ranger - mirror of ranger - a simple, vim-like file manager

	Commit message (Expand)	Author	Age	Files	Lines
*	Fire in the Hole! v1.6.0	hut	2013-02-22	1	-0/+38
*	ceci n'est pas un nouveau version v1.5.5	hut	2012-08-10	1	-0/+3
*	What kind of idiot invented bugs anyway? v1.5.4	hut	2012-05-03	1	-0/+5
*	Always remember to bring your towel. v1.5.3	hut	2012-03-05	1	-0/+22
*	The hard work of the electrons must be respected. v1.5.2	hut	2011-10-24	1	-0/+4
*	The hard work of the computer must be respected. v1.5.1	hut	2011-10-24	1	-0/+11
*	FOR THE HURD v1.5.0	hut	2011-10-11	1	-12/+30
*	api.commands: finished merging ext.command_parser into api	hut	2011-10-10	1	-0/+13
*	CHANGELOG: sorted items	hut	2011-10-02	1	-7/+7
*	One in five ranger users use it exclusively for browsing porn v1.4.4	hut	2011-10-02	1	-4/+17
*	This gonna be cool once it's finished v1.4.3	hut	2011-04-05	1	-0/+5
*	Next version will run on pure Lisp Machines only v1.4.2	hut	2011-03-05	1	-0/+5
*	If ranger won't run, you still got peanut butter to live for. v1.4.1	hut	2011-01-04	1	-0/+4
*	We are not authorized to disclose the commit message v1.4.0	hut	2010-12-22	1	-3/+20
*	HOW DO I TURN OFF CAPS LOCK? v1.2.3	hut	2010-12-13	1	-0/+7
*	Tea, Earl Grey, Hot. v1.2.2	hut	2010-10-10	1	-8/+16
*	CHANGELOG updated	hut	2010-09-16	1	-0/+3
*	Beware of low-flying butterflies v1.2.0	hut	2010-09-13	1	-0/+18
*	Why did the astrophysicist order three hamburgers? v1.1.2	hut	2010-07-12	1	-0/+6
*	version = version + 1 v1.1.1	hut	2010-06-18	1	-0/+1
*	added CHANGELOG	hut	2010-06-18	1	-0/+21

<?xml version="1.0" encoding="utf-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"> <head>  <meta http-equiv="Content-Type" content="text/html;charset=utf-8" /> <meta name="viewport" content="width=device-width, initial-scale=1" /> <title>x86 Assembly from my understanding</title> <meta name="author" content="Crystal" /> <meta name="generator" content="Org Mode" /> <link rel="stylesheet" type="text/css" href="../../src/css/colors.css"/> <link rel="stylesheet" type="text/css" href="../../src/css/style.css"/> <link rel="icon" type="image/x-icon" href="../../../favicon.png"> </head> <body> <div id="org-div-home-and-up"> <a accesskey="h" href=""> UP </a> | <a accesskey="H" href="https://crystal.tilde.institute/"> HOME </a> </div><div id="content" class="content"> <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-orgbee531b" class="outline-2"> <h2 id="orgbee531b">Memory :</h2> <div class="outline-text-2" id="text-orgbee531b"> <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-org3db3907" class="outline-4"> <h4 id="org3db3907">Important :</h4> <div class="outline-text-4" id="text-org3db3907"> <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="org37b19f8"></a>Example :<br /> <div class="outline-text-5" id="text-org37b19f8"> <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> </div> </li> </ul> </div> <div id="outline-container-org9bc1004" class="outline-3"> <h3 id="org9bc1004">Registers</h3> <div class="outline-text-3" id="text-org9bc1004"> <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-org9277a22" class="outline-4"> <h4 id="org9277a22">General Registers</h4> <div class="outline-text-4" id="text-org9277a22"> <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-org2702b3c" class="outline-3"> <h3 id="org2702b3c">Addressing and registers…again</h3> <div class="outline-text-3" id="text-org2702b3c"> </div> <div id="outline-container-orgef839da" class="outline-4"> <h4 id="orgef839da">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-orgef839da"> <p> Well lets take a step back to the notion of effective addresses VS relative ones. </p> </div> </div> <div id="outline-container-orgad7af48" class="outline-4"> <h4 id="orgad7af48">Effective = 10h x Segment + Offset . Part1</h4> <div class="outline-text-4" id="text-orgad7af48"> <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="org0b2c15e"></a>Answer :<br /> <div class="outline-text-5" id="text-org0b2c15e"> <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 : <img src="../../gifs/lain-dance.gif" alt="lain-dance.gif" /> </p> <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-org0b9e5e9" class="outline-4"> <h4 id="org0b9e5e9">Another example :</h4> <div class="outline-text-4" id="text-org0b9e5e9"> <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-orge060532" class="outline-4"> <h4 id="orge060532">Segment + Register <3</h4> <div class="outline-text-4" id="text-orge060532"> <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="orgc97f34f"></a>Note<br /> <div class="outline-text-5" id="text-orgc97f34f"> <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> <div id="postamble" class="status"> <p class="author">Author: Crystal</p> <p class="date">Created: 2024-03-07 Thu 20:51</p> </div> </body> </html>