assembler tutorial

 
Assembler Tutorial
1996 Edition
University of Guadalajara
Information Systems General
Coordination.
Culture and Entertainment Web
 
 
 
 
June 12th 1995
Copyright(C)1995-1996
This is an introduction for people who want to programming
in assembler language.
Copyright (C) 1995-1996, Hugo Perez. Anyone may
reproduce this document, in whole or in part, provided that: (1) any copy or
republication of the entire document must show University of Guadalajara
as the source, and must include this notice; and (2) any other use of this
material must reference this manual and , and the fact that the material is
copyright by Hugo Perez and is used by permission.
 
 
Table of Contents
1. Introduction
2. Basic Concepts
3. Assembler programming
4. Assembler language instructions
5. Interruptions and file managing
6. Macros and procedures

Program examples
 
 
1. Introduction
Table of contents
1.1 What's new in the Assembler material
1.2 Presentation
1.3 Why learn Assembler language
1.4 We need your opinion
 
1.1 What's new in the Assembler
material
After of one year that we've released the first Assembler
material on-line. We've received a lot of e-mail where each people talk about
different aspects about this material. We've tried to put these comments and
suggestions in this update assembler material. We hope that this new Assembler
material release reach to all people that they interest to learn the most
important language for IBM PC.
In this new assembler release includes:
A complete chapter about how to use debug program
More example of the assembler material
Each section of this assembler material includes a link file
to Free
On-line of Computing by Dennis Howe
Finally, a search engine to look for any topic or item
related with this updated material.
 
1.2 Presentation
The document you are looking at, has the primordial function
of introducing you to assembly language programming, and it has been thought for
those people who have never worked with this language.
The tutorial is completely focused towards the computers
that function with processors of the x86 family of Intel, and considering that
the language bases its functioning on the internal resources of the processor,
the described examples are not compatible with any other
architecture.
The information was structured in units in order to allow
easy access to each of the topics and facilitate the following of the
tutorial.
In the introductory section some of the elemental concepts
regarding computer systems are mentioned, along with the concepts of the
assembly language itself, and continues with the tutorial itself.
 
1.3 Why learn assembler language
The first reason to work with assembler is that it provides
the opportunity of knowing more the operation of your PC, which allows the
development of software in a more consistent manner.
The second reason is the total control of the PC which you
can have with the use of the assembler.
Another reason is that the assembly programs are quicker,
smaller, and have
larger capacities than ones created with other
languages.
Lastly, the assembler allows an ideal optimization in
programs, be it on their size or on their execution.
 
1.4 We need your opinion
Our goal is offers you easier way to learn yourself
assembler language. You send us your comments or suggestions about this 96'
edition. Any comment will be welcome.
 
2. Basic Concepts
Contents
2.1 Basic description of a computer
system.
2.2 Assembler language Basic concepts
2.3 Using debug program
2.1 Basic description of a computer system.

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.


Contents
2.1.1 Central Processor
2.1.2 Central Memory
2.1.3 Input and Output Units
2.1.4 Auxiliary Memory
Units
Computer System.
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.
2.1.1 Central Processor.
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.
 
2.1.2 Central Memory.
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 ROM.
2.1.3 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.
2.1.4 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.
2.2 Assembler language Basic
concepts




Contents
2.2.1 Information in the computers
2.2.2 Data representation methods
 
2.2.1 Information in the computer




Contents
2.2.1.1 Information units
2.2.1.2 Numeric systems
2.2.1.3 Converting binary numbers to
decimal
2.2.1.4 Converting decimal numbers to
binary
2.2.1.5 Hexadecimal
system
2.2.1.1 Information Units
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.
2.2.1.2 Numeric systems
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 following formula:

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 numeric base.
 
2.2.1.3 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 are:
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.
 
2.2.1.4 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 divide.
 
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
101011
2.2.1.5 Hexadecimal system
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 with zeros.
Taking as an example the binary number of 101011, we divide
it in 4 bits groups and we are left with:
10;1011
Filling the last group with zeros (the one from the
left):
0010;1011
Afterwards we take each group as an independent number and
we consider its
decimal value:
0010=2;1011=11
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.
2.2.2 Data representation methods in a
computer.




Contents
2.2.2.1.ASCII code
2.2.2.2 BCD method
2.2.2.3 Floating point
representation
2.2.2.1 ASCII code
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.
 
2.2.2.2 BCD Method
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.
2.2.2.3 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 result.
2.3 Using Debug program


Contents
2.3.1 Program creation process
2.3.2 CPU registers
2.3.3 Debug program
2.3.4 Assembler structure
2.3.5 Creating basic assembler program
2.3.6 Storing and loading the programs
2.3.7 More debug program
examples
2.31 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.
 
 
2.3.2 CPU Registers
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:
AX Accumulator
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
 
2.3.3 Debug program
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 operations:
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:
C:/>Debug [Enter]
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:
-r[Enter]
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:
-rbx
BX 0000
:
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.
 
 
2.3.4 Assembler structure
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:
add al,[170]
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.
2.3.5 Creating basic assembler program
The first step is to initiate the Debug, this step only
consists of typing debug[Enter] on the operative system prompt.
To assemble a program on the Debug, the "a" (assemble)
command is used; when this command is used, the address where you want the
assembling to begin can be given as a parameter, if the parameter is omitted the
assembling will be initiated at the locality specified by CS:IP, usually 0100h,
which is the locality where programs with .COM extension must be initiated. And
it will be the place we will use since only Debug can create this specific type
of programs.
Even though at this moment it is not necessary to give the
"a" command a parameter, it is recommendable to do so to avoid problems once the
CS:IP registers are used, therefore we type:
a 100[enter]
mov ax,0002[enter]
mov bx,0004[enter]
add ax,bx[enter]
nop[enter][enter]
What does the program do?, move the value 0002 to the ax
register, move the value 0004 to the bx register, add the contents of the ax and
bx registers, the instruction, no operation, to finish the program.
In the debug program. After this is done, the screen will
produce the following lines:
C:\>debug
-a 100
0D62:0100 mov ax,0002
0D62:0103 mov bx,0004
0D62:0106 add ax,bx
0D62:0108 nop
0D62:0109
Type the command "t" (trace), to execute each instruction of
this program, example:
-t
AX=0002 BX=0000 CX=0000 DX=0000 SP=FFEE
BP=0000 SI=0000 DI=0000
DS=0D62 ES=0D62 SS=0D62 CS=0D62 IP=0103 NV EI
PL NZ NA PO NC
0D62:0103 BB0400 MOV BX,0004
You see that the value 2 move to AX register. Type the
command "t" (trace), again, and you see the second instruction is
executed.
-t
AX=0002 BX=0004 CX=0000 DX=0000 SP=FFEE
BP=0000 SI=0000 DI=0000
DS=0D62 ES=0D62 SS=0D62 CS=0D62 IP=0106 NV EI
PL NZ NA PO NC
0D62:0106 01D8 ADD AX,BX
Type the command "t" (trace) to see the instruction add is
executed, you will see the follow lines:
 
-t
AX=0006 BX=0004 CX=0000 DX=0000 SP=FFEE
BP=0000 SI=0000 DI=0000
DS=0D62 ES=0D62 SS=0D62 CS=0D62 IP=0108 NV EI
PL NZ NA PE NC
0D62:0108 90 NOP
The possibility that the registers contain different values
exists, but AX and BX must be the same, since they are the ones we just
modified.
To exit Debug use the "q" (quit) command.
 
2.3.6 Storing and loading the programs
It would not seem practical to type an entire program each
time it is needed, and to avoid this it is possible to store a program on the
disk, with the enormous advantage that by being already assembled it will not be
necessary to run Debug again to execute it.
The steps to save a program that it is already stored on
memory are:
Obtain the length of the program subtracting the final
address
from the initial address, naturally in hexadecimal
system.
Give the program a name and extension.
Put the length of the program on the CX register.
Order Debug to write the program on the disk.
By using as an example the following program, we will have a
clearer idea
of how to take these steps:
When the program is finally assembled it would look like
this:
0C1B:0100 mov ax,0002
0C1B:0103 mov bx,0004
0C1B:0106 add ax,bx
0C1B:0108 int 20
0C1B:010A
To obtain the length of a program the "h" command is used,
since it will show us the addition and subtraction of two numbers in
hexadecimal. To obtain the length of ours, we give it as parameters the value of
our program's final address (10A), and the program's initial address (100). The
first result the command shows us is the addition of the parameters and the
second is the subtraction.
-h 10a 100
020a 000a
The "n" command allows us to name the program.
-n test.com
The "rcx" command allows us to change the content of the CX
register to the value we obtained from the size of the file with "h", in this
case 000a, since the result of the subtraction of the final address from the
initial address.
-rcx
CX 0000
:000a
Lastly, the "w" command writes our program on the disk,
indicating how many bytes it wrote.
-w
Writing 000A bytes
To save an already loaded file two steps are
necessary:
Give the name of the file to be loaded.
Load it using the "l" (load) command.
To obtain the correct result of the following steps, it is
necessary that the above program be already created.
Inside Debug we write the following:
-n test.com
-l
-u 100 109
0C3D:0100 B80200 MOV AX,0002
0C3D:0103 BB0400 MOV BX,0004
0C3D:0106 01D8 ADD AX,BX
0C3D:0108 CD20 INT 20
The last "u" command is used to verify that the program was
loaded on memory. What it does is that it disassembles the code and shows it
disassembled. The parameters indicate to Debug from where and to where to
disassemble.
Debug always loads the programs on memory on the address
100H, otherwise indicated.
 
