lecture notes

KS30103 Microprocessor

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INTRODUCTION TO PROGRAMMING

Outcomes

At the end of this chapter, you should be able to:

1. Define software, programme, and programming languages

2. Explain the classification of programming languages

3. Describe the stages of a programme's development life cycle

4. Describe the sequence, selection, and iteration programming constructs

5. Design a simple program using flowchart

Fundamentals

Computers, in general, are divided into two parts: hardware and software. By definition, all

physical components (CPU, memory, I/O ports etc) of the computer are collectively called the

hardware. Software, on the other hand, refers to the instructions that tell the hardware what to

do.

Software

Software refers to computer instructions or data or anything that can be stored electronically. It

can be classified into systems software that includes the operating system and all the utilities

that enable the computer to function. And applications software that includes programs that do

real work for users. For example, word processors, spreadsheets, and database management

systems fall under the category of applications software

Program

A program is an organised lists of instructions in a given programming language that, when

executed, causes the computer to behave in a predetermined manner. An instruction is a

computer command that tell the computer to perform an action.

Programming Languages

A programming language has a vocabulary and a set of grammatical rules for instructing a

computer to perform specific tasks. The vocabulary is a set of unique keywords (words that it

understands). While, the grammatical rules (syntax) specify how instructions are organised.

Programming Languages Classifications


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Programming languages, in general, can be classified into 4

categories called generations. These generations reflect the

historical development of programming languages. As can be

seen in figure 1.1 below, the first level of software above the

hardware is machine code (first generation). The second level

(second generation) is assembly language. While, the third

and fourth levels are high-level (third generation) and object-

oriented (fourth generation) respectively.

First Generation (Machine Code)

Machine code is the lowest-level programming language and is the only languages understood

by computers. Machine language instructions are written in binary or hexadecimal numbers.

This makes it easily understood by computers but almost impossible for humans to understand

and use. Every CPU has its own unique machine language and thus, machine language are

called machine-dependent which means a program written for a given CPU can only runs on

that CPU.

Second Generation (Assembly Language)

Assembly language is the next level above machine code. It has the same structure and set of

commands as machine language, but it allows a programmer to use names (mnemonics)

instead of numbers to write instructions. Each type of CPU has its own machine language and

assembly language, so an assembly language program written for one type of CPU won't run on

another. Thus, assembly language is machine-dependent.

In the early days of programming, all programs were written in assembly language. Now, most

programs are written in a high-level language. Programmers, however, still use assembly

language when speed is essential or when they need to perform an operation that isn't possible

in a high-level language. Assembly language programs need to be translated to machine code

before they can be executed (run) by the CPU. An assembler (translator program) is used to

translate assembly language programs into machine language

Third Generation (High-Level Language)

Machine code and assembly language are called low-level programming languages because

each instruction corresponds to one CPU action. High-level languages, on the other hand, allow

programmers to write powerful instruction that may generate many CPU actions. In addition,

high-level languages enable a programmer to write programs that are more or less independent

of a particular type of computer. Thus, high-level languages are called machine-independent.

Hardware

Machine code

Assembly Language

High-Level Language

Fourth Generation

Figure 1.1


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The main advantage of high-level languages over low-level languages is that they are easier to

read, write, and maintain. Programs written in a high-level language must be translated into

machine language by a compiler or interpreter.

Fourth Generation (Object-Oriented, Query Language)

Fourth-generation languages, often-abbreviated 4GL, are programming languages closer to

human languages than typical high-level programming languages. Most 4GLs are used to access

databases.

Program Development Life Cycle

The phases a software product goes through

between when it is conceived and when it is

no longer available for use is called the

software life cycle. The software life cycle

typically includes the following stages:

Requirements Analysis, Design, Coding,

Testing (validation) and Debugging,

Implementation, Operational Maintenance,

and Retirement. The development process

tends to run iteratively through these phases

rather than linearly as shown in figure 1.2

below. Although all the stages given in

figure 1.2 are important, only those stages

that are very critical for developing assembly

language programs are briefly described

below.

Design Stage

The design stage involves expressing the

solution to the problem in a standard format.

There are several design tools (pseudo-

languages) that can be used by programmer. The most common design tool used by assembly

language programmer is flowchart. While, pseudocode is a common design tool used by third

and fourth generation programmers. These common design tools are briefly described below:

Flowchart

Requirement Analysis

Design

Coding

Testing

Logical

Errors?

Implementation

Maintenance

Retirement

No

Yes

Figure 1.2


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Flowchart is a graphical representation of the program logic. It is one of the oldest methods of

program design. Flowchart uses graphical shapes to represents different actions that the

computer will perform. Arrows that indicate the flow of control connect these shapes. Figure

1.3 shows the common flowchart symbols.

Pseudocode

Pseudocode is a notation resembling a programming language but not intended for actual

compilation. It usually combines some of the structure of a programming language with an

informal natural-language description of the computations to be carried out. It is often

produced by CASE systems as a basis for later hand coding.

Coding

A written computer instruction is called a code and the process of writing codes is called coding.

The code written by programmers is called source code. The programmer then uses a compiler

to compile the source code to produce what is called an object code. The object code is then

converted to a program that is ready to run which is called executable code or machine code.

Debugging

A bug is an error or defect in software or hardware that causes a program to malfunction. The

process of detecting and removing bugs is called debugging. This process is an iterative process

that is carried out every time an error is detected by the testing stage. Thus, the debugging

stage may involve going back through the analysis, design, coding, and testing stages.

Program Constructs

A program can be written using only few constructs (building blocks). These constructs are:

1. Sequence

2. Selection

3. Iteration

PROCESS

PROCEDUR

E

DECISIO

N

INPUT

/ OUTPUT

TERMINATION CONNECTOR

OFF-PAGE

CONNECTOR

DIRECTION OF

CONTROL FLOW

START/END

Figure 1.3


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Sequence

The simplest construct is the sequence. In a sequence, all the statements are executed one

after another in the same order they are found in the program. The diagram below shows a

sequence in flowchart and pseudocode

STATEMENT 1

STATEMENT 2

a) Flowchart

STATEMENT 1

STATEMENT 2

b) PseudocodeSalary = gross-

deduction

deduction = epf

+income-tax

deduction = epf

+income-tax

Salary = gross-

deduction

Selection

There are two constructs that perform selection:

The IF construct.

The CASE construct

The IF Construct

This construct has two forms: IF THEN ELSE and IF

THEN . The condition is evaluated. If the condition evaluates to

logical True then the is executed and the is skipped. If the

condition evaluates to logical False then the is executed and the

is skipped. The diagram below shows an example.


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IF Conditon THEN

True Action

ELSE

False Action

ENDIF

IF Conditon THEN

True Action

ENDIF

IF.THEN.ELSE

True Action

Decision

True?

Yes

No

True Action False Action

Decision

True?

NoYes

IF.THEN

a) Flowchart

b) Pseudocode

The Case Construct

The case construct can be thought of as a group of nested if constructs. In this construct,

several conditions are evaluated. If condition1 evaluates to true then action1 is executed and all

other actions are skipped. If, however, condition2 is true then action2 is executed and all other

actions are skipped. This process continues until all conditions are evaluated. If no condition

evaluates to true then the false action (if there is one) is executed. Like the IF construct, only

one action or no action in the case construct will be executed.

a) Flowchart

Action 1

Select Case Expression

Case 1 Case 2 Case n

..Action 2 Action n

Select Case

Case 1

Do action 1

Case 2

Do action 2

..

Case N

Do action N

[Otherwise

Do falseaction]

Endcase

c) Pseudocode


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Iteration

There are basically two types of Iterations: the While Loop and the Repeat until loop

The While Loop

A condition is evaluated at the beginning of the loop. If the condition is true, all the statements

in the loop are executed and the condition is evaluated again. These processes continue until

the condition evaluates to false.

THE REPEAT-UNTIL LOOP

All the statements in this loop are executed before the condition placed at the end of the loop is

evaluated. If it evaluates to true the loop is terminated. However, if it evaluates to false, the

statements are executed again until the condition is true.

Is Condition

True?

Body of the Loop

No

Yes

Is Condition

True?

Body of the Loop

No

Yes

Figure


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Example.

