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Date : Sept 2000
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Title : Assembly for nerds using linux - part 1
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Source : http://www.AstaLaVista.com
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Writer : made by member Olaf of LSG - Linux Security
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email
- linuxsecgroup@hotmail.com
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...
--------
file
ends
Regards,
Olaf |