 
3 Assembler programming
Contents
3.1 Building Assembler programs
3.2 Assembly process
3.3 More assembler programs
3.4 Types of instructions
 
3.1 Building Assembler programs


Contents
3.1.1 Needed software

Assembler Programming
 
3.1.1 Needed software
In order to be able to create a program, several tools are
needed:
First an editor to create the source program. Second a
compiler, which is nothing more than a program that "translates" the source
program into an object program. And third, a linker that generates the
executable program from the object program.
The editor can be any text editor at hand, and as a compiler
we will use the TASM macro assembler from Borland, and as a linker we will use
the Tlink program.
The extension used so that TASM recognizes the source
programs in assembler is .ASM; once translated the source program, the TASM
creates a file with the .OBJ extension, this file contains an "intermediate
format" of the program, called like this because it is not executable yet but it
is not a program in source language either anymore. The linker generates, from a
.OBJ or a combination of several of these files, an executable program, whose
extension usually is .EXE though it can also be .COM, depending of the form it
was assembled.
3.1.2 Assembler Programming
To build assembler programs using TASM programs is a
different program structure than from using debug program.
It's important to include the following assembler
directives:
.MODEL SMALL
Assembler directive that defines the memory model to use in
the program
.CODE
Assembler directive that defines the program
instructions
.STACK
Assembler directive that reserves a memory space for program
instructions
in the stack
END
Assembler directive that finishes the assembler
program
Let's program
First step
use any editor program to create the source file. Type the
following lines:
First example
; use ; to put comments in the assembler
program
.MODEL SMALL; memory model
.STACK; memory space for program instructions in the
stack
.CODE; the following lines are program
instructions
mov ah,1h; moves the value 1h to register
ah
mov cx,07h; moves the value 07h to register
cx
int 10h;10h interruption
mov ah,4ch; moves the value 4 ch to register
ah
int 21h; 21h interruption
END; finishes the program code
This assembler program changes the size of the computer
cursor.
Second step
Save the file with the following name: examp1.asm
Don't forget to save this in ASCII format.
Third step
Use the TASM program to build the object program.
Example:
C:\>tasm exam1.asm
Turbo Assembler Version 2.0 Copyright (c) 1988, 1990
Borland International
Assembling file: exam1.asm
Error messages: None
Warning messages: None
Passes: 1
Remaining memory: 471k
The TASM can only create programs in .OBJ format, which are
not executable by themselves, but rather it is necessary to have a linker which
generates the executable code.
Fourth step
Use the TLINK program to build the executable program
example:
C:\>tlink exam1.obj
Turbo Link Version 3.0 Copyright (c) 1987, 1990
Borland International
C:\>
Where exam1.obj is the name of the intermediate program,
.OBJ. This generates a file directly with the name of the intermediate program
and the .EXE extension.
Fifth step
Execute the executable program
C:\>exam1[enter]
Remember, this assembler program changes the size of the
cursor.


Assembly process.
Segments
Table of symbols
SEGMENTS
The architecture of the x86 processors forces to the use of
memory segments to manage the information, the size of these segments is of
64kb.
The reason of being of these segments is that, considering
that the maximum size of a number that the processor can manage is given by a
word of 16 bits or register, it would not be possible to access more than 65536
localities of memory using only one of these registers, but now, if the PC's
memory is divided into groups or segments, each one of 65536 localities, and we
use an address on an exclusive register to find each segment, and then we make
each address of a specific slot with two registers, it is possible for us to
access a quantity of 4294967296 bytes of memory, which is, in the present day,
more memory than what we will see installed in a PC.
In order for the assembler to be able to manage the data, it
is necessary that each piece of information or instruction be found in the area
that corresponds to its respective segments. The assembler accesses this
information taking into account the localization of the segment, given by the
DS, ES, SS and CS registers and inside the register the address of the specified
piece of information. It is because of this that when we create a program using
the Debug on each line that we assemble, something like this appears:
1CB0:0102 MOV AX,BX
Where the first number, 1CB0, corresponds to the memory
segment being used, the second one refers to the address inside this segment,
and the instructions which will be stored from that address follow.
The way to indicate to the assembler with which of the
segments we will work with is with the .CODE, .DATA and .STACK
directives.
The assembler adjusts the size of the segments taking as a
base the number of bytes each assembled instruction needs, since it would be a
waste of memory to use the whole segments. For example, if a program only needs
10kb to store data, the data segment will only be of 10kb and not the 64kb it
can handle.
SYMBOLS CHART
Each one of the parts on code line in assembler is known as
token, for example on the code line:
MOV AX,Var
we have three tokens, the MOV instruction, the AX operator,
and the VAR operator. What the assembler does to generate the OBJ code is to
read each one of the tokens and look for it on an internal "equivalence" chart
known as the reserved words chart, which is where all the mnemonic meanings we
use as instructions are found.
Following this process, the assembler reads MOV, looks for
it on its chart and identifies it as a processor instruction. Likewise it reads
AX and recognizes it as a register of the processor, but when it looks for the
Var token on the reserved words chart, it does not find it, so then it looks for
it on the symbols chart which is a table where the names of the variables,
constants and labels used in the program where their addresses on memory are
included and the sort of data it contains, are found.
Sometimes the assembler comes on a token which is not
defined on the program, therefore what it does in these cased is to pass a
second time by the source program to verify all references to that symbol and
place it on the symbols chart.
There are symbols which the assembler will not find since
they do not belong to that segment and the program does not know in what part of
the memory it will find that segment, and at this time the linker comes into
action, which will create the structure necessary for the loader so that the
segment and the token be defined when the program is loaded and before it is
executed.
3.3 More assembler programs
Another example
First step
Use any editor program to create the source file. Type the
following lines:
;example11
.model small
.stack
.code
mov ah,2h ;moves the value 2h to register
ah
mov dl,2ah ;moves de value 2ah to register
dl
;(Its the asterisk value in ASCII format)
int 21h ;21h interruption
mov ah,4ch ;4ch function, goes to operating
system
int 21h ;21h interruption
end ;finishes the program code
Second step
Save the file with the following name: exam2.asm
Don't forget to save this in ASCII format.
Third step
Use the TASM program to build the object program.
C:\>tasm exam2.asm
Turbo Assembler Version 2.0 Copyright (c) 1988, 1990
Borland International
Assembling file: exam2.asm
Error messages: None
Warning messages: None
Passes: 1
Remaining memory: 471k
Fourth step
Use the TLINK program to build the executable
program
C:\>tlink exam2.obj
Turbo Link Version 3.0 Copyright (c) 1987, 1990
Borland International
C:\>
Fifth step
Execute the executable program
C:\>ejem11[enter]
*
C:\>
This assembler program shows the asterisk character on the
computer screen
 
3.4 Types of instructions.


Contents
3.4.1 Data movement
3.4.2 Logic and arithmetic operations
3.4.3 Jumps, loops and procedures
 
3.4.1 Data movement
In any program it is necessary to move the data in the
memory and in the CPU registers; there are several ways to do this: it can copy
data in the memory to some register, from register to register, from a register
to a stack, from a stack to a register, to transmit data to external devices as
well as vice versa.
This movement of data is subject to rules and restrictions.
The following are some of them:
*It is not possible to move data from a memory locality to
another directly; it is necessary to first move the data of the origin locality
to a register and then from the register to the destiny locality.
*It is not possible to move a constant directly to a segment
register; it first must be moved to a register in the CPU.
It is possible to move data blocks by means of the movs
instructions, which copies a chain of bytes or words; movsb which copies n bytes
from a locality to another; and movsw copies n words from a locality to another.
The last two instructions take the values from the defined addresses by DS:SI as
a group of data to move and ES:DI as the new localization of the
data.
To move data there are also structures called batteries,
where the data is introduced with the push instruction and are extracted with
the pop instruction. In a stack the first data to be introduced is the last one
we can take, this is, if in our program we use these instructions:
PUSH AX
PUSH BX
PUSH CX
To return the correct values to each register at the moment
of taking them from the stack it is necessary to do it in the following
order:
 
POP CX
POP BX
POP AX
For the communication with external devices the out command
is used to send information to a port and the in command to read the information
received from a port.
The syntax of the out command is:
OUT DX,AX
Where DX contains the value of the port which will be used
for the communication and AX contains the information which will be
sent.
The syntax of the in command is:
IN AX,DX
Where AX is the register where the incoming information will
be kept and DX contains the address of the port by which the information will
arrive.
3.4.2 Logic and arithmetic operations
The instructions of the logic operations are: and, not, or
and xor. These work on the bits of their operators. To verify the result of the
operations we turn to the cmp and test instructions. The instructions used for
the algebraic operations are: to add, to subtract sub, to multiply mul and to
divide div.
Almost all the comparison instructions are based on the
information contained in the flag register. Normally the flags of this register
which can be directly handled by the programmer are the data direction flag DF,
used to define the operations about chains.
Another one which can also be handled is the IF flag by
means of the sti and cli instructions, to activate and deactivate the
interruptions.
3.4.3 Jumps, loops and procedures
The unconditional jumps in a written program in assembler
language are given by the jmp instruction; a jump is to moves the flow of the
execution of a program by sending the control to the indicated
address.
A loop, known also as iteration, is the repetition of a
process a certain number of times until a condition is fulfilled. These loops
are used (broken sentence).
4 Assembler language Instructions
Contents
4.1 Transfer instructions
4.2 Loading instructions
4.3 Stack instructions
4.4 Logic instructions
4.5 Arithmetic instructions
4.6 Jump instructions
4.7 Instructions for cycles: loop
4.8 Counting Instructions
4.9 Comparison Instructions
4.10 Flag Instructions
 