The program below reads a set of numbers and counts how many odd and even numbers exist

in the set. As it can be seen, the flowchart is made of a single while loop that checks if there

are more numbers in the set. If yes, then a number is read from the set. Within the While

construct there is an IF construct. If the number is odd the odd number counter is incremented

by 1 else the even number counter is incremented by 1. This process continues until no more

numbers in the set. A procedure displays the results before the program is terminated.

Self-Test Questions

1) Computers, in general, are divided into: a) Hardware and programs b) Hardware and software

2) A programming language has: a) Keywords b) Syntax c) All of the above

3) Programming languages are classified into: a) 3 generations b) 4 generations c) 5 generations d) None of the above

Start

More number

?

Increment Odd Number Counter

Print Result

End Is Number odd?

Increment Even Number Counter

Read a Number

No

No

Yes

Yes


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4) The common design tools are: a) Flowchart and coding b) Pseudocode and chart c) Pseudocode and flowchart

5) The common programme constructs are: a) Sequence, IF, and Loop b) If, While loop, and Repeat loop c) Sequence, Selection, and Iteration d) None of the above

6) In a While loop, the condition that control the execution of the loop is placed at: a) The beginning of the loop b) The end of the loop c) Anywhere in the loop

Review Questions

1) Define Software

2) Define Program

3) Define Programming Languages

4) Describe 2nd generation programming languages

5) List the stages in the programme development life cycle

6) Describe the flowchart symbols given in figure 3.3

7) Draw a flowchart for the program whose pseudocode is given below

Input age

If age > 21 then

Display "You are eligible to vote"

Else

Display " Sorry! You are under-age"

Endif

8) Draw a flowchart for a program that displays the odd number from 1 to 9

9) A program that perform the following tasks:

a) Accept a long sentence

b) Convert it to upper case

c) Display the result on the screen

You are required to:

Design this program using flowchart and pseudocode


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INTRODUCTION TO ASSEMBLY LANGUAGE PROGRAMMING

Outcomes

1. Explain the basic structure of an assembly language programme

2. List and explain the tools required for assembly language programming

3. Describe the steps in creating assembly language programmes

Fundamentals

The Structure of an Assembly Language Program

An assembly program contains:

1. Statements or instructions that the programmer wants them executed by the

microprocessor. Each statement may have up to 4 fields

2. Directives to the Assembler (translator program) that informs the assembler how to

assemble (translate) the program

3. Remarks that explains the working of the program to other programmers. These remarks

are optional and will be ignored by the assembler.

Assembly Language Programs Statements

An assembly language statement may have up to 4 fields as shown in example 2.1. The first

field (new:) is the label field. It is used for flow control of the program. A colon (:) must

terminate the label field. This field is not required unless the programmer wants to refer to that

line somewhere in the program. The next field is the operation code (Add), which must be a

valid operation code for the given microprocessor. The operation codes are given by the

microprocessor manufacturer and are called the instructions set. The operation code tells the

microprocessor what action to perform. This field is required unless that line of the program is a

remark line.

The third field is the operand field (AX,5H). This contains the data on which the operation will

be performed. The type and the number of operands depend on the operation code, as some

may required no operand, one, or two operands. If more than one operand is required they


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must separated by commas. The last field is the remark field (; Add percentage). This may

contains anything as it is intended for the programmer and not the computer. The objectives of

the remark field are to aid the readability of the program; help the programmer understands

the program and thus improve maintainability. The comment field starts with a semicolon (;).

This field is not compulsory but recommended.

Example 2.1

New: Add AX,5H ; Add percentage

Mov dx,ax

Jmp new

Field

No

Contents Comment

1 Label field Used for reference in program execution control

Must be a single word

Must be terminated by a colon (: )

Optional i.e. not compulsory

2 Operation

Code

A mnemonic taken from the microprocessor

instructions set

Compulsory except for empty lines

3 Operand Depends on the operation code

Not all operation code requires an operand

Can be a constant or a variable

4 Comment Must start with a semicolon (;)

Used to explain the statement line

Optional


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Assembler Directives

Assembler directives are pseudo-operations that control the assembly process but they do not

generates any instructions in the assembled program. They merely instruct the assembler on

how to process the assembly programs and as such they are specific to each particular

assembler. Some common uses of assembler directives are: -

To declare variables (reserve memory space to store data),

To define segment, and

To define procedures (subroutines)

Assembly Language Programming Tools

To be able to effectively develop assembly programs, the following tools are needed:

Text Editor

A text editor is a program that allows you to create a file containing the assembly programs

(source code). Some of the operations that you can perform with a text: -

1. Type the assembly program

2. Correct any typing error in the program

3. Insert new statements or delete unwanted ones

4. Save the program on magnetic medium for later uses

; Exp2_1.asm ; display a message on the screen .MODEL SMALL .STACK 200h .DATA Prompt db 13,10,"This is the message $" .CODE .startup mov ax, seg prompt ;moves the segment of data into ax mov ds,ax mov ah,9 mov dx,offset Prompt int 21h ;print the message .exit end

Figure 2.1 An Example of an Assembly Language Program


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5. Rearrange the order of the statements by moving them from one place in the program to

another

6. Copy part of the program and insert it in another part of the program or in another program

7. Some text editors may have more advanced facilities like find and replace, etc.

Assembler

The assembler is a program that translates assembly language statements into machine code

instructions that can be executed by the microprocessor. The assembler takes the source code

as input and will produces an object file containing the machine code instructions if the

assembler did not detect any syntax error during the assembly process. Other optional files may

be generated depending on the assembler in use. If the assembler detects any error during the

assembly process only a list file containing the assembly instructions, the error code, and an

error message for each error encountered will be produced.

Hand written

Assembly

Program

Manual

Input

Text

Editor

Source

Code

Source

Code

Figure 2.2


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LINKER

A linker is a program that links several object files into an executable file (a program) that can

be executed. The output of the linker is an executable file if no error encountered during the

linking process. If any error encountered the linking process is halted and an error message

describing the error is displayed

Creating Assembly Programs

To create an assembly program, follow the steps below:

1. Design your program using any of the design tools mentioned earlier

2. Use a text editor or a word processor that can produce text file to type the assembly

language programs

3. Save the program (as a DOS text file) on a disk

4. Assemble the program to check for syntax errors

Linker Object Code

Figure 2.4

Executable Code

Library Files

Assembler Source Code

Figure 2.3

Syntax Errors?

Source code Listing + error messages

Object Code

Source code Listing


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5. If the program assembles properly, Link the program to produce an executable file

6. Run the executable file

Self-Test Questions

1) An assembly program may contain

a) Assembly language instructions

b) Assembler directives

c) Comments

d) All the above

2) An assembly language statement may have up to ______ fields

3) Assembler directives can be used to:

a) Declare variables

b) Define segments

c) Define procedures

d) All the above

4) The input file to an assembler is called

a) Source code

b) Object code

c) Executable code

Review Questions

1) Describe the structure of an assembly language program

2) Describe the structure of an assembly language statement


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ARCHITECTURE OF THE 80386 MICROPROCESSORS

Outcomes

1. Describe the RISC and CISC microprocessor architectures

2. Highlight the major events in the history of microprocessor

3. Give an overview of the software view of the 80386 microprocessor

4. Describe the functions of the 80386 registers

5. Describe the usage of the 80386 flags

The Architecture of a Microprocessor

A central processing unit (CPU) on a single integrated circuit is often called a microprocessor.

The CPU is divided into several smaller units such as the Arithmetic and Logic Unit (ALU), the

decoding unit, etc. Each of these units is designed to perform specific function such as

arithmetic and logic operations performed by the (ALU).

Architecture

The architecture of a microprocessor refers to its internal design. Microprocessors can be

classified according to their architecture as either Reduced Instruction Set Computing (RISC) or

Complex Instruction Set Computing (CISC). However, microprocessors can also be classified

according to the type of applications they are used for to two main categories general-purpose

and special-purpose. General-purpose microprocessors are used in computers while special-

purpose ones are used in embedded systems.

Reduced Instruction Set Computing

A processor whose design is based on the rapid execution of a sequence of simple instructions

rather than on the provision of a large variety of complex instructions is called a RISC processor

Features that are generally found in RISC designs are:

Uniform instruction encoding which allows faster decoding

A homogenous register set, allowing any register to be used in any context and simplifying

compiler design

Simple addressing modes with more complex modes replaced by sequences of simple

arithmetic instructions.

