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#+title: x86 Assembly from my understanding
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#+AUTHOR: Crystal
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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
* Memory :
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)
- The Segment is a set of 64 consecutive Koctets (1 Koctet = 1024 octets).
- And the offset is to specify the particular octet in that segment.
The offset and segment are encoded in 16bits, so they take a value between 0 and 65535
*** Important :
The relation between the Effective Address and the Segment & Offset is as follow :
**Effective address = 16 x segment + offset** keep in mind that this equation is encoded in decimal, which will change soon as we use Hexadecimal for convention reasons.
**** Example :
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*
Now if you are a clever one ( I know you are, since you are reading this <3 ) you may say that the physical address *12345h* can be written in more than one way....and you are right, more precisely : *2^{12} = 4096* different ways !!!
** Registers
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).
*** General Registers
General registers contribute to arithmetic's and logic and addressing too.
Each half-register is accessible as a register of 8bits, therefor making the 8086 backwards compatible with the 8080 (which had 8bit registers)
Now here are the Registers we can find in this section:
*AX*: 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:
#+BEGIN_SRC asm
ADD AX, AX ;(AX = AX + AX)
#+END_SRC
*BX*: 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:
#+BEGIN_SRC asm
MOV BL, [500] ;(BL = 500H)
#+END_SRC
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