From a3ae9e5d0ed8aa1ab10f559ee04916ec8aec89d4 Mon Sep 17 00:00:00 2001 From: Crystal Date: Thu, 7 Mar 2024 20:55:36 +0100 Subject: Add stuff --- blog/asm/1.html | 77 ++++++++++++++++++++++++++++++--------------------------- 1 file changed, 41 insertions(+), 36 deletions(-) (limited to 'blog/asm') diff --git a/blog/asm/1.html b/blog/asm/1.html index c4b0bf6..d46759f 100644 --- a/blog/asm/1.html +++ b/blog/asm/1.html @@ -3,7 +3,7 @@ "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> - + x86 Assembly from my understanding @@ -23,9 +23,9 @@

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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Memory :

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Memory :

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Memory is a sequence of octets (Aka 8bits) that each have a unique integer assigned to them called The Effective Address (EA), in this particular CPU Architecture (the i8086), the octet is designated by a couple (A segment number, and the offset in the segment)

@@ -40,9 +40,9 @@ Memory is a sequence of octets (Aka 8bits) that each have a unique integer assig The offset and segment are encoded in 16bits, so they take a value between 0 and 65535

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Important :

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Important :

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The relation between the Effective Address and the Segment & Offset is as follow :

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  • Example :
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  • Example :
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    Let the Physical address (Or Effective Address, these two terms are enterchangeable) 12345h (the h refers to Hexadecimal, which can also be written like this 0x12345), the register DS = 1230h and the register SI = 0045h, the CPU calculates the physical address by multiplying the content of the segment register DS by 10h (or 16) and adding the content of the register SI. so we get : 1230h x 10h + 45h = 12345h

    @@ -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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Registers

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Registers

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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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General Registers

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General Registers

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General registers contribute to arithmetic’s and logic and addressing too.

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Addressing and registers…again

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Addressing and registers…again

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I realized what I wrote here before was almost gibberish, sooo here we go again I guess ?

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I realized what I wrote here before was almost gibberish, sooo here we go again I guess ?

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Well lets take a step back to the notion of effective addresses VS relative ones.

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Effective = 10h x Segment + Offset . Part1

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Effective = 10h x Segment + Offset . Part1

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When trying to access a specific memory space, we use this annotation [Segment:Offset], so for example, and assuming DS = 0100h. We want to write the value 0x0005 to the memory space defined by the physical address 1234h, what do we do ?

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    MOV [DS:0234h], 0x0005
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    Why ? Let’s break it down : -lain-dance.gif

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    lain-dance.gif +

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    We Already know that Effective = 10h x Segment + Offset, So here we have : 1234h = 10h x DS + Offset, we already know that DS = 0100h, we end up with this simple equation 1234h = 1000h + Offset, therefor the Offset is 0234h

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Another example :

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Another example :

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What if we now have this instruction ?

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Segment + Register <3

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Segment + Register <3

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Consider DS = 0100h and BX = BP = 0234h and this code snippet:

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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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    Author: Crystal

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    Created: 2024-03-07 Thu 20:54

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    Created: 2024-03-07 Thu 20:55

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