Assembly Language Tutor:
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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 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 od Guadalajara as
the source and must include this notice; and (2) any other use of this material must reference this manual and University of Guadalajara and the fact that the material is copyright by Hugo Perez and is used by permission.

Assembler Tutorial
1996 Edition


Table of Contents

1 Introduction
2 Basic Concepts
3 Assembler programming
4 Assembler language instructions
5 Interruptions and file managing
6 Macros and procedures
7 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

Table of 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.


Table of 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


Table of Contents

2.2.1 Information in the computers
2.2.2 Data representation methods


2.2.1 Information in the computers

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.

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

Table of 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 to do this appear on the screen some like the
follow 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

Table of Contents

3.1 Building Assembler programs
3.2 Assembly process
3.3 More assembler programs
3.4 Types of instructions
3.5 Click here to get more assembler programs

3.1 Building Assembler programs

3.1.1 Needed software
3.1.2 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.

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

4 Assembler language Instructions


Table of 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

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
4.7 Instructions for cycles: loop
4.8 Counting Instructions
4.9 Comparison Instructions
4.10 Flag Instructions


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

Table of 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

Table of 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

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.

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.

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 pa

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 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

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.

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
ill be 1 and AX will contain the error code: 3 path not found 4 there
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 03

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

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 gro 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

Table of 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

table of 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.

6.2 Macros

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 gro 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 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 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 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 progrma 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.