(Editor's Note - this article requires additional formatting work and is incomplete)

Assembly Tutorial
by Hugo Perez University of Guadalajara
Information Sistems General Coordination
Culture and Entertainment Web

June 12th 1995
Copyright(C) 1995-1996

This is an introduction for people who want to programming in assembler language.

Copyright (C) 1995-1996, Hugo Perez Perez. Anyone may reproduce this document, in whole or in part, provided that: (1) any copy or republication of the entire document must show University od Guadalajara as the source, and must include this notice; and (2) any other use of this material must reference this manual and University of Guadalajara, and the fact that the material is copyright by Hugo Perez and is used by permission.

Assembler Tutorial
1996 Edition

Table of Contents

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


1. Introduction
Table of contents

1.1 What's new in the Assembler material
1.2 Presentation
1.3 Why learn Assembler language
1.4 We need your opinion


1.1 What's new in the Assembler material
After of one year that we've released the first Assembler material on-line. We've received a lot of e-mail where each people talk about different aspects about this material. We've tried to put these comments and suggestions in this update assembler material. We hope that this new Assembler material release reach to all people that they interest to learn the most important language for IBM PC.

In this new assembler release includes:

A complete chapter about how to use debug program
More example of the assembler material
Each section of this assembler material includes a link file to Free
On-line of Computing by Dennis Howe
Finally, a search engine to look for any topic or item related with this updated material.


1.2 Presentation
The document you are looking at, has the primordial function of introducing you to assembly language programming, and it has been thought for those people who have never worked with this language.

The tutorial is completely focused towards the computers that function with processors of the x86 family of Intel, and considering that the language bases its functioning on the internal resources of the processor, the described examples are not compatible with any other architecture.

The information was structured in units in order to allow easy access to each of the topics and facilitate the following of the tutorial.

In the introductory section some of the elemental concepts regarding computer systems are mentioned, along with the concepts of the assembly language itself, and continues with the tutorial itself.


1.3 Why learn assembler language
The first reason to work with assembler is that it provides the opportunity of knowing more the operation of your PC, which allows the development of software in a more consistent manner.

The second reason is the total control of the PC which you can have with the use of the assembler.

Another reason is that the assembly programs are quicker, smaller, and have larger capacities than ones created with other languages.

Lastly, the assembler allows an ideal optimization in programs, be it on their size or on their execution.


1.4 We need your opinion
Our goal is offers you easier way to learn yourself assembler language. You send us your comments or suggestions about this 96' edition. Any comment will be welcome.


2. Basic Concepts
Table of Contents

2.1 Basic description of a computer system.
2.2 Assembler language Basic concepts
2.3 Using debug program


2.1 Basic description of a computer system.
This section has the purpose of giving a brief outline of the main components of a computer system at a basic level, which will allow the user a greater understanding of the concepts which will be dealt with throughout the tutorial.

Table of Contents

2.1.1 Central Processor
2.1.2 Central Memory
2.1.3 Input and Output Units
2.1.4 Auxiliary Memory Units

Computer System.

We call computer system to the complete configuration of a computer, including the peripheral units and the system programming which make it a useful and functional machine for a determined task.


2.1.1 Central Processor.
This part is also known as central processing unit or CPU, which in turn is made by the control unit and the arithmetic and logic unit. Its functions consist in reading and writing the contents of the memory cells, to forward data between memory cells and special registers, and decode and execute the instructions of a program. The processor has a series of memory cells which are used very often and thus, are part of the CPU. These cells are known with the name of registers. A processor may have one or two dozen of these registers. The arithmetic and logic unit of the CPU realizes the operations related with numeric and symbolic calculations. Typically these units only have capacity of performing very elemental operations such as: the addition and subtraction of two whole numbers, whole number multiplication and division, handling of the registers' bits and the comparison of the content of two registers. Personal computers can be classified by what is known as word size, this is, the quantity of bits which the processor can handle at a time.


2.1.2 Central Memory.
It is a group of cells, now being fabricated with semi-conductors, used for general processes, such as the execution of programs and the storage of information for the operations.

Each one of these cells may contain a numeric value and they have the property of being addressable, this is, that they can distinguish one from another by means of a unique number or an address for each cell.

The generic name of these memories is Random Access Memory or RAM. The main disadvantage of this type of memory is that the integrated circuits lose the information they have stored when the electricity flow is interrupted. This was the reason for the creation of memories whose information is not lost when the system is turned off. These memories receive the name of Read Only Memory or ROM.


2.1.3 Input and Output Units.
In order for a computer to be useful to us it is necessary that the processor communicates with the exterior through interfaces which allow the input and output of information from the processor and the memory. Through the use of these communications it is possible to introduce information to be processed and to later visualize the processed data.

Some of the most common input units are keyboards and mice. The most common output units are screens and printers.


2.1.4 Auxiliary Memory Units.
Since the central memory of a computer is costly, and considering today's applications it is also very limited. Thus, the need to create practical and economical information storage systems arises. Besides, the central memory loses its content when the machine is turned off, therefore making it inconvenient for the permanent storage of data.