4.1 Transfer instructions
They are used to move the contents of the operators. Each
instruction can be used with different modes of addressing.
MOV
MOVS (MOVSB) (MOVSW)
MOV INSTRUCTION
Purpose: Data transfer between memory cells,
registers and the accumulator.
Syntax:
MOV Destiny, Source
Where Destiny is the place where the data will be
moved and Source is the place where the data is.
The different movements of data allowed for this instruction
are:
*Destiny: memory. Source: accumulator
*Destiny: accumulator. Source: memory
*Destiny: segment register. Source:
memory/register
*Destiny: memory/register. Source: segment
register
*Destiny: register. Source: register
*Destiny: register. Source: memory
*Destiny: memory. Source: register
*Destiny: register. Source: immediate data
*Destiny: memory. Source: immediate data
Example:
MOV AX,0006h
MOV BX,AX
MOV AX,4C00h
INT 21H
This small program moves the value of 0006H to the AX
register, then it moves the content of AX (0006h) to the BX register, and lastly
it moves the 4C00h value to the AX register to end the execution with the 4C
option of the 21h interruption.
MOVS (MOVSB) (MOVSW) Instruction
Purpose: To move byte or word chains from the source,
addressed by SI, to
the destiny addressed by DI.
Syntax:
MOVS
This command does not need parameters since it takes as
source address the
content of the SI register and as destination the content of
DI. The following sequence of instructions illustrates this:
MOV SI, OFFSET VAR1
MOV DI, OFFSET VAR2
MOVS
First we initialize the values of SI and DI with the
addresses of the VAR1 and VAR2 variables respectively, then after executing MOVS
the content of VAR1 is copied onto VAR2.
The MOVSB and MOVSW are used in the same way as MOVS, the
first one moves one byte and the second one moves a word.
4.2 Loading instructions
They are specific register instructions. They are used to
load bytes or chains of bytes onto a register.
LODS (LODSB) (LODSW)
LAHF
LDS
LEA
LES
LODS (LODSB) (LODSW) INSTRUCTION
Purpose: To load chains of a byte or a word into the
accumulator.
Syntax:
LODS
This instruction takes the chain found on the address
specified by SI, loads it to the AL (or AX) register and adds or subtracts ,
depending on the state of DF, to SI if it is a bytes transfer or if it is a
words transfer.
MOV SI, OFFSET VAR1
LODS
The first line loads the VAR1 address on SI and the second
line takes the content of that locality to the AL register.
The LODSB and LODSW commands are used in the
same way, the first one loads a byte and the second one a word (it uses the
complete AX register).
LAHF INSTRUCTION
Purpose: It transfers the content of the flags to the
AH register.
Syntax:
LAHF
This instruction is useful to verify the state of the flags
during the execution of our program.
The flags are left in the following order inside the
register:
SF ZF ?? AF ?? PF ?? CF
The "??" means that there will be an undefined value
in those bits.
LDS INSTRUCTION
Purpose: To load the register of the data
segment
Syntax:
LDS destiny, source
The source operator must be a double word in memory. The
word associated with the largest address is transferred to DS, in other words it
is taken as the segment address. The word associated with the smaller address is
the displacement address and it is deposited in the register indicated as
destiny.
LEA INSTRUCTION
Purpose: To load the address of the source
operator
Syntax:
LEA destiny, source
The source operator must be located in memory, and its
displacement is placed on the index register or specified pointer in
destiny.
To illustrate one of the facilities we have with this
command let us write an equivalence:
MOV SI,OFFSET VAR1
Is equivalent to:
LEA SI,VAR1
It is very probable that for the programmer it is much
easier to create extensive programs by using this last format.
LES INSTRUCTION
Purpose: To load the register of the extra
segment
Syntax:
LES destiny, source
The source operator must be a double word operator in
memory. The content of the word with the larger address is interpreted as the
segment address and it is placed in ES. The word with the smaller address is the
displacement address and it is placed in the specified register on the destiny
parameter.
4.3 Stack instructions
These instructions allow the use of the stack to store or
retrieve data.
POP
POPF
PUSH
PUSHF
POP INSTRUCTION
Purpose: It recovers a piece of information from the
stack
Syntax:
POP destiny
This instruction transfers the last value stored on the
stack to the destiny operator, it then increases by 2 the SP
register.
This increase is due to the fact that the stack grows from
the highest memory segment address to the lowest, and the stack only works with
words, 2 bytes, so then by increasing by two the SP register, in reality two are
being subtracted from the real size of the stack.
POPF INSTRUCTION
Purpose: It extracts the flags stored on the
stack
Syntax:
POPF
This command transfers bits of the word stored on the higher
part of the stack to the flag register.
The way of transference is as follows:
BIT FLAG
0 CF
2 PF
4 AF
6 ZF
7 SF
8 TF
9 IF
10 DF
11 OF
These localities are the same for the PUSHF
command.
Once the transference is done, the SP register is increased
by 2, diminishing the size of the stack.
 
PUSH INSTRUCTION
Purpose: It places a word on the stack.
Syntax:
PUSH source
The PUSH instruction decreases by two the value of SP and
then transfers the content of the source operator to the new resulting address
on the recently modified register.
The decrease on the address is due to the fact that when
adding values to the stack, this one grows from the greater to the smaller
segment address, therefore by subtracting 2 from the SP register what we do is
to increase the size of the stack by two bytes, which is the only quantity of
information the stack can handle on each input and output of
information.
PUSHF INSTRUCTION
Purpose: It places the value of the flags on the
stack.
Syntax:
PUSHF
This command decreases by 2 the value of the SP register and
then the content of the flag register is transferred to the stack, on the
address indicated by SP.
The flags are left stored in memory on the same bits
indicated on the POPF command.
4.4 Logic instructions
They are used to perform logic operations on the
operators.
AND
NEG
NOT
OR
TEST
XOR
 
AND INSTRUCTION
Purpose: It performs the conjunction of the operators
bit by bit.
Syntax:
AND destiny, source
With this instruction the "y" logic operation for both
operators is carried out:
Source Destiny | Destiny
1 1 | 1
1 0 | 0
0 1 | 0
0 0 | 0
The result of this operation is stored on the destiny
operator.
NEG INSTRUCTION
Purpose: It generates the complement to 2.
Syntax:
NEG destiny
This instruction generates the complement to 2 of the
destiny operator and stores it on the same operator.
For example, if AX stores the value of 1234H,
then:
NEG AX
This would leave the EDCCH value stored on the AX
register.
NOT INSTRUCTION
Purpose: It carries out the negation of the destiny
operator bit by bit.
Syntax:
NOT destiny
The result is stored on the same destiny
operator.
OR INSTRUCTION
Purpose: Logic inclusive OR
Syntax:
OR destiny, source
The OR instruction carries out, bit by bit, the logic
inclusive disjunction of the two operators:
Source Destiny | Destiny
1 1 | 1
1 0 | 1
0 1 | 1
0 0 | 0
 
TEST INSTRUCTION
Purpose: It logically compares the operators

Syntax:
TEST destiny, source
It performs a conjunction, bit by bit, of the operators, but
differing from AND, this instruction does not place the result on the destiny
operator, it only has effect on the state of the flags.
XOR INSTRUCTION
Purpose: OR exclusive
Syntax:
XOR destiny, source
Its function is to perform the logic exclusive disjunction
of the two operators bit by bit.
Source Destiny | Destiny
1 1 | 0
0 0 | 1
0 1 | 1
0 0 | 0
4.5 Arithmetic instructions
They are used to perform arithmetic operations on the
operators.
ADC
ADD
DIV
IDIV
MUL
IMUL
SBB
SUB
 