Examples of RISC processors are the Berkeley RISC, HP-PA, Clipper, i960, AMD 29000, RISC

System 6000 and SP/2 lines


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Complex Instruction Set Computing

A processor where each instruction can perform several low-level operations such as memory

access, arithmetic operations or address calculations is called a CISC processor. The aim of this

design is to bridge the "semantic gap". Thus, to design instruction sets to support high-level

languages by providing "high-level" instructions such as procedure call and return, loop

instructions such as "decrement and branch if non-zero" and complex addressing modes to

allow data structure and array accesses to be compiled into single instructions. While these

architectures achieved their aim of allowing high-level language constructs to be expressed in

fewer instructions, it was observed that they did not always result in improved performance.

Examples of CISC processors are the Motorola 680x0 family and the Intel 80186 through Intel

486 and Pentium.

Embedded Systems

An embedded system is a system in which the microprocessor performs a dedicated task. A

typical embedded system consists of a single-board microcomputer with software in ROM,

which starts running some special purpose application program as soon as it is turned on and

will not stop until it is turned off. An embedded system may include some kind of operating

system but often it will be simple enough to be written as a single program. It will not usually

have any of the normal peripherals such as a keyboard, monitor, serial connections, mass

storage, etc. or any kind of user interface software unless these are required by the overall

system of which it is a part. Often it must provide real-time response.

Embedded systems can be classified as either event controller or data controller. Event

controller as often called Microcontroller such as the Intel 8051 while data controller are called

embedded microprocessor such as the 80386EX. The control of an air condition unit is an

example of event control. The Microcontroller is used primarily to detect the temperature and to

switch the air condition unit on and off. The hard-disk controller however, is an example of data

control because the primary function of the microprocessor is to transfer as fast as possible

data between the hard disk and the computer memory.

The History of Microprocessors

Microprocessors have evolved very fast during the introduction of the first microprocessor in

1971 by Intel. A brief history of the evolution of the microprocessor is given in the table below

Year Microprocessor Family Remarks

1971 Intel 4004 and 4040 4-bit address up 4k*4 memory has only 45 instructions speed of 50 KIPs (Kilo Instructions per

second)


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1972 Intel 8008 8-bit address up 16k*8 memory has only 48 instructions can manipulate data and characters

1974 Intel 8080 Motorola MC6800, Zilog Z80

8-bit address up 64k*8 memory has only 48 instructions 10 times faster than the 8008

1977 Intel 8085 8-bit An updated version and slightly faster than the 8080

has 246 instructions

1978 Intel 8086, Motorola 68000

16-bit address up 1M*8 memory has 20,000 instructions

speed of 2.5MIPs (Million Instructions per second)

has an instruction cache or queue

1983 Intel 80286 16-bit address up 16M*8 memory has a few additional instructions than

the 8086 speed of 4.0 MIPs (Million Instructions

per second)

1986 Intel 80386, Motorola 68020

32-bit address up 4G*8 memory

clock speed of 16 to 33 MHz

1989 Intel 80486 32-bit address up to 4G*8 memory clock speed of 25 to 100 MHz has an 8K internal cache speed of up to 50 MIPs

1993 Intel Pentium, PowerPC

64-bit address up to 4G*8 memory clock speed of 60 to 166 MHz has an 16K internal cache speed of up to 110 MIPs

1997 Intel Pentium II 64-bit address up to 64 Gigabytes Clock speed from 233 to 450 MHz Has two levels caches 512K level 2 and

32K level 1

Bus speed of 66 and 100 MHz

1999 Intel Pentium III 64-bit address up to 64 Gigabytes Clock speed from 450 to 550 MHz Has two levels caches 512K level 2 and

32K level 1 Bus speed of 100 MHz 70 new instructions

The Software View of the 80386 Microprocessor

The 80386 Microprocessor Family

The 80386 family contains several microprocessors to satisfy the needs of different types of

users. The most common family members are the 80386DX and the 80386SX. Table 3.2 below

gives a brief description of some of the features of both DX and SX versions of the 80386. Both


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the SX and the DX versions of the 80386 can operate in either real mode or protected mode. In

the real mode the 80386 microprocessor operates like a powerful 80286 (16-bit microprocessor)

and thus can address only 1 MB of memory. In the protected mode the 80386 can access the

entire memory space. In this book only the real mode will be discussed.

Real Mode Model

To enable a programmer to write assembly programs for the 80386 family of microprocessors,

a software model of the internal architecture of the microprocessor is needed. Figure 2.1 shows

software view of the real mode model of the 80386 microprocessor. In this model, it can be

seen that all the registers that are available in the 80286 are available in the 80386 real mode

for upward compatibility.

Registers

The 80386 microprocessor has a large number of registers as can be seen in figure 3.1. We can

broadly classify these into two groups: General-purpose registers, and Special purpose registers

GENERAL PURPOSE REGISTERS

The general-purpose registers can also be divided into two groups: Data registers, and index

and pointers registers.

80386 DX 80386 SX

32-bit data bus

32-bit address bus

132 pin package

can address up to 4 Gigabyte

32 bit data transfer

16-bit data bus

24-bit address bus

100 pin package

can address up to 16 megabyte

16 bit data transfer

Table 3. 2


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CS

DS

SS

ES

FS

GS

0 15 31

Segment

Register

EAX

AX

AL AH

BL BH

CL CH

DL DH

AX

BX

CX

DX

EAX

EBX

ECX

EDX

SP ESP

BP EBP

SI ESI

DI EDI

IP

Flags

CR0

Figure 3.1


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Data Registers

There are 4 data registers that can be used to store temporary data during execution of

program instructions. They can be used as destination or source for any data movement

instruction. These register can be used as 8-bit (for example AL or AH) so that programs

written on Intel 8-bit microprocessor can be executed on the 80386 in real mode. They can also

be used as 16-bit (for example AX) to allow 80286 codes to run by the 80386. These registers

also allow 32-bit operations (for example EAX).

Although, these registers are called general purpose, there are certain operations that only

possible on a given register. Table 3.2 below lists the 4 general-purpose data registers and the

operations specific to each of them.

REGISTER

NAME

ABBREVIATION USAGE

8 8 16 32

Accumulator AL AH AX EAX Arithmetic operations like addition, subtraction,

division and multiplication, data conversion

between byte, word, double word, and

quadruple word. Input/output operations and

memory addressing

Base Index BL BH BX EBX Memory addressing

Count CL CH CX ECX Looping and repeating string operations,

reading and writing data to disk files

Data DL DH DX EDX Input/output operations with external devices,

multiplication and division operations (in

conjunction with accumulator), and string

operations

Table 3.2

Index and Pointer Registers

The 4 registers in this group can be used either as 16-bit or 32-bit registers. These registers are

used to hold the offset of the address of the data held in memory. The other part of the


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address, the segment address, is normally held by a segment registers. Table 3.3 list the name

and abbreviation of these registers.

Register Name Abbreviation Usage

16 32

Base Pointer BP EBP Used to hold the offset address of the stack

as well as the memory

Destination

Index

DI EDI Used to hold the offset address of a

destination operand

Source Index SI ESI Used to hold the offset address of a source

operand

Stack Pointer SP ESP Used to hold the offset address of the stack

Table 3.3

SPECIAL PURPOSE REGISTERS

The 80386 microprocessor has 4 types of special purpose registers. These are the Segment

registers, the Flags register, Instruction Pointer, and Control registers.

The Segment Registers

The six segment registers are 16 bits wide. Each segment register holds the base memory

address for a 64K-memory segment. Table 3.4 below briefly describes the usage of the

segment registers.


