University of Guadalajara
Information Sistems General
Coordination.
Culture and Entertainment Web
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: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.