ADC INSTRUCTION
Purpose: Cartage addition
Syntax:
ADC destiny, source
It carries out the addition of two operators and adds one to
the result in case the CF flag is activated, this is in case there is
carried.
The result is stored on the destiny operator.
ADD INSTRUCTION
Purpose: Addition of the operators.
Syntax:
ADD destiny, source
It adds the two operators and stores the result on the
destiny operator.
DIV INSTRUCTION
Purpose: Division without sign.
Syntax:
DIV source
The divider can be a byte or a word and it is the operator
which is given the instruction.
If the divider is 8 bits, the 16 bits AX register is taken
as dividend and if the divider is 16 bits the even DX:AX register will be taken
as dividend, taking the DX high word and AX as the low.
If the divider was a byte then the quotient will be stored
on the AL register and the residue on AH, if it was a word then the quotient is
stored on AX and the residue on DX.
IDIV INSTRUCTION
Purpose: Division with sign.
Syntax:
IDIV source
It basically consists on the same as the DIV instruction,
and the only difference is that this one performs the operation with
sign.
For its results it used the same registers as the DIV
instruction.
MUL INSTRUCTION
Purpose: Multiplication with sign.
Syntax:
MUL source
The assembler assumes that the multiplicand will be of the
same size as the
multiplier, therefore it multiplies the value stored on the
register given as operator by the one found to be contained in AH if the
multiplier is 8 bits or by AX if the multiplier is 16 bits.
When a multiplication is done with 8 bit values, the result
is stored on the AX register and when the multiplication is with 16 bit values
the result is stored on the even DX:AX register.
IMUL INSTRUCTION
Purpose: Multiplication of two whole numbers with
sign.
Syntax:
IMUL source
This command does the same as the one before, only that this
one does take
into account the signs of the numbers being
multiplied.
The results are kept in the same registers that the MOV
instruction uses.
SBB INSTRUCTION
Purpose: Subtraction with cartage.
Syntax:
SBB destiny, source
This instruction subtracts the operators and subtracts one
to the result if CF is activated. The source operator is always subtracted from
the destiny.
This kind of subtraction is used when one is working with 32
bits quantities.
SUB INSTRUCTION
Purpose: Subtraction.
Syntax:
SUB destiny, source
It subtracts the source operator from the
destiny.
4.6 Jump instructions
They are used to transfer the flow of the process to the
indicated
operator.
JMP
JA (JNBE)
JAE (JNBE)
JB (JNAE)
JBE (JNA)
JE (JZ)
JNE (JNZ)
JG (JNLE)
JGE (JNL)
JL (JNGE)
JLE (JNG)
JC
JNC
JNO
JNP (JPO)
JNS
JO
JP (JPE)
JS
JMP INSTRUCTION
Purpose: Unconditional jump.
Syntax:
JMP destiny
This instruction is used to deviate the flow of a program
without taking into account the actual conditions of the flags or of the
data.
JA (JNBE) INSTRUCTION
Purpose: Conditional jump.
Syntax:
JA Label
After a comparison this command jumps if it is or jumps if
it is not down or if not it is the equal.
This means that the jump is only done if the CF flag is
deactivated or if the ZF flag is deactivated, that is that one of the two be
equal to zero.
JAE (JNB) INSTRUCTION
Purpose: Conditional jump.
Syntax:
JAE label
It jumps if it is or it is the equal or if it is not
down.
The jump is done if CF is deactivated.
JB (JNAE) INSTRUCTION
Purpose: Conditional jump.
Syntax:
JB label
It jumps if it is down, if it is not , or if it is the
equal.
The jump is done if CF is activated.
JBE (JNA) INSTRUCTION
Purpose: Conditional jump.
Syntax:
JBE label
It jumps if it is down, the equal, or if it is not
.
The jump is done if CF is activated or if ZF is activated,
that any of them be equal to 1.
JE (JZ) INSTRUCTION
Purpose: Conditional jump.
Syntax:
JE label
It jumps if it is the equal or if it is zero.
The jump is done if ZF is activated.
JNE (JNZ) INSTRUCTION
Purpose: Conditional jump.
Syntax:
JNE label
It jumps if it is not equal or zero.
The jump will be done if ZF is deactivated.
JG (JNLE) INSTRUCTION
Purpose: Conditional jump, and the sign is taken into
account.
Syntax:
JG label
It jumps if it is larger, if it is not larger or
equal.
The jump occurs if ZF = 0 or if OF = SF.
JGE (JNL) INSTRUCTION
Purpose: Conditional jump, and the sign is taken into
account.
Syntax:
JGE label
It jumps if it is larger or less than, or equal
to.
The jump is done if SF = OF
JL (JNGE) INSTRUCTION
Purpose: Conditional jump, and the sign is taken into
account.
Syntax:
JL label
It jumps if it is less than or if it is not larger than or
equal to.
The jump is done if SF is different than OF.
JLE (JNG) INSTRUCTION
Purpose: Conditional jump, and the sign is taken into
account.
Syntax:
JLE label
It jumps if it is less than or equal to, or if it is not
larger.
The jump is done if ZF = 1 or if SF is defferent than
OF.
JC INSTRUCTION
Purpose: Conditional jump, and the flags are taken
into account.
Syntax:
JC label
It jumps if there is cartage.
The jump is done if CF = 1
JNC INSTRUCTION
Purpose: Conditional jump, and the state of the flags
is taken into
account.
Syntax:
JNC label
It jumps if there is no cartage.
The jump is done if CF = 0.
JNO INSTRUCTION
Purpose: Conditional jump, and the state of the flags
is taken into
account.
Syntax:
JNO label
It jumps if there is no overflow.
The jump is done if OF = 0.
JNP (JPO) INSTRUCTION
Purpose: Conditional jump, and the state of the flags
is taken into
account.
Syntax:
JNP label
It jumps if there is no parity or if the parity is
uneven.
The jump is done if PF = 0.
JNS INSTRUCTION
Purpose: Conditional jump, and the state of the flags
is taken into
account.
Syntax:
JNP label
It jumps if the sign is deactivated.
The jump is done if SF = 0.
JO INSTRUCTION
Purpose: Conditional jump, and the state of the flags
is taken into
account.
Syntax:
JO label
It jumps if there is overflow.
The jump is done if OF = 1.
JP (JPE) INSTRUCTION
Purpose: Conditional jump, the state of the flags is
taken into account.
Syntax:
JP label
It jumps if there is parity or if the parity is
even.
The jump is done if PF = 1.
JS INSTRUCTION
Purpose: Conditional jump, and the state of the flags
is taken into
account.
Syntax:
JS label
It jumps if the sign is on.
The jump is done if SF = 1.
4.7 Instructions for cycles:loop
They transfer the process flow, conditionally or
unconditionally, to a
destiny, repeating this action until the counter is
zero.
 
 
LOOP
LOOPE
LOOPNE
LOOP INSTRUCTION
Purpose: To generate a cycle in the
program.
Syntax:
LOOP label
The loop instruction decreases CX on 1, and transfers the
flow of the
program to the label given as operator if CX is different
than 1.
 
LOOPE INSTRUCTION
Purpose: To generate a cycle in the program
considering the state of ZF.
Syntax:
LOOPE label
This instruction decreases CX by 1. If CX is different to
zero and ZF is equal to 1, then the flow of the program is transferred to the
label indicated as operator.
LOOPNE INSTRUCTION
Purpose: To generate a cycle in the program,
considering the state of ZF.
Syntax:
LOOPNE label
This instruction decreases one from CX and transfers the
flow of the program only if ZF is different to 0.
4.8 Counting instructions
They are used to decrease or increase the content of the
counters.
DEC
INC
DEC INSTRUCTION
Purpose: To decrease the operator.
Syntax:
DEC destiny
This operation subtracts 1 from the destiny operator and
stores the new
value in the same operator.
 
INC INSTRUCTION
Purpose: To increase the operator.
Syntax:
INC destiny The instruction adds 1 to the destiny operator
and keeps the
result in the same destiny operator.
4.9 Comparison instructions
They are used to compare operators, and they affect the
content of the
flags.
CMP
CMPS (CMPSB) (CMPSW)
 
CMP INSTRUCTION
Purpose: To compare the operators.
Syntax:
CMP destiny, source
This instruction subtracts the source operator from the
destiny operator but without this one storing the result of the operation, and
it only affects the state of the flags.
 
 
CMPS (CMPSB) (CMPSW) INSTRUCTION
Purpose: To compare chains of a byte or a
word.
Syntax:
CMP destiny, source
With this instruction the chain of source characters is
subtracted from the destiny chain.
DI is used as an index for the extra segment of the source
chain, and SI as an index of the destiny chain.
It only affects the content of the flags and DI as well as
SI are incremented.
4.10 Flag instructions
They directly affect the content of the flags.
CLC
CLD
CLI
CMC
STC
STD
STI
 
 
CLC INSTRUCTION
Purpose: To clean the cartage flag.
Syntax:
CLC
This instruction turns off the bit corresponding to the
cartage flag, or in other words it puts it on zero.
CLD INSTRUCTION
Purpose: To clean the address flag.
Syntax:
CLD
This instruction turns off the corresponding bit to the
address flag.
CLI INSTRUCTION
Purpose: To clean the interruption flag.
Syntax:
CLI
This instruction turns off the interruptions flag, disabling
this way those maskarable interruptions.
A maskarable interruptions is that one whose functions are
deactivated when IF=0.
CMC INSTRUCTION
Purpose: To complement the cartage flag.
Syntax:
CMC
This instruction complements the state of the CF flag, if CF
= 0 the instructions equals it to 1, and if the instruction is 1 it equals it to
0.
We could say that it only "inverts" the value of the
flag.
STC INSTRUCTION
Purpose: To activate the cartage flag.
Syntax:
STC
This instruction puts the CF flag in 1.
STD INSTRUCTION
Purpose: To activate the address flag.
Syntax:
STD
The STD instruction puts the DF flag in 1.
STI INSTRUCTION
Purpose: To activate the interruption
flag.
Syntax:
STI
The instruction activates the IF flag, and this enables the
maskarable external interruptions ( the ones which only function when IF =
1).
5 Interruptions and file managing
Contents
5.1 Internal hardware interruptions
5.2 External hardware interruptions
5.3 Software interruptions
5.4 Most Common interruptions
 
5.1 Internal hardware interruptions
Internal interruptions are generated by certain events which
come during the execution of a program.
This type of interruptions are managed on their totality by
the hardware and it is not possible to modify them.
A clear example of this type of interruptions is the one
which actualizes the counter of the computer internal clock, the hardware makes
the call to this interruption several times during a second in order to maintain
the time to date.
Even though we cannot directly manage this interruption,
since we cannot control the time dating by means of software, it is possible to
use its effects on the computer to our benefit, for example to create a "virtual
clock" dated continuously thanks to the clock's internal counter. We only have
to write a program which reads the actual value of the counter and to translates
it into an understandable format for the user.
 
5.2 External hardware interruptions
External interruptions are generated by peripheral devices,
such as keyboards, printers, communication cards, etc. They are also generated
by coprocessors. It is not possible to deactivate external
interruptions.
These interruptions are not sent directly to the CPU, but
rather they are sent to an integrated circuit whose function is to exclusively
handle this type of interruptions. The circuit, called PIC8259A, is controlled
by the CPU using for this control a series of communication ways called
paths.
 