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REGISTER NAME ABBREVIATION USAGE

Stack Segment SS Holds the Base address of the stack

memory

Code Segment CS Holds the Base address of the code

memory

Data Segment DS Holds the Base address of the data

memory

Extra Segment ES Holds the Base address of the extra

memory

Additional Segments GS and FS Hold the Base address for two

additional segments of memory

Table 3.4

The Flags

The 80386 microprocessor has a 32-bit flag register (EFLAGS). However, in real-mode only the

first nine bits are used. These are the same as those for the 80286 microprocessor. The first six

are called status flags because their states (set or reset) change as a result of executing certain

instructions. The last three flags are control flags used to control the operations of the 80386

microprocessor. The name and usage of these status flags are summarised in table 3.5 below

NAME ABBREVIATION USAGE

Carry C This flag is set (1) if there is a carryout or a borrow-

from the most significant bit of an arithmetic

instruction such as Add. Otherwise, it is reset (0)

Parity P If the result of an instruction has an even number of

1's (even Parity) then this flag is set. Otherwise, it is

reset

Auxiliary

Carry

A This flag is set if there is a carryout from the low

nibble to the high nibble or a borrow-from the high

nibble to the low nibble during BCD addition and

subtraction. Otherwise, it is reset

Zero Z This flag is set if the result of an instruction is 0


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otherwise, it is reset

Sign S This flag is set if the result of logic or an arithmetic

operation is negative. Otherwise, it is reset.

Overflow O This flag is set when an overflow occurs during signed

number arithmetic

Trap T Used to enable trapping errors during program

debugging. If this flag is set, the single-step mode of

the 80386 is enabled

Interrupt I If set to 1 interrupt is enabled otherwise interrupt is

disabled

Direction D If set to 1 the DI and SI registers are automatically

decremented otherwise they are automatically

incremented

Table 3.5

The Instruction Pointer

The instruction pointer (IP) is a 16-bit register. This register is used with the code segment

register (CS) to hold the address of the next instruction to be fetched from the code segment of

the memory.

Control Register

Although, the 80386 microprocessor has several control registers, only one of them control

register zero is active in real-mode. The five least-significant digits are called the machine

status word (MSW). In real-mode, only bit 0 of the control register zero is active. This bit is

called the protection enable and is used to switch between real-mode and protected-mode.

Self-Test Questions

1. Microprocessors are classified according to their architecture as:

a. General-purpose, and special-purpose

b. Complex instruction set computing, and reduced instruction set computing

2. The 80386EX is an example of event controller?

a. True

b. False


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3. RISC microprocessors decode instructions faster than CISC microprocessor of the same

class?

a. True

b. False

4) The registers of the 80386 microprocessor can be classified into:

a) General-purpose and special-purpose

b) General-purpose, data, index and pointer registers

c) Special-purpose, data, index and pointer registers

d) None of the above

5) The number of data registers in the 80386DX microprocessor is?

a) 6

b) 4

c) 8

6) The data registers in the 80386 microprocessor allows:

a) 8-bit access

b) 16-bit access

c) 32-bit access

d) All the above

7) Which of the following operations can be performed on the accumulator?

a) Arithmetic operation

b) Input/output operation

c) Data conversion

d) All the above

8) The number of index registers in the 80386DX microprocessor is?

a) 6

b) 4

c) 2


9) Which two of the index and pointer registers can be used to hold the offset for the stack

segment?

a) SP and SI

b) BP and DI

c) SP and BP

d) All the above

10) The special-purpose registers of the 80386 microprocessor are:


26

a) Segment registers, Instruction pointer, and Control register

b) Segment registers, Flags register, Instruction pointer, and Control register

c) Segment registers and Flags register

Review Questions

1. Contrast the RISC and CISC microprocessor architectures

2. Highlight the main features of Intel 32-bits microprocessors

3. List the 4 data registers and briefly explain their usage

4. List the 4 index and pointer registers and briefly explain their usage

5. List the 6 segment registers and briefly explain their usage

6. Explain the usage of the Carry flag


27

DATA STORAGE AND MOVEMENT

Objectives

1. Show how numeric data are represented in computers

2. Show how alphanumeric data (Characters) are represented in computers

Data Representation within a Computer

Data can be classified into numeric and alphanumeric. The decimal digits from 0 to 9 are called

numeric data because arithmetic operations can be performed on them. Alphanumeric data, on

the other hand, comprises alphabets, numbers, and symbols.

Alphanumeric Data

The most common way to represent alphanumeric data is the ASCII code. The 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.

In most microcomputer, placing a 1 in the most significant bit uses an extended ASCII

character set. The extended ASCII is used to represent foreign letters, Greek and mathematical

symbols as well as box drawing and other special characters. Table 4.1 below shows the ASCII

code. Notice that the codes from 0H to 1FH (Grey background) are called control codes and are

used to control the operation of the computer. The control codes are non-printable. Only 4 of

the control codes are shown here.


28

MSB 0 1 2 3 4 5 6 7

LSB 0 SP 0 @ P ' P

1 ! 1 A Q a q

2 " 2 B R b r

3 # 3 C S c s

4 $ 4 D T d t

5 % 5 E U e u

6 & 6 F V f v

7 BEL ' 7 G W g w

8 ( 8 H X h x

9 ) 9 I Y i y

A LF * : J Z j z

B ESC + ; K [ k

C , < L \ l |

D CR _ = M ] m

E . > N ^ n ~

F / ? O - o DEL

Table 4.1

Numeric Data

Numeric data can be classified as either integer or real. Integer numbers are whole number

with no decimal point, while real numbers have decimal point. The 80386 can only perform

arithmetic operations on integers and therefore, only the representation of integers in computer

will be discussed here.

Integers can be represented in the computer as pure binary digits or as a binary coded decimal

(BCD).


29

BCD Method

BCD is an acronym for Binary Coded Decimal. In BCD a group of 4 bits are used to represent

each decimal digit from 0 to 9. Data in BCD can be stored either as packed BCD or as unpacked

BCD. In packed BCD, each nibble of a byte represents one decimal digit. Thus, we can store

two digits per byte of information. While, in unpacked BCD, the upper nibble is set to 0 and the

lower nibble is used to store the decimal digit. Thus, we can only store one digit per byte of

information. Table 4.2 below shows how integers are stored as packed and unpacked BCD.

DECIMAL PACKED UNPACKED

25 0010 0101 0000 0010 0000 0101

427 0000 0100 0010 0111 0000 0100 0000 0010 0000 0111

Table 4.2 Storage of decimal numbers as Packed and unpacked BCD

Integer Representation In Binary Format

Integers, signed and unsigned can be represented as a single byte, single word (2 bytes) or as

a double word (4 bytes).

Byte-Sized Data

An integer can be stored as a single byte (8-bits). The range for unsigned integer is from 0 to

28-1 i.e. from 0 to 255. While the range for the signed integer is from -27 to 27-1 i.e. from -128

to 127. Signed integers are stored in 2's complement, thus, the most significant bit is 1 for

negative numbers and 0 for positive numbers. Figure 4.2 below shows how signed and

unsigned integers are stored as a single byte.


30

Word-Sized Data

An integer can be stored as a single word (16-bits). The range for unsigned integer is from 0 to

216-1 i.e. from 0 to 65535. While the range for the signed integer is from -215 to 215-1 i.e.

from -32786 to 32785. Figure 4.3 below shows how signed and unsigned integers are stored

as a single word.

1 2 4 8 16 32 64 128 256 32768

0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Binary Weights

Figure 4.3 a) Storage of an unsigned integer in a 16-bit (word) format

Figure 4.3 b) Storage of a signed integer in a 16-bit (word) format

1 2 4 8 16 32 64 128 256 -32768

Binary Weights

Signed byte

1 2 4 8 16 32 64 128 1 2 4 8 16 32 64 -128 Binary Weights

Unsigned byte

Binary Weights

1 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1

0 0 0 0 0 0 0 0

0 1 1 1 1 1 1 1

Smallest

Largest

Figure 4.2 Single byte storage of signed and unsigned integers


31

Double-Word Sized Data

An integer can be stored as a double word (32-bits). ). The range for unsigned integer is from 0

to 232-1 i.e. from 0 to 4294967295. While the range for the signed integer is from -231 to 231-

1 i.e. from -2147483648 to 2147483647.

Data Storage in Memory

The Intel family of microprocessors uses the little Endian technique when storing data in

memory. This technique stores the least significant byte of data in the low memory address and

the most significant byte in the high memory address. Figure 4.4 below shows how the data

FA3BH is stored in memory starting at address 0000EH.