These and other inconvenience give place for the creation of peripheral units of memory which receive the name of auxiliary or secondary memory. Of these the most common are the tapes and magnetic discs.

The stored information on these magnetic media means receive the name of files. A file is made of a variable number of registers, generally of a fixed size; the registers may contain information or programs.


2.2 Assembler language Basic concepts
Table of Contents

2.2.1 Information in the computers
2.2.2 Data representation methods


2.2.1 Information in the computer
2.2.1.1 Information units
2.2.1.2 Numeric systems
2.2.1.3 Converting binary numbers to decimal
2.2.1.4 Converting decimal numbers to binary
2.2.1.5 Hexadecimal system

2.2.1.1 Information Units

In order for the PC to process information, it is necessary that this information be in special cells called registers. The registers are groups of 8 or 16 flip-flops.

A flip-flop is a device capable of storing two levels of voltage, a low one, regularly 0.5 volts, and another one, commonly of 5 volts. The low level of energy in the flip-flop is interpreted as off or 0, and the high level as on or 1. These states are usually known as bits, which are the smallest information unit in a computer.

A group of 16 bits is known as word; a word can be divided in groups of 8 bits called bytes, and the groups of 4 bits are called nibbles.

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:

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:

Filling the last group with zeros (the one from the left):

Afterwards we take each group as an independent number and we consider its decimal value:

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:

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


2.3.1 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:


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:

It is possible to visualize the values of the internal registers of the CPU using the Debug program. To begin working with Debug, type the following prompt in your computer:

On the next line a dash will appear, this is the indicator of Debug, at this moment the instructions of Debug can be introduced using the following command:

All the contents of the internal registers of the CPU are displayed; an alternative of viewing them is to use the "r" command using as a parameter the name of the register whose value wants to be seen. For example:

This instruction will only display the content of the BX register and the Debug indicator changes from "-" to ":"

When the prompt is like this, it is possible to change the value of the register which was seen by typing the new value and [Enter], or the old value can be left by pressing [Enter] without typing any other value.


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:

Here "add" is the command to be executed, in this case an addition, and "ah" as well as "bh" are the parameters.

For example:

In the above example, we are using the instruction mov, it means move the value 25 to al register.

The name of the instructions in this language is made of two, three or four letters. These instructions are also called mnemonic names or operation codes, since they represent a function the processor will perform.

Sometimes instructions are used as follows:

The brackets in the second parameter indicate to us that we are going to work with the content of the memory cell number 170 and not with the 170 value, this is known as direct addressing.


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:

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:

Type the command "t" (trace), to execute each instruction of this program, example:

You see that the value 2 move to AX register. Type the command "t" (trace), again, and you see the second instruction is executed.

Type the command "t" (trace) to see the instruction add is executed, you will see the follow lines:

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:

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.

The "n" command allows us to name the program.

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.

Lastly, the "w" command writes our program on the disk, indicating how many bytes it wrote.

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:

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

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:

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:

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

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:

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:

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 structur


3.3 More assembler programs
Another example

first step

use any editor program to create the source file. Type the following lines:

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.

fourth step

Use the TLINK program to build the executable program

fifth step

Execute the executable program

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:

1. 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.
2. 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:

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:

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:

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:

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:

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:

Example:

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:

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:

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:

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.

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:

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:

The "??" means that there will be an undefined value in those bits.


LDS Instruction
Purpose: To load the register of the data segment

Syntax:

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:

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:

Is equivalent to:

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:

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:

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:

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:

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:

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:

With this instruction the "y" logic operation for both operators is carried out:

The result of this operation is stored on the destiny operator.


NEG Instruction
Purpose: It generates the complement to 2.

Syntax:

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:

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:

The result is stored on the same destiny operator.


OR Instruction
Purpose: Logic inclusive OR

Syntax:

The OR instruction carries out, bit by bit, the logic inclusive disjunction of the two operators:


TEST Instruction
Purpose: It logically compares the operators

Syntax:

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:

Its function is to perform the logic exclusive disjunction of the two operators bit by bit.


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:

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:

It adds the two operators and stores the result on the destiny operator.


DIV Instruction
Purpose: Division without sign.

Syntax:

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:

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:

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:

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:

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:

It subtracts the source operator from the destiny.


4.6 Jump instructions
They are used to transfer the flow of the process to the indicated operator.

JMP
JA (JNBE)
JAE (JNBE)
JB (JNAE)
JBE (JNA)
JE (JZ)
JNE (JNZ)
JG (JNLE)
JGE (JNL)
JL (JNGE)
JLE (JNG)
JC
JNC
JNO
JNP (JPO)
JNS
JO
JP (JPE)
JS


JMP Instruction
Purpose: Unconditional jump.

Syntax:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

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:

This instruction turns off the corresponding bit to the address flag.


CLI Instruction
Purpose: To clean the interruption flag.

Syntax:

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:

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:

This instruction puts the CF flag in 1.


STD Instruction
Purpose: To activate the address flag.

Syntax:

The STD instruction puts the DF flag in 1.


STI Instruction
Purpose: To activate the interruption flag.

Syntax:

The instruction activates the IF flag, and this enables the maskarable external interruptions ( the ones which only function when IF = 1).


5. Interruptions and