5.3 Software interruptions
Software interruptions can be directly activated by the
assembler invoking the number of the desired interruption with the INT
instruction.
The use of interruptions helps us in the creation of
programs, and by using them our programs are shorter, it is easier to understand
them and they usually have a better performance mostly due to their smaller
size.
This type of interruptions can be separated in two
categories: the operative system DOS interruptions and the BIOS
interruptions.
The difference between the two is that the operative system
interruptions are easier to use but they are also slower since these
interruptions make use of the BIOS to achieve their goal, on the other hand the
BIOS interruptions are much faster but they have the disadvantage that since
they are part of the hardware, they are very specific and can vary depending
even on the brand of the maker of the circuit.
The election of the type of interruption to use will depend
solely on the characteristics you want to give your program: speed, using the
BIOS ones, or portability, using the ones from the DOS.
5.4 Most common interruptions


Contents
5.4.1 Int 21H (DOS interruption)
Multiple calls to DOS functions.
5.4.2 Int 10H (BIOS interruption)
Video input/output.
5.4.3 Int 16H (BIOS interruption)
Keyboard input/output.
5.4.4 Int 17H (BIOS interruption)
Printer input/output.
 
5.41 21H Interruption
Purpose: To call on diverse DOS functions.
Syntax:
Int 21H
Note: When we work in TASM program is necessary to specify
that the value we
are using is hexadecimal.
This interruption has several functions, to access each one
of them it is necessary that the function number which is required at the moment
of calling the interruption is in the AH register.


Functions to display information to the
video.
02H Exhibits output
09H Chain Impression (video)
40H Writing in device/file
Functions to read information from the
keyboard.
01H Input from the keyboard
0AH Input from the keyboard using
buffer
3FH Reading from device/file
Functions to work with files.
In this section only the specific task of each function
is exposed, for a
reference about the concepts used, refer to unit 7,
titled : "Introduction
to file handling".
FCB Method
0FH Open file
14H Sequential reading
15H Sequential writing
16H Create file
21H Random reading
22H Random writing
Handles
3CH Create file
3DH Open file
3EH Close file driver
3FH Reading from file/device
40H Writing in file/device
42H Move pointer of reading/writing in
file
 
 
VIDEO DISPLAY FUNCTIONS
 
 
02H FUNCTION
Use:
It displays one character to the screen.
Calling registers:
AH = 02H
DL = Value of the character to display.
Return registers:
None.
This function displays the character whose hexadecimal code
corresponds to the value stored in the DL register, and no register is modified
by using this command.
The use of the 40H function is recommended instead of this
function.
09H FUNCTION
Use:
It displays a chain of characters on the screen.
Call registers:
AH = 09H
DS:DX = Address of the beginning of a chain of
characters.
Return registers:
None.
This function displays the characters, one by one, from the
indicated address in the DS:DX register until finding a $ character, which is
interpreted as the end of the chain.
It is recommended to use the 40H function instead of this
one.
40H FUNCTION
Use:
To write to a device or a file.
Call registers:
AH = 40H
BX = Path of communication
CX = Quantity of bytes to write
DS:DX = Address of the beginning of the data to
write
Return registers:
CF = 0 if there was no mistake
AX = Number of bytes written
CF = 1 if there was a mistake
AX = Error code
The use of this function to display information on the
screen is done by giving the BX register the value of 1 which is the preassigned
value to the video by the operative system MS-DOS.
 
 
KEYBOARD INFORMATION
FUNCTIONS
 
01H FUNCTION
Use:
To read a keyboard character and to display it.
Call registers
AH = 01H
Return registers:
AL = Read character
It is very easy to read a character from the keyboard with
this function, the hexadecimal code of the read character is stored in the AL
register. In case it is an extended register the AL register will contain the
value of 0 and it will be necessary to call on the function again to obtain the
code of that character.
0AH FUNCTION
Use:
To read keyboard characters and store them on the
buffer.
Call registers:
AH = 0AH
DS:DX = Area of storage address
BYTE 0 = Quantity of bytes in the area
BYTE 1 = Quantity of bytes read
from BYTE 2 till BYTE 0 + 2 = read characters
Return characters:
None.
The characters are read and stored in a predefined space on
memory. The structure of this space indicate that in the first byte are
indicated how many characters will be read. On the second byte the number of
characters already read are stored, and from the third byte on the read
characters are written.
When all the indicated characters have been stored the
speaker sounds and any additional character is ignored. To end the capture of
the chain it is necessary to hit [ENTER].
3FH FUNCTION
Use:
To read information from a device or file.
Call registers:
AH = 3FH
BX = Number assigned to the device
CX = Number of bytes to process
DS:DX = Address of the storage area
Return registers:
CF = 0 if there is no error and AX = number of read
bytes.
CF = 1 if there is an error and AX will contain the error
code.
FILE WORKING FUNCTIONS:
FCB FUNCTIONS:
0FH FUNCTION
Use:
To open an FCB file
Call registers:
AH = 0FH
DS:DX = Pointer to an FCB
Return registers:
AL = 00H if there was no problem, otherwise it returns to
0FFH
14H FUNCTION
Use:
To sequentially read an FCB file.
Call registers:
AH = 14H
DS:DX = Pointer to an FCB already opened.
Return registers:
AL = 0 if there were no errors, otherwise the corresponding
error code will be returned: 1 error at the end of the file, 2 error on the FCB
structure and 3 partial reading error.
What this function does is that it reads the next block of
information from the address given by DS:DX,
What this function does is that it reads the next block of
information from the address given by DS:DX, and dates this register.
15H FUNCTION
Use:
To sequentially write and FCB file.
Call registers:
AH = 15H
DS:DX = Pointer to an FCB already opened.
Return registers:
AL = 00H if there were no errors, otherwise it will contain
the error code: 1 full disk or read-only file, 2 error on the formation or on
the specification of the FCB.
The 15H function dates the FCB after writing the register to
the present
block.
16H FUNCTION
Use:
To create an FCB file.
Call registers:
AH = 16H
DS:DX = Pointer to an already opened FCB.
Return registers:
AL = 00H if there were no errors, otherwise it will contain
the 0FFH value.
It is based on the information which comes on an FCB to
create a file on a disk.
21H FUNCTION
Use:
To read in an random manner an FCB file.
Call registers:
AH = 21H
DS:DX = Pointer to and opened FCB.
Return registers:
A = 00H if there was no error, otherwise AH will contain the
code of the error: 1 if it is the end of file, 2 if there is an FCB
specification error and 3 if a partial register was read or the file pointer is
at the end of the same.
This function reads the specified register by the fields of
the actual block and register of an opened FCB and places the information on the
DTA, Disk Transfer Area.
22H FUNCTION
Use:
To write in an random manner an FCB file.
Call registers:
AH = 22H
DS:DX = Pointer to an opened FCB.
Return registers:
AL = 00H if there was no error, otherwise it will contain
the error code: 1 if the disk is full or the file is an only read and 2 if there
is an error on the
It writes the register specified by the fields of the actual
block and register of an opened FCB. It writes this information from the content
of the DTA.
FILE WORKING FUNCTIONS:
HANDLES:
3CH FUNCTION
Use:
To create a file if it does not exist or leave it on 0
length if it exists,
Handle.
Call registers:
AH = 3CH
CH = File attribute
DS:DX = Pointer to an ASCII specification.
Return registers:
CF = 0 and AX the assigned number to handle if there is no
error, in case there is, CF will be 1 and AX will contain the error code: 3 path
not found, 4 there
CF will be 1 and AX will contain the error code: 3 path not
found, 4 there are no handles available to assign and 5 access
denied.
This function substitutes the 16H function. The name of the
file is specified on an ASCII chain, which has as a characteristic being a
conventional chain of bytes ended with a 0 character.
The file created will contain the attributes defined on the
CX register in the following manner:
Value Attributes
00H Normal
02H Hidden
04H System
06H Hidden and of system
The file is created with the reading and writing
permissions. It is not possible to create directories using this
function.
 
3DH FUNCTION
Use:
It opens a file and returns a handle.
Call registers:
AH = 3DH
AL = manner of access
DS:DX = Pointer to an ASCII specification
Return registers:
CF = 0 and AX = handle number if there are no errors,
otherwise CF = 1 and AX = error code: 01H if the function is not valid, 02H if
the file was not found, 03H if the path was not found, 04H if there are no
available handles, 05H in case access is denied, and 0CH if the access code is
not valid.
The returned handled is 16 bits.
The access code is specified in the following
way:
BITS
7 6 5 4 3 2 1
. . . . 0 0 0 Only reading
. . . . 0 0 1 Only writing
. . . . 0 1 0 Reading/Writing
. . . x . . . RESERVED
3EH FUNCTION
Use:
Close file (handle).
Call registers:
AH = 3EH
BX = Assigned handle
Return registers:
CF = 0 if there were no mistakes, otherwise CF will be 1 and
AX will contain the error code: 06H if the handle is invalid.
This function dates the file and frees the handle it was
using.
3FH FUNCTION
Use:
To read a specific quantity of bytes from an open file and
store them on a specific buffer.
 
5.4.2 10H INTERRUPTION
Purpose: To call on diverse BIOS video
function
Syntax:
Int 10H
This interruption has several functions, all of them control
the video input/output, to access each one of them it is necessary that the
function number which is required at the moment of calling the interruption is
in the Ah register.
In this tutorial we will see some functions of the 10h
interruption.
Common functions of the 10h interruption
02H Function, select the cursor position
09H Function, write attribute and character of the
cursor
0AH Function, write a character in the cursor
position
0EH Function, Alphanumeric model of the writing
characters
02H FUNCTION
Use:
Moves the cursor on the computer screen using text
model.
Call registers:
AH = 02H
BH = Video page where the cursor is positioned.
DH = row
DL = Column
Return Registers:
None.
The cursor position is defined by its coordinates, starting
from the position 0,0 to position 79,24. This means from the left per computer
screen corner to right lower computer screen. Therefore the numeric values that
the DH and DL registers get in text model are: from 0 to 24 for rows and from 0
to 79 for columns.
09H FUNCTION
Use:
Shows a defined character several times on the computer
screen with a defined attribute, starting with the actual cursor
position.
Call registers:
AH = 09H
AL = Character to display
BH = Video page, where the character will display
it;
BL = Attribute to use
number of repetition.
Return registers:
None
This function displays a character on the computer screen
several times, using a specified number in the CX register but without changing
the cursor position on the computer screen.
0AH FUNCTION
Use:
Displays a character in the actual cursor
position.
Call registers:
AH = 0AH
AL = Character to display
BH = Video page where the character will display
it
BL = Color to use (graphic mode only).
CX = number of repetitions
Return registers:
None.
The main difference between this function and the last one
is that this one doesn't allow modifications on the attributes neither does it
change the cursor position.
0EH FUNCTION
Use:
Displays a character on the computer screen dates the cursor
position.
Call registers:
AH = 0EH
AL = Character to display
BH = Video page where the character will display
it
BL = Color to use (graphic mode only).
Return registers:
None
5.4.3 16H INTERRUPTION
We will see two functions of the 16 h interruption, these
functions are
called by using the AH register.


