chap 01[1]

Chapter 1: Context of Assembly Language

Copyright 1999-2000, Prentice-Hall Incorporated

Updated 08/00

Slides to Accompany

Assembly Language for Intel-Based Computers,

Third Edition

Table 1. Software Hierarchy Levels.

Level Description

Application ProgramSoftware designed for a particular class ofapplications.

High-Level Language (HLL)

Programs are compiled into either assembly languageor machine language. Each statement usuallytranslates into multiple machine language instructions.Examples are C++, Pascal, Java, and Visual Basic.

Operating System

Contains procedures that can be called from programswritten in either high-level language or assemblylanguage. This system may also contain an applicationprogramming interface (API).

Assembly Language (ASM)Uses instruction mnemonics that have a one-to-onecorrespondence with machine language.

Machine Language (ML)Numeric instructions and operands that can be storedin memory and directly executed by the computerprocessor.

Essential Tools

• An assembler is a program that converts source-code

programs into a machine language (object file).

• A linker joins together two or more object files and

produces a single executable file.

• A debugger loads an executable program, displays

the source code, and lets the programmer step

through the program one instruction at a time, and

display and modify memory.

ASM ML

ML

ML

ML

ML

HLL

Figure 1. Machine Language Generation by ASM and HLL programs.

Table 2. Comparison of Assembly Language to HLLs.

Type of Application High-Level Language Assembly Language

Business applicationsoftware, written forsingle platform, mediumto large size.

Formal structures make iteasy to organize andmaintain large sections ofcode.

No formal structure.Programmer must impose anartificial structure.

Hardware device driver. Language may not providefor direct hardware access.Awkward coding techniquesmust be used, resulting inpossible maintenanceproblems.

Hardware access isstraightforward and simple.Easy to maintain when theprograms are short and welldocumented.

Business applicationwritten for multipleplatforms (differentoperating systems).

Usually very portable. Thesource code can berecompiled on each targetoperating system withminimal changes.

Must be recoded separatelyfor each platform, oftenusing an assembler with adifferent syntax. Difficult tomaintain.

Embedded systems andcomputer gamesrequiring directhardware access.

Produces too muchexecutable code, and maynot run efficiently.

Ideal, because theexecutable code is smalland runs quickly.

Figure 2. Assembly Language Subroutines Used as Hardware Interfaces.

ApplicationProgram

InterfaceSubroutine

OperatingSystem

Device DriverSubroutine

Hardware

Machine Language

• Consists of binary numbers• The "native" language of the computer• Each ML instruction contains an op code (operation

code) and zero or more operands. • Examples:

Opcode Operand Meaning-------------------------------------------------40 increment the AX register05 0005 add 0005 to AX

Bits, Bytes, and Doublewords:

byte byte

wordbit01 000 10 1 10 1 010 10

Each 1 or 0 is called a bit.

Storage Type Bits Range (low - high)

Unsigned byte 8 0 to 255

Unsigned word 16 0 to 65,535

Unsigned doubleword 32 0 to 4,294,967,295

Unsigned quadword 64 0 to 18,446,744,073,709,551,615

Table 3. Storage Sizes and Ranges of Unsigned Integers.

System Base Possible Digits

Binary 2 0 1

Octal 8 0 1 2 3 4 5 6 7

Decimal 10 0 1 2 3 4 5 6 7 8 9

Hexadecimal 16 0 1 2 3 4 5 6 7 8 9 A B C D E F

Table 4. Digits in Various Number Systems.

Format Value

ASCII binary "01000001"

ASCII decimal "65"

ASCII hexadecimal "41"

ASCII octal "101"

Page 9. ASCII Digit String.

2n Decimal Value 2n Decimal Value

20 1 28 256

21 2 29 512

22 4 210 1024

23 8 211 2048

24 16 212 4096

25 32 213 8192

26 64 214 16384

27 128 215 32768

Table 5. Binary Bit Position Values.

