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
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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.
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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.
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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.
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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.
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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 computer
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.
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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:
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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
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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.
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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.
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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.
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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
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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]
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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
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:
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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:
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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
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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
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.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.
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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]
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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.
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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.
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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
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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
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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:
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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.
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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)
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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:
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.
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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
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LODS (LODSB) (LODSW) INSTRUCTION
Purpose: To load chains of a byte or a word into the accumulator.
Syntax:
LODS
This instruction takes the chain found on the address specified by SI,
loads it to the AL (or AX) register and adds or subtracts , depending on
the state of DF, to SI if it is a bytes transfer or if it is a words
transfer.
MOV SI, OFFSET VAR1
LODS
The first line loads the VAR1 address on SI and the second line takes the
content of that locality to the AL register.
The LODSB and LODSW commands are used in the same way, the first one loads a byte and the second one a word (it uses the complete AX register).
LAHF INSTRUCTION
Purpose: It transfers the content of the flags to the AH register.
Syntax:
LAHF
This instruction is useful to verify the state of the flags during the
execution of our program.
The flags are left in the following order inside the register:
SF ZF ?? AF ?? PF ?? CF
The "??" means that there will be an undefined value in those bits.
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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.
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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.
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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.
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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.
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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.
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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
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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
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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
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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.
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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.
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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
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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.
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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.
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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
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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
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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.
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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.
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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.
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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
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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.
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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:
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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
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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].
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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:
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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.
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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.
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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 will be 1 and AX will contain the error code: 3 path not found, 4 there
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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:
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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.
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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
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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
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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
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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
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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.
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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.
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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.
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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".
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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
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More debug program examples
In this section we provide you several assembler programs to run in the
debug program. You can execute each assembler program using the "t" (trace) command, to see what each instruction does.
First example
-a0100
297D:0100 MOV AX,0006 ; Puts value 0006 at register AX
297D:0103 MOV BX,0004 ;Puts value 0004 at register BX
297D:0106 ADD AX,BX ;Adds BX to AX contents
297D:0108 INT 20 ;Causes end of the Program
The only thing that this program does is to save two values in two
registers and add the value of one to the other.
Second example
- a100
0C1B:0100 jmp 125 ; Jumps to direction 125H
0C1B:0102 [Enter]
- e 102 'Hello, How are you ?' 0d 0a '$'
- a125
0C1B:0125 MOV DX,0102 ; Copies string to DX register
0C1B:0128 MOV CX,000F ; Times the string will be displayed
0C1B:012B MOV AH,09 ; Copies 09 value to AH register
0C1B:012D INT 21 ; Displays string
0C1B:012F DEC CX ; Reduces in 1 CX
0C1B:0130 JCXZ 0134 ; If CX is equal to 0 jumps to 0134
0C1B:0132 JMP 012D ; Jumps to direction 012D
0C1B:0134 INT 20 ; Ends the program
This program displays on the screen 15 times a character string.
Third example
-a100
297D:0100 MOV AH,01 ;Function to change the cursor
297D:0102 MOV CX,0007 ;Forms the cursor
297D:0105 INT 10 ;Calls for BIOS
297D:0107 INT 20 ;Ends the program
This program is good for changing the form of the cursor.
Fourth example
-a100
297D:0100 MOV AH,01 ; Funtion 1 (reads keyboard)
297D:0102 INT 21 ; Calls for DOS
297D:0104 CMP AL,0D ; Compares if what is read is a carriage return
297D:0106 JNZ 0100 ; If it is not, reads another character
297D:0108 MOV AH,02 ; Funtion 2 (writes on the screen)
297D:010A MOV DL,AL ; Character to write on AL
297D:010C INT 21 ; Calls for DOS
297D:010E INT 20 ; Ends the program
This program uses DOS 21H interruption. It uses two functions of the same:
the first one reads the keyboard (function 1) and the second one writes on
the screen. It reads the keyboard characters until it finds a carriage
return.
Fifth example
-a100
297D:0100 MOV AH,02 ; Function 2 (writes on the screen)
297D:0102 MOV CX,0008 ; Puts value 0008 on register CX
297D:0105 MOV DL,00 ; Puts value 00 on register DL
297D:0107 RCL BL,1 ; Rotates the byte in BL to the left by one bit through the ;carry flag
297D:0109 ADC DL,30 ; Converts flag register to1
297D:010C INT 21 ; Calls for DOS
297D:010E LOOP 0105 ; Jumps if CX > 0 to direction 0105
297D:0110 INT 20 ; Ends the program
This program displays on the screen a binary number through a conditional
cycle (LOOP) using byte rotation.
Sixth example
-a100
297D:0100 MOV AH,02 ; Function 2 (writes on the screen)
297D:0102 MOV DL,BL ; Puts BL's value on DL
297D:0104 ADD DL,30 ; Adds value 30 to DL
297D:0107 CMP DL,3A ; Compares 3A value with DL's contents without affecting ; its value only modifying the state of the car
297D:010A JL 010F ; jumps if < direction 010f
297D:010C ADD DL,07 ; Adds 07 value on DL
297D:010F INT 21 ; Calls for Dos
297D:0111 INT 20 ; Ends the Program
This program prints a zero value on hex digits
Seventh example
-a100
297D:0100 MOV AH,02 ; Function 2 (writes on the screen)
297D:0102 MOV DL,BL ; Puts BL value on DL
297D:0104 AND DL,0F ; Carries ANDing numbers bit by bit
297D:0107 ADD DL,30 ; Adds 30 to Dl
297D:010A CMP DL,3A ; Compares Dl with 3A
297D:010D JL 0112 ; Jumps if < 0112 direction
297D:010F ADD DL, 07 ; Adds 07 to DL
297D:0112 INT 21 ; Calls for Dos
297D:0114 INT 20 ;Ends the program
This program is used to print two digit hex numbers.
Eight example
-a100
297D:0100 MOV AH,02 ; Function 2 (writes on the screen)
297D:0102 MOV DL,BL ; Puts BL value on DL
297D:0104 MOV CL,04 ; Puts 04 value on CL
297D:0106 SHR DL,CL ; Moves per four bits of your number to the rightmost ;nibble
297D:0108 ADD DL,30 ; Adds 30 to DL
297D:010B CMP 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
