Basic description of a computer system
A D V E R T I S E M E N T
This section has the purpose of giving a brief outline of the main components
of a computer system at a basic level, which will allow the user a greater
understanding of the concepts which will be dealt with throughout the tutorial.
We call computer system to the complete configuration of a computer,
including the peripheral units and the system programming which make it a useful
and functional machine for a determined task.
This part is also known as central processing unit or CPU, which in turn is
made by the control unit and the arithmetic and logic unit. Its functions
consist in reading and writing the contents of the memory cells, to forward data
between memory cells and special registers, and decode and execute the
instructions of a program. The processor has a series of memory cells which are
used very often and thus, are part of the CPU. These cells are known with the
name of registers. A processor may have one or two dozen of these registers. The
arithmetic and logic unit of the CPU realizes the operations related with
numeric and symbolic calculations. Typically these units only have capacity of
performing very elemental operations such as: the addition and subtraction of
two whole numbers, whole number multiplication and division, handling of the
registers' bits and the comparison of the content of two registers. Personal
computers can be classified by what is known as word size, this is, the quantity
of bits which the processor can handle at a time.
It is a group of cells, now being fabricated with semi-conductors, used for
general processes, such as the execution of programs and the storage of
information for the operations.
Each one of these cells may contain a numeric value and they have the
property of being addressable, this is, that they can distinguish one from
another by means of a unique number or an address for each cell.
The generic name of these memories is Random Access Memory or RAM. The main
disadvantage of this type of memory is that the integrated circuits lose the
information they have stored when the electricity flow is interrupted. This was
the reason for the creation of memories whose information is not lost when the
system is turned off. These memories receive the name of Read Only Memory or
Input and Output Units
In order for a computer to be useful to us it is necessary that the processor
communicates with the exterior through interfaces which allow the input and
output of information from the processor and the memory. Through the use of
these communications it is possible to introduce information to be processed and
to later visualize the processed data.
Some of the most common input units are keyboards and mice. The most common
output units are screens and printers.
Auxiliary Memory Units
Since the central memory of a computer is costly, and considering today's
applications it is also very limited. Thus, the need to create practical and
economical information storage systems arises. Besides, the central memory loses
its content when the machine is turned off, therefore making it inconvenient for
the permanent storage of data.These and other inconvenience give place for the
creation of peripheral units of memory which receive the name of auxiliary or
secondary memory. Of
these the most common are the tapes and magnetic discs.
The stored information on these magnetic media means receive the name of
files. A file is made of a variable number of registers, generally of a fixed
size; the registers may contain information or programs.
Assembler language Basic concepts
In order for the PC to process information, it is necessary that this
information be in special cells called registers. The registers are groups of 8
or 16 flip-flops.
A flip-flop is a device capable of storing two levels of voltage, a low one,
regularly 0.5 volts, and another one, commonly of 5 volts. The low level of
energy in the flip-flop is interpreted as off or 0, and the high level as on or
1. These states are usually known as bits, which are the smallest information
unit in a computer.
A group of 16 bits is known as word; a word can be divided in groups of 8
bits called bytes, and the groups of 4 bits are called nibbles.
The numeric system we use daily is the decimal system, but this system is not
convenient for machines since the information is handled codified in the shape
of on or off bits; this way of codifying takes us to the necessity of knowing
the positional calculation which will allow us to express a number in any base
where we need it.
It is possible to represent a determined number in any base through the
Where n is the position of the digit beginning from right to left and
numbering from zero. D is the digit on which we operate and B is the used
Converting binary numbers to decimals
When working with assembly language we come on the necessity of converting
numbers from the binary system, which is used by computers, to the decimal
system used by people.
The binary system is based on only two conditions or states, be it on(1) or
off(0), thus its base is two.
For the conversion we can use the positional value formula:
For example, if we have the binary number of 10011, we take each digit from
right to left and multiply it by the base, elevated to the new position they
Binary: 1 1 0 0 1
Decimal: 1*2^0 + 1*2^1 + 0*2^2 + 0*2^3 + 1*2^4
= 1 + 2 + 0 + 0 + 16 = 19 decimal.
The ^ character is used in computation as an exponent symbol and the *
character is used to represent multiplication.
Converting decimal numbers to binary
There are several methods to convert decimal numbers to binary; only one will
be analyzed here. Naturally a conversion with a scientific calculator is much
easier, but one cannot always count with one, so it is convenient to at least
know one formula to do it.
The method that will be explained uses the successive division of two,
keeping the residue as a binary digit and the result as the next number to
Let us take for example the decimal number of 43.
43/2=21 and its residue is 1
21/2=10 and its residue is 1
10/2=5 and its residue is 0
5/2=2 and its residue is 1
2/2=1 and its residue is 0
1/2=0 and its residue is 1
Building the number from the bottom , we get that the binary result is
On the hexadecimal base we have 16 digits which go from 0 to 9 and from the
letter A to the F, these letters represent the numbers from 10 to 15. Thus we
count 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E, and F.
The conversion between binary and hexadecimal numbers is easy. The first
thing done to do a conversion of a binary number to a hexadecimal is to divide
it in groups of 4 bits, beginning from the right to the left. In case the last
group, the one most to the left, is under 4 bits, the missing places are filled
Taking as an example the binary number of 101011, we divide it in 4 bits
groups and we are left with:
Filling the last group with zeros (the one from the left):
Afterwards we take each group as an independent number and we consider its
But since we cannot represent this hexadecimal number as 211 because it would
be an error, we have to substitute all the values greater than 9 by their
respective representation in hexadecimal, with which we obtain:
2BH, where the H represents the hexadecimal base.
