Wednesday, July 9, 2008

Tutorial Assembly for nerds using linux - part 1

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Tutorial Assembly for nerds using linux - part 1

made by member Olaf of LNG - Linux security group

email – linuxsecgroup@hotmail.com

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If you want to join LNG mailing list go to

http://www.egroups.com/group/linuxsecgroup



############ Introduction ############



Assembly nowadays is a hard thing to learn, not because it's difficult but

because people thinks that there is no reason to learn assembly! That's

not true... With assembly you can have total power above the computer, and

know exactly what he's doing. Try to remember that while trying to learn!

You may wondering: Why to read this tut? Good point... There are thousands

of papers for programming assembly in x86. That's true... But did they

teach you how to make apps for linux? Did they talked about linux

interrupts? I don't think so... There are many tutorials about programming

assembly for x86 in dos and windows, but very few on linux. I want to

change that. So this is the first issue of a collection of papers about

that. If you find that this paper has any error, please contact me and let

me know.



##### Index #####



1. Numbering systems

1.1. Decimal system

1.2. Binary system

1.2.1. Converting a binary number to a decimal number

1.2.2. Converting a decimal number to a binary number

1.3. Hexadecimal system

1.4. Conventions

2. Binaries in computers

2.1. Bit

2.2. Nibble

2.3. Byte

2.4. Word

2.5. Double word



###################### 1. Numbering systems ######################



1.1. Decimal system



Nowadays we use the decimal numbering system in almost everything that is

related to numbers. We use it so often and in a natural way that we forget

it's meaning. What is decimal system?



. Every decimal number, has only digits between zero and nine,

making a total of 10 digits

Note: how many fingers do you have? ...10. In fact the decimal

system is bound to human anatomy.

. Ok, and what is the meaning of each digit? Consider the

following numbers: 234 and 234,43

We do some transformations

-> 234 i.e. 200 + 30 + 4 i.e. 2 * 10^2 + 3 * 10^1 + 4 * 10^0

-> 234,43 = 2 * 10^2 + 3 * 10^1 + 4 * 10^0 + 0,43 = 2 * 10^2 + 3 *

10^1 + 4 * 10^0 + 4 * 10^-1 + 3 * 10^-2

Do you see the relation? Each digit appearing to the left of the

decimal point represents a value between zero and nine times an increasing

power of ten. Digits appearing to the right of the decimal point represent

a value between zero and nine times a decreasing power of ten.



1.2. Binary system



Binary system uses only two digits, by convention the digits are 0 and 1.

This system is so widely used in computers... By coincidence or not this

system adjusts perfectly to computers... Computers operate using binary

logic. The computer represents values using two different voltage levels,

in this way we can represent 0 and 1. Like I said before the same applies

to binary system, it is well adjust to computer anatomy!



1.2.1. Converting a binary number to a decimal number

Apply the same rule we saw in 1.1, but with powers of two.

Example: 1010 -> 1 * 2^3 + 0 * 2^2 + 1 * 2^1 + 0 * 2^0 = 10

1.2.2. Converting a decimal number to a binary number



We have two ways to do it:



1.2.2.1 We consecutively divide the decimal value by a power

two(keeping the remainder), while the result of the division is different

than zero. The binary representation is obtained by the sequence of

remainders in the inverse order of the divisions.



Consider the number 10(in decimal):



10 / 2

0 5 / 2

1 2 / 2

0 1 / 2

1 0



So in binary we write 1010



1.2.2.2 You can try to find out the number by adding powers of two,

that added will produce the decimal result.



Consider for example number 123... hmmm it's a number not less than 2^0

and not greater then 2^7. Cool…



2^7 2^6 2^5 2^4 2^3 2^2 2^1 2^0

0 1 1 1 1 0 1 1 because 1 * 2^6 + 1 * 2^5 + 1 *

2^4 + 1 * 2^3 + 0 * 2^2 + 1*2^1 + 1 * 2^0

= 123

Our result is 1111011.



1.3. Hexadecimal system



You saw how many digits took to represent the number 123 in binary. 7

digits! Imagine 1200, 10000,... it hurts. So programmers had to choose

another numbering system, just to "talk" to the machine... and no... it's

not the decimal system!! You saw the trouble we had to convert one simple

number like 10 between decimal and binary... I think you don't want to

spend half of your life doing that. Engineers thought on that and they

elected the hexadecimal system... Hexadecimals is the "english" for

computers. They have two special features:

- They're very compact

- it's simple to convert them to binary and vice-versa. A

hexadecimal number has digits with a value between 0 and 15 times a

certain power of sixteen. Because we only know digits between 0-9 we have

to use six more digits! We can use the 6 first letters of the alphabet.

Let's see a example: FF = 15 * 16^1 + 15 * 16^0 = 255 (16) (10)



Converting between binary and hexadecimal is very easy! To convert binary

to hexadecimal remember that every four digits correspond to a single

hexadecimal digit... to convert back to binary just apply the inverse

rule! Let's take a look at the next example:



110 1011 = 0110 1011 =6B

(2) (16)



It's very easy!! To make things easier, take a look at the following

table:



################

# D # H # B #

################

# 0 # 0 # 0000 #

# 1 # 1 # 0001 #

# 2 # 2 # 0010 #

# 3 # 3 # 0011 #

# 4 # 4 # 0100 #

# 5 # 5 # 0101 #

# 6 # 6 # 0110 #

# 7 # 7 # 0111 #

# 8 # 8 # 1000 #

# 9 # 9 # 1001 #

#10 # A # 1010 #

#11 # B # 1011 #

#12 # C # 1100 #

#13 # D # 1101 #

#14 # E # 1110 #

#15 # F # 1111 #

################



1.4. Conventions



Programming in assembly, requires you to obey some rules when using

numbers, because you can use three different numbering systems.



When writing a number:



- all numbers have to start with a decimal digit

- all numbers end with a letter, indicating the type of number:

. for hexadecimals the letter is h

. binary numbers end with b

. decimals end with t or d We will use the following notation:



Xn Xn-1 ... X2 X1 -> Xi represents a bit, and i<-[0,1,...,n] represents

it's position.



######################## 2. Binaries in computers ########################



2.1. Bit is the abbreviation to binary digit. We'll use this abbreviation

all the time. As you may have guessed that “bit” is the smallest unit of

data on a binary computer.



2.2. Nibble is a set of four bits. For example 1110, 1101, 1111 is a

nibble... every thing that has 4 bits is a nibble.

X3 X2 X1 X0 -> X1 X0 are the low order bits in the nibble, while

X2X3 are the high order bits.



2.3. Byte -> This is the most used data structure in computers. A byte is

a collection of eight bits:

X7 X6 X5 X4 X3 X2 X1 X0 When referring to a byte we say that it's

low order nibble is X0 X1 X2 X3, while the high order nibble is X4 X5 X6

X7. Note: bytes are used to represent characters using the ASCII

character set.



2.4. A word consists of 2 bytes, i.e. a group of 16 bits. In a byte you

have a low order byte and high order byte.



2.5. A double word consists of 2 words, i.e. 4 bytes i.e. 32 bits. In a

double word you have a L.O. word and a H.O. word.



In the next tutorial I will talk about:



- Arithmetic operations - Signed and unsigned numbers - System

Organization - Flags and registers - Logical operations on bits - Some

instructions like: mov, jmp, call, and...

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