Functions of the 16h interruption
00H Function, reads a character from the
keyboard.
01H Function, reads the keyboard
state.
00H FUNCTION USE:
Reads a character from the keyboard.
Call registers:
AH = 00H
Return registers:
AH = Scan code of the keyboard
AL = ASCII value of the character
When we use this interruption, the program executing is
halted until a character is typed, if this is an ASCII value; it is stored in
the Ah register, Else the scan code is stored in the AL register and the AH
register contents the value 00h.
The proposal of the scan code is to use it with the keys
without ASCII representation as [ALT][CONTROL], the function keys and so
on.
01H FUNCTION
Use:
Reads the keyboard state
Call registers:
AH = 01H
Return registers:
If the flag register is zero, this means, there is
information on the buffer memory, else, there is no information in the buffer
memory. Therefore the value of the Ah register will be the value key stored in
the buffer memory.
 
5.4.4 17H INTERRUPTION
Purpose: Handles the printer input/output.
Syntax:
Int 17H
This interruption is used to write characters on the
printer, sets printer and reads the printer state.


Functions of the 16h interruptions
00H Function, prints value ASCII out
01H Function, sets printer
02H Function, the printer
state
00H FUNCTION
Use:
Writes a character on the printer.
Call registers:
AH = 00H
AL = Character to print.
DX = Port to use.
Return registers:
AH = Printer device state.
The port to use is in the DX register, the different values
are: LPT1 = 0,
LPT2 = 1, LPT3 = 2 ...
The printer device state is coded bit by bit as
follows:
BIT 1/0 MEANING
----------------------------------------
0 1 The waited time is over
1 -
2 -
3 1 input/output error
4 1 Chosen printer
5 1 out-of-paper
6 1 communication recognized
7 1 The printer is ready to use
1 and 2 bits are not relevant
Most BIOS sport 3 parallel ports, although there are BIOS
which sport 4 parallel ports.
01H FUNCTION
Use:
Sets parallel port.
Call registers:
AH = 01H
DX = Port to use
Return registers:
AH = Printer status
Port to use is defined in the DX register, for example:
LPT=0, LPT2=1, and so on.
The state of the printer is coded bit by bit as
follows:
BIT 1/0 MEANING
----------------------------------------
0 1 The waited time is over
1 -
2 -
3 1 input/output error
4 1 Chosen printer
5 1 out-of-paper
6 1 communication recognized
7 1 The printer is ready to use
1 and 2 bits are not relevant
Most BIOS sport 3 parallel ports, although there are BIOS
which sport 4 parallel ports.
02H FUNCTION
Uses:
Gets the printer status.
Call registers:
AH = 01H
DX = Port to use
Return registers
AH = Printer status.
Port to use is defined in the DX register, for example:
LPT=0, LPT2=1, and
so on
The state of the printer is coded bit by bit as
follows:
BIT 1/0 MEANING
----------------------------------------
0 1 The waited time is over
1 -
2 -
3 1 input/output error
4 1 Chosen printer
5 1 out-of-paper
6 1 communication recognized
7 1 The printer is ready to use
1 and 2 bits are not relevant
Most BIOS sport 3 parallel ports, although there are BIOS
which sport 4
parallel ports.
5.5 Ways of working with files
There are two ways to work with files, the first one is by
means of file control blocks or "FCB" and the second one is by means of
communication channels, also known as "handles".
The first way of file handling has been used since the CPM
operative system, predecessor of DOS, thus it assures certain compatibility with
very old files from the CPM as well as from the 1.0 version of the DOS, besides
this method allows us to have an unlimited number of open files at the same
time. If you want to create a volume for the disk the only way to achieve this
is by using this method.
Even after considering the advantages of the FCB, the use of
the communication channels it is much simpler and it allows us a better handling
of errors, besides, since it is much newer it is very probable that the files
created this way maintain themselves compatible through later versions of the
operative system.
For a greater facility on later explanations I will refer to
the file control blocks as FCBs and to the communication channels as
handles.
 
5.6 FCB method


Contents
5.6.1 Introduction
5.6.2 Open files
5.6.3 Create a new file
5.6.4 Sequential writing
5.6.5 Sequential reading
5.6.6Random reading and writing
5.6.7 Close a file
 
5.6.1 Introduction
There are two types of FCB, the normal, whose length is 37
bytes and the extended one of 44 bytes. On this tutorial we will only deal with
the first type, so from now on when I refer to an FCB, I am really talking about
a 37 bytes FCB.
The FCB is composed of information given by the programmer
and by information which it takes directly from the operative system.
When thesetypes of files are used it is only possible to
work on the current directory since the FCBs do not provide sport for the use of
the organization by directories of DOS.
The FCB is formed by the following fields:
POSITION LENGTH MEANING
00H 1 Byte Drive
01H 8 Bytes File name
09H 3 Bytes Extension
0CH 2 Bytes Block number
0EH 2 Bytes Register size
10H 4 Bytes File size
14H 2 Bytes Creation date
16H 2 Bytes Creation hour
18H 8 Bytes Reserved
20H 1 Bytes Current register
21H 4 Bytes Random register
To select the work drive the next format is followed: drive
A = 1; drive B = 2; etc. If 0 is used the drive being used at that moment will
be taken as option.
The name of the file must be justified to the left and in
case it is necessary the remaining bytes will have to be filled with spaces, and
the extension of the file is placed the same way.
The current block and the current register tell the computer
which register will be accessed on reading or writing operations. A block is a
group of 128 registers. The first block of the file is the block 0. The first
register is the register 0, therefore the last register of the first block would
be the 127, since the numbering started with 0 and the block can contain 128
registers in total.
5.6.2 Opening files
To open an FCB file the 21H interruption, 0FH function is
used. The unit, the name and extension of the file must be initialized before
opening it.
The DX register must point to the block. If the value of FFH
is returned on the AH register when calling on the interruption then the file
was not found, if everything came out well a value of 0 will be
returned.
If the file is opened then DOS initializes the current block
to 0, the size of the register to 128 bytes and the size of the same and its
date are filled with the information found in the directory.
5.6.3 Creating a new file
For the creation of files the 21H interruption 16H function
is used. DX must point to a control structure whose requirements are that at
least the logic unit, the name and the extension of the file be defined. In case
there is a problem the FFH value will be returned on AL, otherwise this register
will contain a value of 0.
5.6.4 Sequential writing
Before we can perform writing to the disk it is necessary to
define the data transfer area using for this end the 1AH function of the 21H
interruption.
The 1AH function does not return any state of the disk nor
or the operation, but the 15H function, which is the one we will use to write to
the disk, does it on the AL register, if this one is equal to zero there was no
error and the fields of the current register and block are dated.
5.6.5 Sequential reading
Before anything we must define the file transfer area or
DTA. In order to sequentially read we use the 14H function of the 21H
interruption.
The register to be read is the one which is defined by the
current block and register. The AL register returns to the state of the
operation, if AL contains a value of 1 or 3 it means we have reached the end of
the file. A value of 2 means that the FCB is wrongly structured.
In case there is no error, AL will contain the value of 0
and the fields of the current block and register are dated.
5.6.6 Random reading and writing
The 21H function and the 22H function of the 21H
interruption are the ones in charge of realizing the random readings and
writings respectively.
The random register number and the current block are used to
calculate the relative position of the register to read or write.
The AL register returns the same information for the
sequential reading of writing. The information to be read will be returned on
the transfer area of the disk, likewise the information to be written resides on
the DTA.
5.6.7 Closing a file
To close a file we use the 10H function of the 21H
interruption.
If after invoking this function, the AL register contains
the FFH value, this means that the file has changed position, the disk was
changed or there is error of disk access.
 
 
 
 
 
5.7 Channels of communication


Contents
5.7.1 Working with handles
5.7.2 Functions to use handles
 
5.7.1 Working with handles
The use of handles to manage files greatly facilitates the
creation of files and programmer can concentrate on other aspects of the
programming without worrying on details which can be handled by the operative
system.
The easy use of the handles consists in that to operate o a
file, it is only necessary to define the name of the same and the number of the
handle to use, all the rest of the information is internally handled by the
DOS.
When we use this method to work with files, there is no
distinction between sequential or random accesses, the file is simply taken as a
chain of bytes.
5.7.2 Functions to use handles
The functions used for the handling of files through handles
are described in unit 6: Interruptions, in the section dedicated to the 21H
interruption.
 
