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diff --git a/blog/asm/1.html b/blog/asm/1.html deleted file mode 100644 index 4ed3574..0000000 --- a/blog/asm/1.html +++ /dev/null @@ -1,538 +0,0 @@ -<?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> -<!-- 2024-04-10 Wed 21:05 --> -<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-orgd66d87f" class="outline-2"> -<h2 id="orgd66d87f">Memory :</h2> -<div class="outline-text-2" id="text-orgd66d87f"> -<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-orgb9ec69c" class="outline-4"> -<h4 id="orgb9ec69c">Important :</h4> -<div class="outline-text-4" id="text-orgb9ec69c"> -<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="org407193f"></a>Example :<br /> -<div class="outline-text-5" id="text-org407193f"> -<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-orgb81ab14" class="outline-3"> -<h3 id="orgb81ab14">Registers</h3> -<div class="outline-text-3" id="text-orgb81ab14"> -<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-orgcd127f7" class="outline-4"> -<h4 id="orgcd127f7">General Registers</h4> -<div class="outline-text-4" id="text-orgcd127f7"> -<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-orgde83b9e" class="outline-3"> -<h3 id="orgde83b9e">Addressing and registers…again</h3> -<div class="outline-text-3" id="text-orgde83b9e"> -</div> -<div id="outline-container-org598f23b" class="outline-4"> -<h4 id="org598f23b">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-org598f23b"> -<p> -Well lets take a step back to the notion of effective addresses VS relative ones. -</p> -</div> -</div> -<div id="outline-container-orga54d5c9" class="outline-4"> -<h4 id="orga54d5c9">Effective = 10h x Segment + Offset . Part1</h4> -<div class="outline-text-4" id="text-orga54d5c9"> -<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="org330429e"></a>Answer :<br /> -<div class="outline-text-5" id="text-org330429e"> -<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="orge9d2dab" 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-org21257b6" class="outline-4"> -<h4 id="org21257b6">Another example :</h4> -<div class="outline-text-4" id="text-org21257b6"> -<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-org7c948b1" class="outline-4"> -<h4 id="org7c948b1">Segment + Register <3</h4> -<div class="outline-text-4" id="text-org7c948b1"> -<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="orgec605fb"></a>Note<br /> -<div class="outline-text-5" id="text-orgec605fb"> -<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-org30acf72" class="outline-2"> -<h2 id="org30acf72">The ACTUAL thing :</h2> -<div class="outline-text-2" id="text-org30acf72"> -<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-org03a7d0f" class="outline-3"> -<h3 id="org03a7d0f">Structure of an assembly program :</h3> -<div class="outline-text-3" id="text-org03a7d0f"> -<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-orgbea80df" class="outline-3"> -<h3 id="orgbea80df">MOV dst, src</h3> -<div class="outline-text-3" id="text-orgbea80df"> -<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-orge229cf5" class="outline-4"> -<h4 id="orge229cf5">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-org05f299b" class="outline-3"> -<h3 id="org05f299b">Variables :</h3> -<div class="outline-text-3" id="text-org05f299b"> -<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-orga473d7b" class="outline-4"> -<h4 id="orga473d7b">Example code :</h4> -<div class="outline-text-4" id="text-orga473d7b"> -<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-org8ceedbb" class="outline-4"> -<h4 id="org8ceedbb">Arrays :</h4> -<div class="outline-text-4" id="text-org8ceedbb"> -<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. -</p> - -<p> -for example: -</p> -<div class="org-src-container"> -<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> -</pre> -</div> -<p> -one more example: -</p> -<div class="org-src-container"> -<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> -</div> -<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. -</p> -</div> -</div> -<div id="outline-container-org8fefb4b" class="outline-4"> -<h4 id="org8fefb4b">LEA</h4> -<div class="outline-text-4" id="text-org8fefb4b"> -<p> -LEA stands for (Load Effective Address) is an instruction used to get the offset of a specific