Figure 3. Converting Binary to Decimal.

9

8+ 1

Binary Decimal Hexadecimal Binary Decimal Hexadecimal

0000 0 0 1000 8 8

0001 1 1 1001 9 9

0010 2 2 1010 10 A

0011 3 3 1011 11 B

0100 4 4 1100 12 C

0101 5 5 1101 13 D

0110 6 6 1110 14 E

0111 7 7 1111 15 F

Table 6. Binary, Decimal, and Hexadecimal Equivalents.

Table 7. Powers of 16, in Decimal.

16n Decimal Value 16n Decimal Value

160 1 164 65,536

161 16 165 1,048,576

162 256 166 16,777,216

163 4096 167 268,435,456

3 * 4096 = 12,288

11 * 256 = 2,816

10 * 16 = 160

4 * 1 = + 4

Total: 15,268

4AB3

Figure 4. Converting 3BA4 Hexadecimal to Decimal.

1 1 1 1 0 1 1 0

0 0 0 0 1 0 1 0

=

=

sign bit

+ 10

– 10

1.2.4 Signed Numbers

Table 8. Signed Integer Storage and Ranges.

Storage Type Bits Range (low - high)

Signed byte 7 -128 to +127

Signed word 15 –32,768 to +32,767

Signed doubleword 31 –2,147,483,648 to 2,147,483,647

Signed quadword 63 –9,223,372,036,854,775,808 to+9,223,372,036,854,775,807

00110001 00110010 00110011

01111011

123

' 1 ' ' 2 ' ' 3 '

=

=

"1 2 3"

1 2 3

1.2.5 Character Storage

Page 14. ASCII Representation of 123:

1.3 Introducing Assembly Language

• An instruction is a symbolic representation of a single machine instruction

• Consists of:– label always optional– mnemonic always required– operand(s) required by some instructions– comment always optional

• Examples:

start: mov ax,20 ; initialize the AX register

inc bx ; increment the BX register

stc ; set the Carry flag

1.

2.

3.

4.

mov ax, 5

add ax, 10

add ax, 20

mov [0120], ax ax

ax

ax

ax 05

15

35

35

Memory

35 0120

5. int 20

011E

011C

0122

0124

0126

Figure 5. Sample Program Written in Debug.

Running DEBUG.EXE, Assembling a Program

C:\>debug

-A 100

0AFE:0100 mov ax,5

0AFE:0103 add ax,10

0AFE:0106 add ax,20

0AFE:0109 mov [120],ax

0AFE:010C int 20

0AFE:010E

assemble, starting at offset 100

Press ENTER to return to command mode

Tracing the Sample Program.

-T

AX=0005 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000

DS=0AFE ES=0AFE SS=0AFE CS=0AFE IP=0103 NV UP EI PL NZ NA PO NC

0AFE:0103 051000 ADD AX,0010

-T


DS=0AFE ES=0AFE SS=0AFE CS=0AFE IP=0106 NV UP EI PL NZ NA PO NC

0AFE:0106 052000 ADD AX,0020

-T


DS=0AFE ES=0AFE SS=0AFE CS=0AFE IP=0109 NV UP EI PL NZ NA PE NC

0AFE:0109 A32001 MOV [0120],AX

Tracing the Sample Program (2)

MOV [0120],AX

-D 120,121

0AFE:0120 35 00


DS=0AFE ES=0AFE SS=0AFE CS=0AFE IP=010C NV UP EI PL NZ NA PE NC

0AFE:010C CD20 INT 20

-G

Program terminated normally

Table 9. Commonly Used Debug Commands.

Command Description

A Starts assembling a program, placing each instruction inmemory. Optionally, an integer argument can besupplied, which specifies the hexadecimal locationwhere the first instruction is to be inserted.

G Executes the remainder of the program.

Q Quits Debug

R Displays the CPU registers.

T Traces (execute) one program instruction.

The End

chap 01[1]

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