e46" class="outline-4">
<h4 id="org6254e46">Segment + Register <3</h4>
<div class="outline-text-4" id="text-org6254e46">
<p>
Consider <b>DS = 0100h</b> and <b>BX = BP = 0234h</b> and this code snippet:
</p>
<div class="org-src-container">
<pre class="src src-asm"> <span style="color: #cba6f7;">MOV</span> [BX], 0x0005 <span style="color: #6c7086;">; </span><span style="color: #a6e3a1; font-weight: bold;">NOTE</span><span style="color: #6c7086;"> : ITS NOT THE SAME AS MOV BX, 0x0005. Refer to earlier paragraphs</span>
</pre>
</div>
<p>
Well you guessed it right, it also does the same thing, but now consider this :
</p>
<div class="org-src-container">
<pre class="src src-asm"> <span style="color: #cba6f7;">MOV</span> [BP], 0x0005
</pre>
</div>
<p>
If you answered that its the same one, you are wrong. And this is because the segment used changes according to the offset as I said before in an implicit way. Here is the explicit equivalent of the two commands above:
</p>
<div class="org-src-container">
<pre class="src src-asm"> <span style="color: #cba6f7;">MOV</span> [DS:BX], 0x0005
<span style="color: #cba6f7;">MOV</span> [SS:BP], 0x0005
</pre>
</div>
<p>
The General rule of thumb is as follows :
</p>
<ul class="org-ul">
<li>If the offset is : DI SI or BX, the Segment used is DS.</li>
<li>If its BP or SP, then the segment is SS.</li>
</ul>
</div>
<ul class="org-ul">
<li><a id="orgb5208d9"></a>Note<br />
<div class="outline-text-5" id="text-orgb5208d9">
<p>
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
</p>
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">DS</span>, 0x0005 <span style="color: #6c7086;">; </span><span style="color: #6c7086;">Is INVALID</span>
<span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">DS</span>, AX <span style="color: #6c7086;">; </span><span style="color: #6c7086;">This one is VALID</span>
</pre>
</div>
</div>
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-org660d1f4" class="outline-2">
<h2 id="org660d1f4">The ACTUAL thing :</h2>
<div class="outline-text-2" id="text-org660d1f4">
<p>
Enough technical rambling, and now we shall go to the fun part, the ACTUAL CODE. But first, some names you should be familiar with :
</p>
<ul class="org-ul">
<li><b>Mnemonics</b> : Or <b>Instructions</b>, are the…well…Instructions executed by the CPU like <b>MOV</b> , <b>ADD</b>, <b>MUL</b>…etc, they are case <b>insensitive</b> but i like them better in UPPERCASE.</li>
<li><b>Operands</b> : These are the options passed to the instructions, like <b>MOV dst, src</b>, and they can be anything from a memory location, to a variable to an immediate address.</li>
</ul>
</div>
<div id="outline-container-orgf7c0650" class="outline-3">
<h3 id="orgf7c0650">Structure of an assembly program :</h3>
<div class="outline-text-3" id="text-orgf7c0650">
<p>
While there is no “standard” structure, i prefer to go with this one :
</p>
<div class="org-src-container">
<pre class="src src-asm"> <span style="color: #cba6f7;">org</span> 100h
<span style="color: #cba6f7;">.data</span>
<span style="color: #6c7086;">; </span><span style="color: #6c7086;">variables and constants</span>
<span style="color: #cba6f7;">.code</span>
<span style="color: #6c7086;">; </span><span style="color: #6c7086;">instructions</span>
</pre>
</div>
</div>
</div>
<div id="outline-container-orgc1466ec" class="outline-3">
<h3 id="orgc1466ec">MOV dst, src</h3>
<div class="outline-text-3" id="text-orgc1466ec">
<p>
The MOV instruction copies the Second operand (src) to the First operand (dst)… The source can be a memory location, an immediate value, a general-purpose register (AX BX CX DX). As for the Destination, it can be a general-purpose register or a memory location.
</p>
<p>
these types of operands are supported:
</p>
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">REG</span>, memory
<span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">memory</span>, REG
<span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">REG</span>, REG
<span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">memory</span>, immediate
<span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">REG</span>, immediate
</pre>
</div>
<p>
<b>REG</b>: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.
</p>
<p>
<b>memory</b>: [BX], [BX+SI+7], variable
</p>
<p>
<b>immediate</b>: 5, -24, 3Fh, 10001101b
</p>
<p>
for segment registers only these types of MOV are supported:
</p>
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">SREG</span>, memory
<span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">memory</span>, SREG
<span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">REG</span>, SREG
<span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">SREG</span>, REG
<span style="color: #89b4fa;">SREG</span>: <span style="color: #cba6f7;">DS</span>, ES, SS, and only as second operand: CS.
</pre>
</div>
<p>
<b>REG</b>: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.
</p>
<p>
<b>memory</b>: [BX], [BX+SI+7], variable
</p>
</div>
<div id="outline-container-org508d45b" class="outline-4">
<h4 id="org508d45b">Note : The MOV instruction <b>cannot</b> be used to set the value of the CS and IP registers</h4>
</div>
</div>
<div id="outline-container-orgb475e10" class="outline-3">
<h3 id="orgb475e10">Variables :</h3>
<div class="outline-text-3" id="text-orgb475e10">
<p>
Let’s say you want to use a specific value multiple times in your code, do you prefer to call it using something like <b>var1</b> or <b>E4F9:0011</b> ? If your answer is the second option, you can gladly skip this section, or even better, seek therapy.