Address Content

Binary

Content

Hexadecimal

00010H

0000FH 1111 1010 FA

0000EH 0011 1011 3B

0000DH

Data Movement

The move (MOV) instruction will be used to illustrate the addressing modes available in the

real-mode programming of the 80386 family of microprocessors. This Instruction

can be used to move, actually copy, data between registers and/ or memory. The general

syntax is:

Mov destination, source

The destination can be a register or a memory address where the data will be stored. While the

source can be a register, a memory address, or an immediate value

Figure 4.4


32

Example 4.1

Mov BL, 23 ; Store the number 23 decimal in the BL register

Mov AL, 10010010B ; The binary number 10010010 is stored in the AL register

mov cl, 0F3H ; The hexadecimal number F3 is stored in CL register

Mov AX, BX ; The content of the BX register is copied into the AX register

Mov DX, 7234O ; The Octal number 7234 is stored in the DX register

mov BX, 'AB'; The ASCII code for the letters A and B are stored in the BX Register

mov BX, AB

Self- Test Questions

1) What is the ASCII code for the 'Intel 80386'?

a) 496E74656C203830333836H

b) 696E74656C203830333836H

c) 696E74656C3830333836H

2) What is the alphanumeric data represented by the ASCII code

696E74656C203830343836H

3) What is the Decimal equivalence of the packed BCD byte 10010111?

a) 79

b) 97

c) 151

4) What is the Decimal equivalence of the signed integer 10001000?

a) 136

b) -120

5) The largest unsigned integer that can be stored a single byte is?

a) 256

b) 255

c) 127

Review Questions

I. What is the largest memory that can be accessed by the 80386 microprocessor in real

mode?

II. List the 4 data registers and briefly explain their usage

III. List the 4 index and pointer registers and briefly explain their usage

IV. List the 6 segment registers and briefly explain their usage

V. Explain the usage of the Carry flag

VI. Contrast the flat and segmented addressing techniques


33

VII. Explain the 80386 real-mode memory addressing scheme

VIII. Show how the data 'Intel 80386' is stored in memory starting at address 00FC0H

IX. Show how the signed integer -512 is stored as a word in memory starting at address

00FFFH

X. Show how the unsigned integer 234 is stored in memory starting at address 0F000H

A. As unpacked BCD

B. As packed BCD

C. As binary


34

ADDRESSING MODES

Objectives

1. Explain flat and segment addressing techniques

2. Explain the 80386 real mode addressing schemes

Memory Addressing

Memory can be addressed in two ways:

1. Segment Addressing as used by Intel

2. Flat Addressing as used by other microprocessor manufacturers

Segment Addressing

Segment addressing scheme used on the Intel 8086 and later Intel microprocessors where all

memory references are formed by adding a 16 bit offset to a 16 bit base address held in a

segment base registers. Each instruction has a default segment, which determines which

segment register is used. Special prefix instructions allow this default to be overridden.

The effect is to segment memory into blocks. Blocks may overlap either partially or completely,

depending on the contents of the segment registers but normally they would be distinct to give

access to the maximum total range of addresses.

Flat Addressing

The memory architecture in which any memory location can be selected from a single

contiguous block by a single integer offset is called flat addressing. A flat address space greatly

simplifies programming because of the simple correspondence between addresses (pointers)

and integers.

80386 Real Mode Memory Addressing

In real mode, the 80386 can address up to 1M bytes of memory. Thus, the address space in

real mode ranges from 00000H to FFFFFH. The memory is divided into two sections: a

dedicated-use section and a general-use section. The dedicated-use section occupies the


35

address space from 00000H to 003FFH and is reserved for the storage of the microprocessor

interrupt-table and cannot be used for the storage of program or data. While, the general-use

section uses address range from 00400H to FFFFFH and is used for the storage of program and

data.

As mentioned earlier, the Intel microprocessor use segmented addressing technique. Thus, The

general-use section of memory is divided into blocks called segment. Each segment is 64K-byte.

The segments can be contiguous, adjacent, disjointed or even overlapping. As the 386

microprocessor has 6 segment registers, only 6 segments can be active at any given time.

Thus, the total memory that can be active at any given time is 6 x 64 for a total of 384K-byte.

To address a memory location, a logical address consisting of a segment address and an offset

(displacement) address is specified. The actual address (Physical address) is obtained by

appending 0H to the segment address and adding it to the offset address. If the offset address

generated by combining the contents of two registers is bigger than FFFFH the extra bit is

ignored. A memory address can only begin at a 16-byte boundary in the memory system. This

16- byte address is sometimes called a paragraph. Table 5.1 shows few examples of logical and

physical addresses.

Segment Offset Logical Address Physical Address

1000H 1234H 1000:1234H 10000H +1234H =11234H

23FBH FAB3H 23FB:FAB3H 23FB0H + FAB3H = 33A63H

0FF1H 1324H 0FF1:1324H 0FF10H + 1324H = 11234H

Table 5.1

Segments and their Default Offset Registers

As mentioned earlier, the logical address is formed from a segment address held in a segment

register and an offset address. The offset address is obtained from one of the general purpose

or special purpose registers or from a combination of both depending on the type of access to


36

be performed by the microprocessor. Each segment register has a default offset register as

shown in table 5.2 below. This default segment-offset relationship can be overridden for certain

operations by specifying the name of the segment to be used in front of the offset register.

Type of operation Offset Default

Segment

Comment

Instruction EIP CS Cannot be changed

Stack ESP, EBP SS

Data and String Source EAX, EBX, EDX, ESI,

EDI

DS Can use any other

data segment

Destination String EDI ES Cannot be changed

Table 5.2

Addressing Modes

The Intel 80386 family of microprocessors supports several addressing techniques in real-mode

programming. The following addressing modes can be used in the real-mode:

Register addressing

Immediate addressing

Direct addressing

Register indirect addressing

Base plus index addressing

Register relative addressing

Base relative plus index addressing

Scaled index addressing

Register Addressing

The register-addressing mode is very simple. It allows data to be moved between the 80386

registers. In this mode:


37

1. The source and destination of the data is an 80386 register such as AH, AL, BH, BL, CH, CL,

DH and DL or their 16-bit counterparts i.e. AX, BX, CX, DX, SP, BP, SI and DI. In addition,

the 32 bit registers EAX, EBX, ECX, EDX, ESP, EBP, ESI, EDI, and EDI can also be used.

2. Never mix registers of different sizes. For ex: MOV AX, BL is not allowed because BL is an 8-

bit register while AX is a 16-bit register

3. Segment to segment register is not allowed. For ex: MOV CS, DS is not allowed

4. The code segment register (CS) cannot be used as a destination register

Example 5.1

Mov AX, CX ; the content of CX register is copied into the AX register

Mov CS, AX ; Invalid the code segment register cannot be a destination

Mov DS, AX ; the content of AX register is copied into the DS register

Immediate Addressing

The immediate addressing mode allows a constant value to be loaded into a register or to be

stored in memory. The constant value is included in the instruction immediately after the

destination. In this mode:

1. The source data is included in the instruction.

2. The data is constant (cannot change between program runs)

3. The data can be in DECIMAL, BINARY, HEXADECIMAL, or CHARACTER

4. The destination can be a register or a memory location

Example 5.2

Mov AX, 3 ; the decimal value 3 is stored into the AX register

Mov List, 0FFh; the hexadecimal value FF is stored into the memory location given by the

label list

Mov CH, 11001001b ; the binary value 11001001 is stored into the CH register

Mov AX, 'a' ; the ASCII code of the letter a is stored in the AX register

Direct Addressing

There are two basic forms of direct addressing:


38

1. Direct Addressing Between Memory And Accumulator

a) Transfers data from a memory location in the data segment and the accumulator

(AL or AX or EAX)

b) The address of the memory location can be specified by a variable (symbolic

memory location) or can be given as direct memory address in some assembler

2. Displacement Addressing

a) Transfers data from a memory location in the data segment and any of the

general-purpose registers including the accumulator.

b) The address of the memory location can be specified by a variable (symbolic

memory location) or can be given as direct memory address in some assembler

The two forms are identical except for the length of the generated machine code instruction.

Example 5.3

Mov AX, list ; A word is copied from the memory location list to the AX register

Mov name, BX ; the content of the BX register (a word) is copied to the memory starting the

memory address given by the name label

It is important to note that both labels (list and name) must have been declared in the data

segment before being used.