variable. We will see later how its used, but first. here is something we will need : -</p> - -<p> -In order to tell the compiler about data type, -these prefixes should be used: -</p> - -<p> -<b>BYTE PTR</b> - for byte. -<b>WORD PTR</b> - for word (two bytes). -</p> - -<p> -For example: -<b>BYTE PTR [BX]</b> ; byte access. - or -<b>WORD PTR [BX]</b> ; word access. -assembler supports shorter prefixes as well: -</p> - -<ul class="org-ul"> -<li>b. - for BYTE PTR</li> -<li>w. - for WORD PTR</li> -</ul> - -<p> -in certain cases the assembler can calculate the data type automatically. -</p> -</div> -<ul class="org-ul"> -<li><a id="orgd644e48"></a>Example :<br /> -<div class="outline-text-5" id="text-orgd644e48"> -<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;">VAR1</span> db 50h - <span style="color: #cba6f7;">VAR2</span> dw 1234h - <span style="color: #cba6f7;">.code</span> - <span style="color: #cba6f7;">MOV</span> AL, VAR1 <span style="color: #6c7086;">; </span><span style="color: #6c7086;">We check the value of VAR1 by putting it in AL</span> - <span style="color: #cba6f7;">MOV</span> AX, VAR2 <span style="color: #6c7086;">; </span><span style="color: #6c7086;">Same here</span> - <span style="color: #cba6f7;">LEA</span> BX, VAR1 <span style="color: #6c7086;">; </span><span style="color: #6c7086;">BX receives the Address of VAR1</span> - <span style="color: #cba6f7;">MOV</span> b.[BX], 44h - <span style="color: #cba6f7;">MOV</span> AL, VAR1 <span style="color: #6c7086;">; </span><span style="color: #6c7086;">We effectively changed the content of the VAR1 variable</span> - <span style="color: #cba6f7;">LEA</span> BX, VAR2 - <span style="color: #cba6f7;">MOV</span> w.[BX], 5678h - <span style="color: #cba6f7;">MOV</span> AX, VAR2 -</pre> -</div> -</div> -</li> -</ul> -</div> -<div id="outline-container-org99559c2" class="outline-4"> -<h4 id="org99559c2">Constants :</h4> -<div class="outline-text-4" id="text-org99559c2"> -<p> -Constants in Assembly only exist until the code is assembled, meaning that if you disassemble your code later, you wont see your constant definitions. -</p> - -<p> -Defining constants is pretty straight forward : -</p> -<div class="org-src-container"> -<pre class="src src-asm"> <span style="color: #cba6f7;">name</span> EQU value -</pre> -</div> - -<p> -Of course constants cant be changed, and aren’t stored in memory. So they are like little macros that live in your code. -</p> -</div> -</div> -</div> -<div id="outline-container-org9179f72" class="outline-3"> -<h3 id="org9179f72">⚐ :</h3> -<div class="outline-text-3" id="text-org9179f72"> -<p> -Now comes the notion of <b>Flags</b>, which are bits in the <b>Status register</b>, which are used for logical and arithmetical instructions and can take a value of 1 or 0 . Here are the 8 flags that exist for the 8086 CPU : -</p> -<ul class="org-ul"> -<li><b>Carry Flag(CF):</b> Set to 1 when there is an <b>unsigned overflow</b>, for example when you add 255 + 1( not in range [0,255] ). by default its set to 0.</li> -<li><b>Overflow Flag(CF):</b> Set to 1 when there is a <b>signed overflow</b>, for example when you add 100 + 50( not in range [-128, 128[ ). by default its set to 0.</li> -<li><b>Zero Flag(ZF):</b> Set to 1 when the result is 0. by default its set to 0.</li> -<li><b>Auxiliary Flag(AF):</b> Set to 1 when there is an <b>unsigned overflow</b> for low nibble (4bits), or in human words : when there is a carry inside the number. for example when you add 29H + 4CH , 9 + C => 15. So we carry the 1 to 2 + 4 and AF is set to 1.</li> -<li><b>Parity Flag(PF):</b> Set to 1 when the result has an even number of one bits. and 0 if it has an odd number of one bits. Even if a result is a word, only the Low 8bits are analyzed.</li> -<li><b>Sign Flag(SF):</b> Self explanatory, set to 1 if the result is negative and 0 if its positive.</li> -<li><b>Interrupt Enable Flag(IF):</b> When its set to 1, the CPU reacts to interrupts from external devices.</li> -<li><b>Direction Flag(DF):</b> When this flag is set to 0, the processing is done forward, if its set to 1, its done backward.</li> -</ul> -</div> -</div> -</div> -</div> -<div id="postamble" class="status"> -<p class="author">Author: Crystal</p> -<p class="date">Created: 2024-04-10 Wed 21:05</p> -</div> -</body> -</html> |