</p>
<p>
Anyways, we have two types of variables, <b>bytes</b> and <b>words(which are two bytes)</b>, and to define a variable, we use the following syntax
</p>
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #89b4fa;">name</span> <span style="color: #cba6f7;">DB</span> value <span style="color: #6c7086;">; </span><span style="color: #6c7086;">To Define a Byte</span>
<span style="color: #89b4fa;">name</span> <span style="color: #cba6f7;">DW</span> value <span style="color: #6c7086;">; </span><span style="color: #6c7086;">To Define a Word</span>
</pre>
</div>
<p>
<b>name</b> - can be any letter or digit combination, though it should start with a letter. It’s possible to declare unnamed variables by not specifying the name (this variable will have an address but no name).
<b>value</b> - can be any numeric value in any supported numbering system (hexadecimal, binary, or decimal), or “?” symbol for variables that are not initialized.
</p>
</div>
<div id="outline-container-org3d5d0c5" class="outline-4">
<h4 id="org3d5d0c5">Example code :</h4>
<div class="outline-text-4" id="text-org3d5d0c5">
<div class="org-src-container">
<pre class="src src-asm"> <span style="color: #cba6f7;">org</span> 100h
<span style="color: #cba6f7;">.data</span>
<span style="color: #cba6f7;">x</span> db <span style="color: #fab387;">33</span>
<span style="color: #cba6f7;">y</span> dw 1350h
<span style="color: #cba6f7;">.code</span>
<span style="color: #cba6f7;">MOV</span> AL, x
<span style="color: #cba6f7;">MOV</span> BX, y
</pre>
</div>
</div>
</div>
<div id="outline-container-orgd4e5244" class="outline-4">
<h4 id="orgd4e5244">Arrays :</h4>
<div class="outline-text-4" id="text-orgd4e5244">
<p>
We can also define Arrays instead of single values using comma separated vaues. like this for example
</p>
<div class="org-src-container">
<pre class="src src-asm"> <span style="color: #cba6f7;">a</span> db 48h, 65h, 6Ch, 6Fh, 00H
<span style="color: #cba6f7;">b</span> db 'Hello', <span style="color: #fab387;">0</span>
</pre>
</div>
<p>
Surprise Surprise, the arrays a and b are identical, the reason behind it is that characters are first converted to their ASCII values then stored in memory!!! Wonderful right ? And guess what, accessing values in assembly IS THE SAME AS IN C !!!
</p>
<div class="org-src-container">
<pre class="src src-asm"> <span style="color: #cba6f7;">MOV</span> AL, a[<span style="color: #fab387;">0</span>] <span style="color: #6c7086;">; </span><span style="color: #6c7086;">Copies 48h to AL</span>
<span style="color: #cba6f7;">MOV</span> BL, b[<span style="color: #fab387;">0</span>] <span style="color: #6c7086;">; </span><span style="color: #6c7086;">Also Copies 48h to BL</span>
</pre>
</div>
<p>
You can also use any of the memory index registers BX, SI, DI, BP, for example:
</p>
<div class="org-src-container">
<pre class="src src-asm"><span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">SI</span>, <span style="color: #fab387;">3</span>
<span style="color: #89b4fa;">MOV</span> <span style="color: #cba6f7;">AL</span>, a[SI]
</pre>
</div>
<p>
If you need to declare a large array you can use DUP operator.
The syntax for <b>DUP</b>:
</p>
<p>
number DUP ( value(s) )
<b>number</b> - number of duplicate to make (any constant value).
<b>value</b> - expression that DUP will duplicate.
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<p>
for example:
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<pre class="src src-asm"><span style="color: #89b4fa;">c</span> <span style="color: #cba6f7;">DB</span> <span style="color: #fab387;">5</span> DUP(<span style="color: #fab387;">9</span>)
<span style="color: #6c7086;">;</span><span style="color: #6c7086;">is an alternative way of declaring:</span>
<span style="color: #89b4fa;">c</span> <span style="color: #cba6f7;">DB</span> <span style="color: #fab387;">9</span>, <span style="color: #fab387;">9</span>, <span style="color: #fab387;">9</span>, <span style="color: #fab387;">9</span>, <span style="color: #fab387;">9</span>
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<p>
one more example:
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<pre class="src src-asm"><span style="color: #89b4fa;">d</span> <span style="color: #cba6f7;">DB</span> <span style="color: #fab387;">5</span> DUP(<span style="color: #fab387;">1</span>, <span style="color: #fab387;">2</span>)
<span style="color: #6c7086;">;</span><span style="color: #6c7086;">is an alternative way of declaring:</span>
<span style="color: #89b4fa;">d</span> <span style="color: #cba6f7;">DB</span> <span style="color: #fab387;">1</span>, <span style="color: #fab387;">2</span>, <span style="color: #fab387;">1</span>, <span style="color: #fab387;">2</span>, <span style="color: #fab387;">1</span>, <span style="color: #fab387;">2</span>, <span style="color: #fab387;">1</span>, <span style="color: #fab387;">2</span>, <span style="color: #fab387;">1</span>, <span style="color: #fab387;">2</span>
</pre>
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<p>
Of course, you can use DW instead of DB if it’s required to keep values larger then 255, or smaller then -128. DW cannot be used to declare strings.
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<div id="postamble" class="status">
<p class="author">Author: Crystal</p>
<p class="date">Created: 2024-03-22 Fri 14:08</p>
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