Figure 5.2 below shows the AX register and the memory content at address list a) before and b)

after the Mov AX, list instruction is executed.

xxxx AX 7AF3H

F3 List

Figure 5.2

a) b)

7A

Memory

F3

7A


39

Register Indirect Addressing

The direct addressing method discussed earlier is good if you want to read or write a single

value to the memory. However, when several values (a vector or a column) is to accessed the

register indirect addressing is more appropriate than the direct method. This is achieved by

loading the offset of the vector or column (a single dimension array) into one of the base

registers and then incrementing it in a loop to access the other elements. Any of the following

base registers: BX, BP, DI, and SI can be used. The BX, DI, and SI registers use the data

segment by default. While the BP register uses the stack segment by default

Example 5.5

Mov AX,[BX] ; The word whose address in the BX register is copied into AX

Mov list, byte ptr[SI] ; copy the byte addressed by the content of SI into the memory

location addressed by list.

Figure 5.3 shows the content of memory and registers a) before and b) after the Mov AX,

[BX] instruction is executed. Assuming DS = F000 then physical address = F0000+F001 =

FF001

Base-Plus-Index Addressing

The Base plus index addressing method is another indirect addressing method that allows

access to two-dimensional arrays (table). In this method, two registers are required: a base

register such as BX and BP (SP cannot be used) and an index register such SI and DI. One

register will be used as the base and the other as the offset. The effective address (physical

xxxx AX 7AF3H

F3 FF001

Figure 5.3

a) b)

7A

Memory

F3

7A FF002

F001 BX

DS = F000


40

address) is obtained by adding the Base register and the Index register to the default data

segment register. The offset of the table should be loaded into the base register and then

incrementing it in a loop and to load the offset of the first element into the index register and

then incrementing it in an inner loop to access the other elements.

Example 5.6

Mov AX,[BX+DI] ; The word stored at the memory address DS:BX+DI is copied into the

AX register.

Figure 5.4 shows the contents of memory and registers a) before and b) after the instruction

Mov AX, [BX+DI] is executed assuming DS = F000

Register Relative Addressing

This addressing technique is similar to the base plus index addressing technique described

earlier except that here only one register (an index or a base register) is used compared with

two registers (an index and a base register). This technique can address Data in a segment of

memory by adding the displacement to the content of a base or index register like (BP, BX, DI,

and SI). Thus, it can be used to access data stored in table (array) by:

1. Loading the offset of the table into the displacement (a variable) and

2. Loading the offset of the first element of the table into one of the base registers and then

incrementing it in a loop to access the other elements

xxxx AX 7AF3H

F3 FF0A4

Figure 5.4

a) b)

7A

Memory

F3

7A FF0A5

F001 BX

DS = F000

00A3 DI

F001

00A3


41

Example 5.7

Mov AX, [BX+100] ; copy a word from the memory addressed by DS:BX+100 to the AX

register

Mov CX, list[DI] ; copy a word from the memory addressed by DS:list+DI to the CX

register

Figure 5.5 shows the content of memory and registers a) before and b) after the Mov

CX,list[DI] is executed. Assuming that DS = 1000H and list = F000H. Thus, the physical

memory address of this instruction is DS*10+list+DI = 10000+F000+00A3 = 1F0A3H.

Base Relative plus Index Addressing

In this technique, adding the content of a displacement (offset) to a base register and an index

register forms the physical address. Thus, it can be used to access data stored in table (one or

two dimensions array) by:

1. Loading the offset of the table into the base registers and then incrementing it in a loop and

2. Loading the offset of the first element into the index register and then incrementing it in an

inner loop to access the other elements.

Thus, it can be seen that this technique is almost identical to the base-plus-index addressing

mode except for the addition of the displacement. The addition of the displacement allows the

addressing of 3 dimensional table or arrays.

xxxx CX BA3F

3F 1F0A3

Figure 5.5

a) b)

BA

Memory

3F

BA 1F0A4

DS = 1000 and list = F000

00A3 DI 00A3


42

Example 5.8

Mov AX,list[BX+DI] ; The word stored at the memory address DS:list+BX+DI is copied into

the AX register.

Figure 5.6 shows the contents of memory and registers a) before and b) after the instruction

Mov AX, list[BX+DI] is executed assuming DS = F000 and list = 0100. Thus, the physical

memory addressed by this instruction is DS*10+list+DI+BX = F0000+100+A3+C300 = FC4A3.

Self-Test Questions

1) The 80386 microprocessor uses:

a) Flat addressing techniques

b) Segment addressing techniques

c) All of the above

2) In real-mode, the memory-address range reserved for the interrupt table is?

a) 0000H to 003FFH

b) 00000H to 003FH

c) 00000H to 003FFH

3) What is the total active memory that can be accessed by the 80386DX microprocessor

in real-mode?

a) 256K-byte

b) 384K-byte

c) 1M-byte


4) What is the physical address for the logical address 1234:FFB3?

a) 222F3H

xxxx AX BA3F

3F

Figure 5.6

a) b)

BA

Memory

3F

BA 00A3 DI 00A3

FC4A3

DS = F000 and list = 0100

C300 BX C300

FC4A4


43

b) 22F23H

c) None of the above

5) Which pairs of registers are used to hold the address of instructions in memory?

a) CS:IP

b) CS: BP

c) DS:IP


6) Which addressing mode is used in the instruction Mov Ax, [BX]

a) Direct Addressing

b) Immediate Addressing

c) Register Addressing

d) Register Indirect Addressing

Review Questions

1. List 3 addressing modes and give an example of each

2. If you need to sum the marks obtained by a group of students for the microprocessor

class, which addressing modes are suitable and why?


44

ACCESSING SYSTEM HARDWARE

Outcomes

1. Explain the 3 methods used for accessing the hardware

2. Use the DOS function calls to access the hardware

Fundamentals

You can access the PC system hardware at one of three general levels from assembly

Language.

Programming the hardware directly

Use ROM BIOS routines to access the hardware

Use MS-DOS calls to access the hardware.

Each level of system access has its own set of advantages and disadvantages.

Programming the hardware directly

1) Programming the hardware directly offers two advantages over the other schemes:

Control and

Efficiency.

2) Programming the hardware directly has its drawbacks.

The needs to create different versions of the same program to work with the

same hardware type (monitor for example) produced by different manufacturers

Use ROM BIOS

The Basic Input Output System, or BIOS provide a hardware-independent interface to various

devices in the IBM PC system. For example, one of the BIOS services is a video display driver.

By making various calls to the BIOS video routines, your software will be able to write

characters to the screen regardless of the actual display board installed. The BIOS allows you to

manipulate devices in a very low level fashion


45

Use MS-DOS

MS-DOS provides a high-level interface to many devices. This high-level interface greatly

reduces the amount of effort in accessing the hardware.

Accessing the hardware using BIOS or DOS call requires the use of software interrupts

What is an Interrupt?

1. An interrupt is a signal that tells the CPU to temporarily stop what it is doing and go do

something else.

2. Interrupt can be either External or Internal

3. External interrupts are generated by external hardware devices. While Internal interrupts

are generated by a running program

Invoking an Interrupt

There are 256 different interrupts numbered from 0 to 255.

A program can invoke any of these interrupts with a special instruction, known as a INT,

and is given the number of the interrupt. Thus an

INT 21h instruction invokes interrupt number 33 decimal.

In real mode, the lowest 1024 bytes of memory are reserved for a table (IVT, Interrupt

Vector Table) containing the addresses for each of the 256 possible interrupts.

When an interrupt occurs (hardware or software), the processor multiplies its number by 4

and looks at the resulting memory location to find the address of the piece of code which

handles the interrupt.

It then places the current address in the program and the processor flags on the stack, and

jumps to the beginning of the interrupt handler.

When the interrupt handler finishes, it invokes a special instruction to return from the

interrupt. This instruction takes the previously saved flags and program address off of the

stack and places them back in the appropriate registers in the CPU.

The interrupt handler has to be careful to preserve any registers that it uses which are not

used to communicate results to the program that invoked the interrupt.


46

If the interrupt can be triggered by a hardware interrupt (only certain ones can on IBM PC's,

XT's, and AT's), then the interrupt handler has to preserve ALL registers, since the interrupt

could have happened anywhere.

ACCESSING THE BIOS

The IBM PC BIOS uses software interrupts to accomplish various operations. Most of these

routines require various parameters in the 80x86's registers. Some require additional

parameters in certain memory locations.