6 Macros and procedures
Contents
6.1 Procedures
6.2 Macros
6.1 Procedure
Definition of procedure
A procedure is a collection of instructions to which we can
direct the flow of our program, and once the execution of these instructions is
over control is given back to the next line to process of the code which called
on the procedure.
Procedures help us to create legible and easy to modify
programs.
At the time of invoking a procedure the address of the next
instruction of the program is kept on the stack so that, once the flow of the
program has been transferred and the procedure is done, one can return to the
next line of the original program, the one which called the
procedure.
Syntax of a Procedure
There are two types of procedures, the intrasegments, which
are found on the same segment of instructions, and the inter-segments which can
be stored on different memory segments.
When the intrasegment procedures are used, the value of IP
is stored on the stack and when the intrasegments are used the value of CS:IP is
stored.
To divert the flow of a procedure (calling it), the
following directive is used:
CALL NameOfTheProcedure
The part which make a procedure are:
Declaration of the procedure
Code of the procedure
Return directive
Termination of the procedure
For example, if we want a routine which adds two bytes
stored in AH and AL each one, and keep the addition in the BX
register:
Adding Proc Near ; Declaration of the procedure
Mov Bx, 0 ; Content of the procedure
Mov B1, Ah
Mov Ah, 00
Add Bx, Ax
Ret ; Return directive
Add Endp ; End of procedure declaration
On the declaration the first word, Adding, corresponds to
the name of out procedure, Proc declares it as such and the word Near indicates
to the MASM that the procedure is intrasegment.
The Ret directive loads the IP address stored on the stack
to return to the original program, lastly, the Add Endp directive indicates the
end of the procedure.
To declare an inter segment procedure we substitute the word
Near for the word FAR.
The calling of this procedure is done the following
way:
Call Adding
Macros offer a greater flexibility in programming compared
to the procedures, nonetheless, these last ones will still be used.

Macros

Contents
6.2.1 Definition of a macro
6.2.2 Syntax of a macro
6.2.3 Macro libraries
 
6.2.1 Definition of the macro
A macro is a group of repetitive instructions in a program
which are codified only once and can be used as many times as
necessary.
The main difference between a macro and a procedure is that
in the macro the passage of parameters is possible and in the procedure it is
not, this is only applicable for the TASM - there are other programming
languages which do allow it. At the moment the macro is executed each parameter
is substituted by the name or value specified at the time of the
call.
We can say then that a procedure is an extension of a
determined program, while the macro is a module with specific functions which
can be used by different programs.
Another difference between a macro and a procedure is the
way of calling each one, to call a procedure the use of a directive is required,
on the other hand the call of macros is done as if it were an assembler
instruction.
6.2.2 Syntax of a Macro
The parts which make a macro are:
Declaration of the macro
Code of the macro
Macro termination directive
The declaration of the macro is done the following
way:
NameMacro MACRO [parameter1,
parameter2...]
Even though we have the functionality of the parameters it
is possible to create a macro which does not need them.
The directive for the termination of the macro is:
ENDM
An example of a macro, to place the cursor on a determined
position on the screen is:
Position MACRO Row, Column
PUSH AX
PUSH BX
PUSH DX
MOV AH, 02H
MOV DH, Row
MOV DL, Column
MOV BH, 0
INT 10H
POP DX
POP BX
POP AX
ENDM
To use a macro it is only necessary to call it by its name,
as if it were another assembler instruction, since directives are no longer
necessary as in the case of the procedures.
Example:
Position 8, 6
6.2.3 Macro Libraries
One of the facilities that the use of macros offers is the
creation of libraries, which are groups of macros which can be included in a
program from a different file.
The creation of these libraries is very simple, we only have
to write a file with all the macros which will be needed and save it as a text
file.
To call these macros it is only necessary to use the
following instruction Include NameOfTheFile, on the part of our program where we
would normally write the macros, this is, at the beginning of our program,
before the declaration of the memory model.
The macros file was saved with the name of MACROS.TXT, the
instruction Include would be used the following way:
;Beginning of the program
Include MACROS.TXT
.MODEL SMALL
.DATA
;The data goes here
.CODE
Beginning:
;The code of the program is inserted here
.STACK
;The stack is defined
End beginning
;Our program ends
 
More debug program examples
In this section we provide you several assembler programs to
run in the debug program. You can execute each assembler program using the "t"
(trace) command, to see what each instruction does.
First example
-a0100
297D:0100 MOV AX,0006 ; Puts value 0006 at register
AX
297D:0103 MOV BX,0004 ;Puts value 0004 at register
BX
297D:0106 ADD AX,BX ;Adds BX to AX
contents
297D:0108 INT 20 ;Causes end of the
Program
The only thing that this program does is to save two values
in two registers and add the value of one to the other.
Second example
- a100
0C1B:0100 jmp 125 ; Jumps to direction 125H
0C1B:0102 [Enter]
- e 102 'Hello, How are you ?' 0d 0a '$'
- a125
0C1B:0125 MOV DX,0102 ; Copies string to DX
register
0C1B:0128 MOV CX,000F ; Times the string will be
displayed
0C1B:012B MOV AH,09 ; Copies 09 value to AH
register
0C1B:012D INT 21 ; Displays string
0C1B:012F DEC CX ; Reduces in 1 CX
0C1B:0130 JCXZ 0134 ; If CX is equal to 0 jumps to
0134
0C1B:0132 JMP 012D ; Jumps to direction 012D
0C1B:0134 INT 20 ; Ends the program
This program displays on the screen 15 times a character
string.
Third example
-a100
297D:0100 MOV AH,01 ;Function to change the
cursor
297D:0102 MOV CX,0007 ;Forms the cursor
297D:0105 INT 10 ;Calls for BIOS
297D:0107 INT 20 ;Ends the program
This program is good for changing the form of the
cursor.
Fourth example
-a100
297D:0100 MOV AH,01 ; Funtion 1 (reads
keyboard)
297D:0102 INT 21 ; Calls for DOS
297D:0104 CMP AL,0D ; Compares if what is read is a
carriage return
297D:0106 JNZ 0100 ; If it is not, reads another
character
297D:0108 MOV AH,02 ; Funtion 2 (writes on the
screen)
297D:010A MOV DL,AL ; Character to write on
AL
297D:010C INT 21 ; Calls for DOS
297D:010E INT 20 ; Ends the program
This program uses DOS 21H interruption. It uses two
functions of the same: the first one reads the keyboard (function 1) and the
second one writes on the screen. It reads the keyboard characters until it finds
a carriage
return.
Fifth example
-a100
297D:0100 MOV AH,02 ; Function 2 (writes on the
screen)
297D:0102 MOV CX,0008; Puts value 0008 on register
CX
297D:0105 MOV DL,00 ; Puts value 00 on register
DL
297D:0107 RCL BL,1 ; Rotates the byte in BL to the
left by one bit
; through the carry flag
297D:0109 ADC DL,30 ; Converts flag register
to1
297D:010C INT 21 ; Calls for DOS
297D:010E LOOP 0105 ; Jumps if CX > 0 to
direction 0105
297D:0110 INT 20 ; Ends the program
This program displays on the screen a binary number through
a conditional cycle (LOOP) using byte rotation.
Sixth example
-a100
297D:0100 MOV AH,02 ; Function 2 (writes on the
screen)
297D:0102 MOV DL,BL ; Puts BL's value on
DL
297D:0104 ADD DL,30 ; Adds value 30 to
DL
297D:0107 CMP DL,3A ; Compares 3A value with DL's
contents without
; affecting its value only modifying the state of