In order to convert a hexadecimal number to binary it is only necessary to
invert the steps: the first hexadecimal digit is taken and converted to binary,
and then the second, and so on.
Data representation methods in a computer.
ASCII is an acronym of American Standard Code for Information Interchange.
This code assigns the letters of the alphabet, decimal digits from 0 to 9 and
some additional symbols a binary number of 7 bits, putting the 8th bit in its
off state or 0. This way each letter, digit or special character occupies one
byte in the computer memory.
We can observe that this method of data representation is very inefficient on
the numeric aspect, since in binary format one byte is not enough to represent
numbers from 0 to 255, but on the other hand with the ASCII code one byte may
represent only one digit. Due to this inefficiency, the ASCII code is mainly
used in the memory to represent text.
BCD is an acronym of Binary Coded Decimal. In this notation groups of 4 bits
are used to represent each decimal digit from 0 to 9. With this method we can
represent two digits per byte of information.
Even when this method is much more practical for number representation in the
memory compared to the ASCII code, it still less practical than the binary since
with the BCD method we can only represent digits from 0 to 99.
On the other hand in binary format we can represent all digits from 0 to 255.
This format is mainly used to represent very large numbers in mercantile
applications since it facilitates operations avoiding mistakes.
Floating point representation
This representation is based on scientific notation, this is, to represent a
number in two parts: its base and its exponent.
As an example, the number 1234000, can be represented as 1.123*10^6, in this
last notation the exponent indicates to us the number of spaces that the decimal
point must be moved to the right to obtain the original result.
In case the exponent was negative, it would be indicating to us the number of
spaces that the decimal point must be moved to the left to obtain the original
Using Debug program
Program creation process
For the creation of a program it is necessary to follow five steps:
Design of the algorithm, stage the problem to be solved is established and
the best solution is proposed, creating squematic
diagrams used for the better solution proposal. Coding the algorithm, consists
in writing the program in some programming language; assembly language in this
specific case, taking as a base the proposed solution on the prior step.
Translation to machine language, is the creation of the object program, in other
words, the written program as a sequence of zeros and
ones that can be interpreted by the processor. Test the program, after the
translation the program into machine language, execute the program in the
computer machine. The last stage is the elimination of detected faults on the
program on the test stage. The correction of a fault normally requires the
repetition of all the steps from the first or second.
The CPU has 4 internal registers, each one of 16 bits. The first four, AX,
BX, CX, and DX are general use registers and can also be used as 8 bit
registers, if used in such a way it is necessary to refer to them for example
as: AH and AL, which are the high and low bytes of the AX register. This
nomenclature is also applicable to the BX, CX, and DX registers.
The registers known by their specific names:
BX Base register
CX Counting register
DX Data register
DS Data segment register
ES Extra segment register
SS Battery segment register
CS Code segment register
BP Base pointers register
SI Source index register
DI Destiny index register
SP Battery pointer register
IP Next instruction pointer register
F Flag register
To create a program in assembler two options exist, the first one is to use
the TASM or Turbo Assembler, of Borland, and the second one is to use the
debugger - on this first section we will use this last one since it is found in
any PC with the MS-DOS, which makes it available to any user who has access to a
machine with these characteristics.
Debug can only create files with a .COM extension, and because of the
characteristics of these kinds of programs they cannot be larger that 64 kb, and
they also must start with displacement, offset, or 0100H memory direction inside
the specific segment.
Debug provides a set of commands that lets you perform a number of useful
A Assemble symbolic instructions into machine code
D Display the contents of an area of memory
E Enter data into memory, beginning at a specific location
G Run the executable program in memory
N Name a program
P Proceed, or execute a set of related instructions
Q Quit the debug program
R Display the contents of one or more registers
T Trace the contents of one instruction
U Unassembled machine code into symbolic code
W Write a program onto disk
It is possible to visualize the values of the internal registers of the CPU
using the Debug program. To begin working with Debug, type the following prompt
in your computer:
On the next line a dash will appear, this is the indicator of Debug, at this
moment the instructions of Debug can be introduced using the following command:
AX=0000 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000
DS=0D62 ES=0D62 SS=0D62 CS=0D62 IP=0100 NV EI PL NZ NA PO NC
0D62:0100 2E CS:
0D62:0101 803ED3DF00 CMP BYTE PTR [DFD3],00 CS:DFD3=03
All the contents of the internal registers of the CPU are displayed; an
alternative of viewing them is to use the "r" command using as a parameter
the name of the register whose value wants to be seen. For example:
This instruction will only display the content of the BX register and the
Debug indicator changes from "-" to ":"
When the prompt is like this, it is possible to change the value of the
register which was seen by typing the new value and [Enter], or the old value
can be left by pressing [Enter] without typing any other value.
In assembly language code lines have two parts, the first one is the name of
the instruction which is to be executed, and the second one are the parameters
of the command. For example: add ah bh
Here "add" is the command to be executed, in this case an addition, and "ah"
as well as "bh" are the parameters.
For example:mov al, 25
In the above example, we are using the instruction mov, it means move the
value 25 to al register.
The name of the instructions in this language is made of two, three or four
letters. These instructions are also called mnemonic names or operation codes,
since they represent a function the processor will perform.
Sometimes instructions are used as follows:
The brackets in the second parameter indicate to us that we are going to work
with the content of the memory cell number 170 and not with the 170 value, this
is known as direct addressing.