Ms-Dos Calling Sequence

MS-DOS is called via the int 21h instruction.

To select an appropriate DOS function, load the ah register with a function number before

issuing the int 21h instruction.

Most DOS calls require other parameters as well. Generally, these other parameters are

passed in the CPU's register set.

Terminate Program Execution

Function (ah): 4Ch

Entry parameters: al - return code

Exit parameters: Does not return to your program

This is the function call normally used to terminate your program.

It returns control to the calling process. A return code can be passed to the calling process

in the al register. Exactly what meaning this return code has is entirely up to you.

Display a Character String

Function: AH = 09

Entry Parameter: DS:DX = Address of the character string

Description: This function displays a string of characters on the screen. The address of the

string should be in DS:DX register pair. The string must be terminated with the $. The string

can be of any length and may contain control characters.


47

Buffered Keyboard Input

Function: AH = 0A

Entry Parameter: DS:DX = address of a keyboard input buffer

Description:

The first byte of the buffer must be the size of the buffer, which can be up to 255 bytes.

The second byte is filled by the function when it returns. It contains the number of

character typed.

The remaining bytes contain the typed characters followed by a carriage return (0D).

This function continues to read the keyboard until either the buffer is filled or the carriage

return (Enter key) is pressed.

The program EX6_1.asm shown in Figure 6.1 is an example of using DOS function calls to

access the hardware. The program first displays Message 1, What is your name ?, on

the screen then waits for the user to type his/her name. Then it displays Message 2, It is

nice to meet you, before placing a dollar sign $ at the end of the stored name and then

displaying it after Message 2.


48

; EX6_1.ASM

; input the name and display a message

.MODEL SMALL

.stack 200h

.DATA

message1 DB 'What is your name ? $'

nam db 32, 33 dup(0)

message2 db 10,13,'It is nice to meet you $'

.CODE

START:

mov ax,seg message1 ;get segment of message1

mov ds,ax ; put it in ds

mov dx, offset message1 ; get offset of message1

mov ah,09h ; display message1

int 21h

mov dx,offset nam ; get offset of name

mov ah,0Ah ; get name from user

int 21h

mov ah,09h

mov dx,offset message2 ;get offset message2

int 21h ; display message2

mov DI,offset nam ; get offset of name

inc di ; point to number of character entered

mov bl,[di] ; get no of character

mov bh,0 ; clear bh, bx = no of character

inc di

mov BYTE PTR [Bx+di],'$'; put '$' at end of string

mov dx,di ; offset of name in dx

mov ah,09h ; display name

int 21h

mov ax,4c00h ;Returns control to DOS

int 21h ;

END START


49

Self-Test Questions

1) The advantages of accessing the hardware by direct programming are:

a) Speed and efficiency

b) Control and Speed

c) Control and Efficiency


2) The Access the hardware using DOS or BIOS, you need to:

a) Use interrupts

b) Use DOS function calls

c) None of the above

3) In real mode, the interrupt table

a) Use 1 Kbytes of memory

b) Has 256 interrupts

c) All of the above


Review Questions

1. List 3 addressing methods of accessing the hardwires

2. List one advantage and one disadvantage of each of the three methods listed in 1

3. Draw a flowchart for the program given in Ex6_1.asm


50

PROGRAM CONTROLS

Outcomes

1. Ability to write assembly programs that performs selection

2. Ability to write assembly programs that performs Iteration

Fundamentals

The Jump Commands

The jump commands can be used to control of the flow of statements execution within a

program

There are two main groups of jump commands:- Conditional and Unconditional jumps

The Unconditional Jump

The general syntax of the unconditional jump is: JMP [short/near/far] label

There are 3 types: - Short, Near and Far jumps

For short jump the label must be within +127 or -128 bytes from the JMP label instruction

For near jump the label must be within 32 Kbytes from the JMP label instruction

For far jump the label can be anywhere in the real memory. The far jump can be obtained

by:- using the FAR PTR directive or by defining the label as an external label using the

EXTRN directive

CONDITIONAL JUMPS

Conditional jumps are either short or near jumps

The Carry (C), Sign (S), Zero (0), Parity (P), and Overflow (O) flags are tested to see if they

are set (equal 1) or reset (equal 0)

There are conditional jumps commands for signed and unsigned numbers

Unsigned Conditional Transfers

Mnemonic Condition Tested "Jump If..."

JA/JNBE (CF or ZF) = 0 Above/not below nor equal

JAE/JNB CF = 0 Above or equal/not below

JB/JNAE CF = 1 Below/not above nor equal

JBE/JNA (CF or ZF) = 1 Below or equal/not above

JC CF = 1 Carry

JE/JZ ZF = 1 Equal/zero


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JNC CF = 0 Not carry

JNE/JNZ ZF = 0 Not equal/not zero

JNP/JPO PF = 0 Not parity/parity odd

JP/JPE PF = 1 Parity/parity even

Signed Conditional Transfers

Mnemonic Condition Tested "Jump If..."

JG/JNLE ((SF xor OF) or ZF) = 0 Greater/not less nor equal

JGE/JNL (SF xor OF) = 0 Greater or equal/not less

JL/JNGE (SF xor OF) = 1 Less/not greater nor equal

JLE/JNG ((SF xor OF) or ZF) = 1 Less or equal/not greater

JNO OF = 0 Not overflow

JNS SF = 0 Not sign (positive, including 0)

JO OF = 1 Overflow

JS SF = 1 Sign (negative)

Since the conditional jump commands depend on the state of the flags, the question is how to

set or reset the values of these flags to make the jump instruction transfers control to the

specified location within the program so that we can make selection and iteration? There are

several ways of setting the values of these registers. The most common method of setting the

values of the flags is by using logical and arithmetic command. The most common used

command for performing program control is the Compare command.

Compare

1. The CMP instruction compares two binary data items by subtracting the source from the

destination.

2. Neither the value of the source nor the destination change by this command. Only the

status of the flags will be changed.

3. Almost all addressing methods except memory-to memory and segment registers cannot

be used

4. This command is normally used before a conditional jump to check whether a condition

is true or false

The general syntax of this command is: CMP destination, source Where: destination can be a register or a memory variable. Source can be a register, a memory

variable, or an immediate value.


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Selection

The program vote.asm shown in Figure 6.1 shows how to use the compare (CMP) command

and the conditional jump instructions to perform a selection. The flowchart of the program is

shown in Figure 6.2.

; vote.asm

.MODEL SMALL

.STACK 200h

.DATA

Prompt db 13,10,"Type your age please (0 to 99) $"

age db 3, 4 dup(?)

yesmessage db 10,13, "Congragulation, you are eligible to

vote $"

nomessage db 10,13, "Sorry, you are under age $"

.CODE

START:

;display prompt

mov ax, seg prompt ;

mov ds,ax

mov ah,9

mov dx,offset Prompt

int 21h ;print a message

;input the age

mov ah,0ah

mov dx,offset age

int 21h ;get the age

;move age to ax

mov bx, offset age

mov ah,[bx+2] ;MSB in ah

mov al,[bx+3] ;LSB in al

; check if age is more than 21

cmp ax,'21' ; is age > 21

jae yes ;yes display yesmessage

mov dx,offset nomessage ; no display nomessage

jmp dispmessage

yes:

mov dx,offset yesmessage

dispmessage:

mov ah,9 ; display message

int 21h

;return to dos

mov ax,4c00h ;Returns control to DOS

int 21h ;

END START

Figure 6.1 The Source Code for Vote.asm


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Iteration

The program ASCII.asm shown in Figure 6.3 shows how to use the Compare (CMP) and the

conditional jump statements to perform iteration, loop. The program makes a loop that repeats

itself 256 times to print out the entire ASCII code. The flowchart of the program is shown in

Figure 6.4. Another method of making an iteration is by using the LOOP commands.

Looping the Loop

The loop instructions are conditional jumps that use a value placed in ECX to specify the

number of repetitions of a software loop. All loop instructions automatically decrement ECX and

terminate the loop when ECX=0. Four of the five loop instructions specify a condition involving

ZF that terminates the loop before ECX reaches zero.