; the car
297D:010A JL 010F ; jumps if < direction
010f
297D:010C ADD DL,07 ; Adds 07 value on
DL
297D:010F INT 21 ; Calls for Dos
297D:0111 INT 20 ; Ends the Program
This program prints a zero value on hex digits
Seventh example
-a100
297D:0100 MOV AH,02 ; Function 2 (writes on the
screen)
297D:0102 MOV DL,BL ; Puts BL value on
DL
297D:0104 AND DL,0F ; Carries ANDing numbers bit by
bit
297D:0107 ADD DL,30 ; Adds 30 to Dl
297D:010A CMP DL,3A ; Compares Dl with
3A
297D:010D JL 0112 ; Jumps if < 0112
direction
297D:010F ADD DL, 07 ; Adds 07 to DL
297D:0112 INT 21 ; Calls for Dos
297D:0114 INT 20 ; Ends the program
This program is used to print two digit hex
numbers.
Eight example
-a100
297D:0100 MOV AH,02 ; Function 2 (writes on the
screen)
297D:0102 MOV DL,BL ; Puts BL value on
DL
297D:0104 MOV CL,04 ; Puts 04 value on
CL
297D:0106 SHR DL,CL ; Moves per four bits of your
number to the
; rightmost nibble
297D:0108 ADD DL,30 ; Adds 30 to DL
297D:010B CMP L,3A ; Compares Dl with 3A
297D:010E JL 0113 ; Jumps if < 0113
direction
297D:0110 ADD DL,07 ; Adds 07 to DL
297D:0113 INT 21 ; Calls for Dos
297D:0115 INT 20 ; Ends the program
This program works for printing the first of two digit hex
numbers
Ninth example
-a100
297D:0100 MOV AH,02 ; Function 2 (writes on the
screen)
297D:0102 MOV DL,BL ; Puts BL value on
DL
297D:0104 MOV CL,04 ; Puts 04 value on
CL
297D:0106 SHR DL,CL ; Moves per four bits of your
number to the
;rightmost nibble
297D:0108 ADD DL,30 ; Adds 30 to DL
297D:010B CMP DL,3A ; Compares Dl with
3A
297D:010E JL 0113 ; Jumps if < 0113
direction
297D:0110 ADD DL,07 ; Adds 07 to DL
297D:0113 INT 21 ; Calls for Dos
297D:0115 MOV DL,BL ; Puts Bl value on
DL
297D:0117 AND DL,0F ; Carries ANDing numbers bit by
bit
297D:011A ADD DL,30 ; Adds 30 to DL
297D:011D CMP DL,3A ; Compares Dl with
3A
297D:0120 JL 0125 ; Jumps if < 125
direction
297D:0122 ADD DL,07 ; Adds 07 to DL
297D:0125 INT 21 ; Calls for Dos
297D:0127 INT 20 ; Ends the Program
This program works for printing the second of two digit hex
numbers
Tenth example
-a100
297D:0100 MOV AH,01 ; Function 1 (reads
keyboard)
297D:0102 INT 21 ; Calls for Dos
297D:0104 MOV DL,AL ; Puts Al value on
DL
297D:0106 SUB DL,30 ; Subtracts 30 from
DL
297D:0109 CMP DL,09 ; Compares DL with
09
297D:010C JLE 0111 ; Jumps if <= 0111
direction
297D:010E SUB DL,07 ; Subtracts 07 from
DL
297D:0111 MOV CL,04 ; Puts 04 value on CL
register
297D:0113 SHL DL,CL ; It inserts zeros to the
right
297D:0115 INT 21 ; Calls for Dos
297D:0117 SUB AL,30 ; Subtracts 30 from
AL
297D:0119 CMP AL,09 ; Compares AL with
09
297D:011B JLE 011F ; Jumps if <= 011f
direction
297D:011D SUB AL,07 ; Subtracts 07 from
AL
297D:011F ADD DL,AL ; Adds Al to DL
297D:0121 INT 20 ; Ends the Program
This program can read two digit hex numbers
Eleventh example
-a100
297D:0100 CALL 0200 ; Calls for a
procedure
297D:0103 INT 20 ;Ends the program
-a200
297D:0200 PUSH DX ; Puts DX value on the
stack
297D:0201 MOV AH,08 ; Function 8
297D:0203 INT 21 ; Calls for Dos
297D:0205 CMP AL,30 ; Compares AL with
30
297D:0207 JB 0203 ; Jumps if CF is activated
towards 0203 direction
297D:0209 CMP AL,46 ; Compares AL with
46
297D:020B JA 0203 ; jumps if <> 0203
direction
297D:020D CMP AL,39 ; Compares AL with
39
297D:020F JA 021B ; Jumps if <> 021B
direction
297D:0211 MOV AH,02 ; Function 2 (writes on the
screen)
297D:0213 MOV DL,AL ; Puts Al value on
DL
297D:0215 INT 21 ; Calls for Dos
297D:0217 SUB AL,30 ; Subtracts 30 from
AL
297D:0219 POP DX ; Takes DX value out of the
stack
297D:021A RET ; Returns control to the main
program
297D:021B CMP AL,41 ; Compares AL with
41
297D:021D JB 0203 ; Jumps if CF is activated
towards 0203 direction
297D:021F MOV AH,02 ; Function 2 (writes on the
screen)
297D:022 MOV DL,AL ; Puts AL value on DL
297D:0223 INT 21 ; Calls for Dos
297D:0225 SUB AL,37 ; Subtracts 37 from
AL
297D:0227 POP DX ; Takes DX value out of the
stack
297D:0228 RET ; Returns control to the main
program
This program keeps reading characters until it receives one
that can be
converted to a hex number
More Assembler programs examples( using TASM
program)
 
;name of the program:one.asm
;
.model small
.stack
.code
mov AH,1h ;Selects the 1 D.O.S. function
Int 21h ;reads character and return ASCII code to
register AL
mov DL,AL ;moves the ASCII code to register
DL
sub DL,30h ;makes the operation minus 30h to
convert 0-9 digit number
cmp DL,9h ;compares if digit number it was between
0-9
jle digit1 ;If it true gets the first number digit
(4 bits long)
sub DL,7h ;If it false, makes operation minus 7h to
convert letter A-F
digit1:
mov CL,4h ;prepares to multiply by 16
shl DL,CL ; multiplies to convert into four bits
upper
int 21h ;gets the next character
sub AL,30h ;repeats the conversion
operation
cmp AL,9h ;compares the value 9h with the content
of register AL
jle digit2 ;If true, gets the second digit
number
sub AL,7h ;If no, makes the minus operation
7h
digit2:
add DL,AL ;adds the second number digit
mov AH,4CH
Int 21h ;21h interruption
End ; finishs the program code
This program reads two characters from the keyboard and
prints them on the screen.
;name the program:two.asm
.model small
.stack
.code
PRINT_A_J PROC
MOV DL,'A' ;moves the A character to register
DL
MOV CX,10 ;moves the decimal value 10 to register
cx
;This number value its the time to print out after
the A ;character
PRINT_LOOP:
CALL WRITE_CHAR ;Prints A character out
INC DL ;Increases the value of register
DL
LOOP PRINT_LOOP ;Loop to print out ten
characters
MOV AH,4Ch ;4Ch function of the 21h
interruption
INT 21h ;21h interruption
PRINT_A_J ENDP ;Finishes the procedure
WRITE_CHAR PROC
MOV AH,2h ;2h function of the 21
interruption
INT 21h ;Prints character out from the register
DL
RET ;Returns the control to procedure
called
WRITE_CHAR ENDP ;Finishes the procedure
END PRINT_A_J ;Finishes the program code
This program prints the a character through j character on
the screen
;name of the program :three.asm
.model small
.STACK
.code
TEST_WRITE_HEX PROC
MOV DL,3Fh ;moves the value 3Fh to the register
DL
CALL WRITE_HEX ;Calls the procedure
MOV AH,4CH ;4Ch function
INT 21h ;Returns the control to operating
system
TEST_WRITE_HEX ENDP ;Finishes the
procedure
PUBLIC WRITE_HEX
;........................................................;
; This procedure converts into hexadecimal number
the byte is in the register DL and show the digit number;
;Use:WRITE_HEX_DIGIT ;
;........................................................;
WRITE_HEX PROC
PUSH CX ;pushes the value of the register CX to the
stack memory
PUSH DX ;pushes the value of the register DX to the
stack memory
MOV DH,DL ;moves the value of the register DL to
register DH
MOV CX,4 ;moves the value numeric 4 to register
CX
SHR DL,CL
CALL WRITE_HEX_DIGIT ;shows on the computer screen,
the first hexadecimal number
MOV DL,DH ;moves the value of the register DH to
the register DL
AND DL,0Fh ;ANDing the upper bit
CALL WRITE_HEX_DIGIT ; shows on the computer
screen, the second hexadecimal number
POP DX ;pops the value of the register DX to
register DX
POP CX ; pops the value of the register DX to
register DX
RET ;Returns the control of the procedure
called
WRITE_HEX ENDP
PUBLIC WRITE_HEX_DIGIT
;......................................................................;
; ;
; This procedure converts the lower 4 bits of the
register DL into hexadecimal ;number and show them in the computer screen
;
;Use: WRITE_CHAR ;
;......................................................................;
WRITE_HEX_DIGIT PROC
PUSH DX ;Pushes the value of the register DX in the
stack memory
CMP DL,10 ;compares if the bit number is minus than
number ten
JAE HEX_LETTER ;No , jumps HEX_LETER
ADD DL,"0" ;yes, it converts into digit
number
JMP Short WRITE_DIGIT ;writes the
character
HEX_LETTER:
ADD DL,"A"-10 ;converts a character into
hexadecimal number
WRITE_DIGIT:
CALL WRITE_CHAR ;shows the character in the
computer screen
POP DX ;Returns the initial value of the register
DX to register DL
RET ;Returns the control of the procedure
called
WRITE_HEX_DIGIT ENDP
PUBLIC WRITE_CHAR
;......................................................................;
;This procedure shows the character in the computer
screen using the D.O.S. ;
;......................................................................;
WRITE_CHAR PROC
PUSH AX ;pushes the value of the register AX in the
stack memory
MOV AH,2 ;2h Function
INT 21h ;21h Interruption
POP AX ;Pops the initial value of the register AX
to the register AX
RET ;Returns the control of the procedure
called
WRITE_CHAR ENDP
END TEST_WRITE_HEX ;finishes the program
code
This program prints a predefined value on the
screen
 
;name of the program:five.asm
.model small
.stack
.code
PRINT_ASCII PROC
MOV DL,00h ;moves the value 00h to register
DL
MOV CX,255 ;moves the value decimal number 255.
this decimal number
;will be 255 times to print out after the character
A
PRINT_LOOP:
CALL WRITE_CHAR ;Prints the characters
out
INC DL ;Increases the value of the register DL
content
LOOP PRINT_LOOP ;Loop to print out ten
characters
MOV AH,4Ch ;4Ch function
INT 21h ;21h Interruption
PRINT_ASCII ENDP ;Finishes the procedure
WRITE_CHAR PROC
MOV AH,2h ;2h function to print character
out
INT 21h ;Prints out the character in the register
DL
RET ;Returns the control to the procedure
called
WRITE_CHAR ENDP ;Finishes the procedure
END PRINT_ASCII ;Finishes the program
code
 
This program prints the 256 ASCII code on the
screen
dosseg
.model small
.stack
.code
write proc
mov ah,2h;
mov dl,2ah;
int 21h
mov ah,4ch
int 21h
write endp
end write
This program prints a defined character using an ASCII code
on the screen.
 
.model small; the name of the program is
seven.asm
.stack;
.code;
EEL: MOV AH,01 ; 1 function (reads one character
from the keyboard)
INT 21h ; 21h interruption
CMP AL,0Dh ; compares the value with 0dh
JNZ EEL ;jumps if no equal of the label
eel
MOV AH,2h ; 2 function (prints the character out on
the screen)
MOV DL,AL ;moves the value of the register AL to
the register DL
INT 21h ;21 interruption
MOV AH,4CH ;4C function (returns the control to the
D.O.S. operating system)
INT 21h ;21 interruption
END ;finishes the program
This program reads characters form the keyboard and prints
them on the screen until find the return character.
 

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