The LOOP Instruction

The LOOP command (Loop while ECX Not Zero) is a conditional transfer that automatically

decrements the ECX register before testing ECX for the branch condition. If ECX is non-zero,

the program branches to the target label specified in the instruction. The LOOP instruction

causes the repetition of a code section until the operation of the LOOP instruction decrements

ECX to a value of zero. If LOOP finds ECX=0, control transfers to the instruction immediately

Start

Input Age

Display Enter your age

Is Age > 21?

Display Congratulation

Display Sorry

End

Figure 6.2 Flowchart of the Vote.asm Program

No

Yes


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following the LOOP instruction. If the value of ECX is initially zero then the LOOP is executed

2^(32) times.

Syntax: Loop Label

LOOPE

The LOOPE (Loop While Equal) and LOOPZ (Loop While Zero) are synonyms for the same

instruction. These instructions automatically decrement the ECX register before testing ECX and

ZF for the branch conditions. If ECX is non-zero and ZF=1, the program branches to the target

label specified in the instruction. If LOOPE or LOOPZ finds that ECX=0 or ZF=0, control

transfers to the instruction immediately following the LOOPE or LOOPZ instruction.

Syntax: Loope Label

Condition for looping: ECX # 0 and ZF = 1

LOOPNE

The LOOPNE (Loop While Not Equal) and LOOPNZ (Loop While Not Zero) are synonyms for the

same instruction. These instructions automatically decrement the ECX register before testing

ECX and ZF for the branch conditions. If ECX is non-zero and ZF=0, the program branches to

the target label specified in the instruction. If LOOPNE or LOOPNZ finds that ECX=0 or ZF=1,

control transfers to the instruction immediately following the LOOPNE or LOOPNZ instruction.

Syntax: Loopne Label

Condition for looping: ECX # 0 and ZF = 0

;ascii.asm

.MODEL SMALL

.STACK 200h

.CODE

START:

mov cx,0 ; first number in the ASCII table

mov ah,6 ; DOS function to display a single character

begin:

mov dl,cl ; ascii code of first character

int 21h ; display it

cmp cx,255

jae done

inc cx

jmp begin

done:

.exit

END START

Figure 6.3 The Source Code for the ASCII.asm Program


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EXECUTING A LOOP ZERO TIMES

JCXZ (Jump if ECX Zero) branches to the label specified in the instruction if it finds a value of

zero in ECX. JCXZ is useful in combination with the LOOP instruction and with the string scan

and compare instructions, all of which decrement ECX. Sometimes, it is desirable to design a

loop that executes zero times if the count variable in ECX is initialised to zero. Because the

LOOP instructions (and repeat prefixes) decrement ECX before they test it, a loop will execute

2^(32) times if the program enters the loop with a zero value in ECX. A programmer may

conveniently overcome this problem with JCXZ, which enables the program to branch around

the code within the loop if ECX is zero when JCXZ executes. When used with repeated string

scan and compare instructions, JCXZ can determine whether the repetitions terminated due to

zero in ECX or due to satisfaction of the scan or compare conditions.

Syntax: JCXZ Label

Condition for looping: ECX = 0

The program ASCII2.asm shown in Figure 6.5 shows how to use the Loop command to

perform an iteration. The program makes a loop that repeats itself 256 times to print out the

Start

Display Char(No)

Is

No>255?

End

Figure 6.4 Flowchart of the ASCII Program

No = 0

No = No + 1

Yes

No


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entire ASCII code in reverse order. The flowchart of the program is shown in Figure 6.6

Self-Assessment Questions

1. The jump commands can be classified into 2 groups: _____________ jumps and

_______________________ jumps

;ascii2.asm

.MODEL SMALL

.STACK 200h

.CODE

START:

mov cx,255 ; last number in the ASCII table

mov ah,6 ; DOS function to display a single character

begin:

mov dl,cl ; ascii code of first character

int 21h ; display it

loop begin

done:

.exit

END START

Figure 6.5 The Source Code for the ASCII2.asm Program


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2. The number of bytes between the jmp Short label instruction and the label should be

between ________________________ and ______________________ bytes

3. The conditional jump can only be ____________ and _____________

4. The conditional jump statement JA is used with unsigned number, the equivalent

conditional jump statement used with signed number is ________________

5. For the Repeat until loop, the condition that controls the loop is placed at _________

_________________________ of the loop

Review Questions

1. Draw a flowchart and then write the source code for an assembly language program

that displays the EVEN numbers from 0 to 100 on the screen

2. The ASCII program shown in Figure 6.3 uses the Compare and conditional jumps

commands to display the ASCII codes starting from 0 to 255. You are required to modify

the ASCII program so that it uses the Loop command to display the ASCII code

3. Draw a flowchart and then write the source code for an assembly language program

that:

a. Request the user to enter his or her sex

b. If the user is a male then the message Hi Handsome Guy is displayed. If the

user is a female, then the message Hi Beautiful Lady is displayed.


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ARITHMETIC AND LOGIC OPERATIONS

Outcomes

1. Ability to write assembly programs that perform arithmetic operations on integers

2. Ability to write assembly programs that perform logic operations

Logic Operations

The AND Instruction

1. The AND instruction performs the logical AND operation on the bits of two data items

2. Can be used to clear selected bits of a binary number by using a mask

The general syntax is: AND destination, source

Where: destination can be a register or a memory variable while source can be a register, a

memory variable or an immediate value. However, both source and destination cannot be

memory variables. After the operation is completed the destination = destination AND

source

Example 7.1

and al, 0F0h

and num1,00Fh

and byte ptr[bx],0F5h

and ax, bx

The OR Instruction

1. The OR instruction performs the logical OR operation on the bits of two data items

2. Can be used to set selected bits of a binary number by using a mask

The general syntax is: OR destination, source



memory variables. After the operation is completed the destination = destination OR

source

Example 7.2

OR al, 0F0h

OR num1,00Fh

OR byte ptr[bx],0F5h


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OR ax, bx

The XOR Instruction

1. The XOR instruction performs the logical XOR operation on the bits of two data items

2. Can be used to invert (complement) selected bits of a binary number by using a mask

The general syntax is: XOR destination , source



memory variables. After the operation is completed the destination = destination XOR

source

Example 7.3

XOR al, 0F0h

XOR num1,00Fh

XOR byte ptr[bx],0F5h

XOR ax, bx

The NOT Instruction

The NOT operation performs the logical NOT operation. Thus, it invert or performs the 1s

complement on the binary data item

The general syntax is: NOT destination.

Where: The destination can be a register or a memory variable. After the command is

completed the destination = 1s complement of the destination.

Example 7.4

NOT num1

NOT byte ptr[bx]

NOT ax

The TEST Instruction

1. The TEST instruction performs a logical AND operation on the destination and the source.

However, unlike the AND operation only the flags are changed

2. Like the CMP command, this command is normally followed by a conditional jump

instruction

3. All the addressing mode that can be used for the AND instruction can be used for this

command

The general syntax is: TEST destination, source


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Where: destination can be a register or a memory variable. Source can be a register, a memory

variable, or an immediate value.

Example 7.5

test al, 0F0h

test num1,00Fh

test byte ptr[bx],0F5h

test ax, bx

The BT Instruction

1. The BT instruction is used to test a single bit by placing it in the carry flag (C) which can

then be used to control the flow program execution

2. This command can also be used to clear, set, or complement the bit under test as shown in

the table below

The general syntax is: BT/BTS/BTC/BTR Operand, location

Where: operand can be a register or a memory variable. Location can be a register, or an

immediate value representing the location of the bit as an offset from the low-order end of the

operand

Instruction Effect on CF Effect on Selected Bit

BT (Bit Test) CF BIT (none)

BTS (Bit Test and Set) CF BIT BIT 1

BTR (Bit Test and Reset) CF BIT BIT 0

BTC (Bit Test and Complement) CF BIT BIT NOT(BIT)

Example 7.6

BT ax, 3

BTS num1, 6

BTC ax, 7

BTR num2, ax

The Bit Scan Instructions

The BSF (Bit Scan Forward) and BSR (Bit Scan Reverse) scans a source data item forward or

reverse respectively until a 1 bit (a bit whose value is 1) is found. The position where the 1 is

found is stored in the destination.


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The general syntax is: BSF/BSR destination, source

Where: Destination is a 16-bit register while Source can be a 16-bit register, or a memory

variable.

Example 7.7

BSF BX,AX

BSR BX,num1

Arithmetic Operations

Binary Arithm

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