Developing assembly programs under DOS

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Developing

assembly programs under DOS

ICT 106 Fundamentals of Computer Systems

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Assembler

An assembler is a program for translating assembly language into object code;

Popular assemblers for Intel x86 architecture include:

Microsoft Macro Assemble (MASM)

Borland Turbo Assembler (TASM)

Netwide Assembler (NASM)

Microsoft Macro Assemble (MASM)

Originally developed by Microsoft for MS- DOS operating system;

Support a wide variety of macro facilities and structured programming;

Support for 16-bit and 32-bit versions;

The latest version is called MASM32 V9;

MASM 611 version is used for demonstration and lab practices.

Assembling, Linking and Executing programs

The steps involved are:

1. Edit

A source program consisting of assembly language statements is created using a text editor (eg, Programmer’s File Editor - pfe, which is a public domain editor and can be downloaded from the ICT106 Unit materials site). This results in a source file of ASCII text.

2. Assemble

The assembler program reads the source file and produces an object file, which is a machine-language translation of the source program.

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3. Link

The object file may contain calls to routines (eg, operating system service functions) in an external link library.

The linker then

copies the needed routines from the link library into the object file,

creates a special header record at the beginning of the program, and

produces an executable program.

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4. Load

When the program is run by entering its name on the DOS command line, the DOS loader decodes the header record of the executable program and loads it into memory

5. Execute

The CPU begins executing the program

Hello World program in assembly language

Title Hello World Program

; File: hello.asm

; This program displays “Hello, world!”

.MODEL SMALL ;each of data and code segments

; <=64K

.STACK 100h ;reserve 256 bytes of stack space

; ---data segment ---

.DATA ;indicates beginning of data

; segment

message db “Hello, world!”,0dh,0ah,’$’

; ---code segment ---

.CODE ;indicates beginning of code segment Main PROC ;proc declares the start of a proc

MOV AX, @DATA ;copy the addr. of data MOV DS, AX ;segment into DS register

MOV AH, 9 ;place the number of DOS

;output routine in the AH

;register

MOV DX, offset message ;place address of

;string in DX register INT 21h ;execute the DOS service

;function whose addr. is in AH

MOV AX, 4C00H ;halt the program and INT 21H ;return control to DOS

Main ENDP ;marks the end of current procedure END Main ;the program exit point

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Alternate Version

Title Hello World Program

; File: hello.asm

; Alternate program displays “Hello, world!”

.MODEL SMALL ;each of data and code segments

; <=64K

.STACK 100h ;reserve 256 bytes of stack space

.DATA ;indicates beginning of data

; segment

Message BYTE “Hello, World!”,0dh,0ah

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.CODE ;indicates beginning of code segment Main PROC ;proc declares the start of a proc

MOV AX, @DATA ;copy the addr. of data MOV DS, AX ;segment into DS register

MOV AH, 40h ;write to file/device

;routine in the AH register MOV BX,1 ;output to console

MOV CX,SIZEOF message ;number of bytes MOV DX, offset message ;place address of

;string in DX register INT 21h ;execute the DOS service

;function whose addr. is in AH

MOV AX, 4C00H ;halt the program and INT 21H ;return control to DOS Main ENDP ;marks the end of current procedure END Main ;the program exit point

MS-DOS Function Calls

The main difference between two versions is on the use of MS-DOS function calls (INT 21h)

INT 21h Function 9: write a $-terminated string to standard output;

Received arguments

AH = 9

DS:DX = offset of the string

INT 21h Function 40h: write an array of bytes to a file or device

Received arguments:

AH = 40h

BX = file or device handle (console = 1)

CX = number of bytes to write

DS:DX = address of array

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Hello.c

#include <stdio.h>

int main() {

printf("Hello, World!");

return 0;

}

Example

The table below displays a list of file names that would be created if the Hello World program was assembled and linked

Step Outcome Filename

Edit Source program Hello.asm Assembly Object program Hello.obj

Listing File Hello.lst Link Executable program Hello.exe

Map file Hello.map

Compilation

Use masm.exe to compile hello.asm

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Linking

Produce the executable file using link.exe

Execution

Compiling with gcc compiler

gcc hello.c –o hello_c.exe

Using strip command

Discard symbols from object files

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MSVC++ 6.0

Generate HelloWorld.exe using MSVC

Decremental Counter

Title Decremental timer

.MODEL SMALL ;each of data and code segments <=64K .STACK 100h ;reserve 256 bytes of stack space

.DATA ;indicates beginning of data segment count EQU 65535 ;initialise counter

count2 EQU 10000 ;second counter Message db "Final result is: $" ;output string

.CODE ;indicates beginning of code segment Main PROC ;proc declares the start of a proc

MOV AX, @DATA ;copy the addr. of data MOV DS, AX ;segment into DS register MOV bx, count ;load counter to BX register L1: MOV ax, count2

L2: DEC ax JNZ L2

DEC bx ;decrement counter by 1 JNZ L1 ;Jump to L1 if BX is not zero MOV AH, 9 ;place the number of DOS output

;routine in the AH register MOV DX, offset message ;place address of

INT 21H ;execute the DOS service

;function whose addr. is in AH MOV ah, 6 ;write a character to standard output ADD bl, 48 ;convert integer to character MOV dl, bl ;output character value INT 21H ;execute the DOS service MOV AX, 4C00H ;halt the program and INT 21H ;return control to DOS

Main ENDP ;marks the end of current procedure END Main ;the program exit point

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Equivalent C Code

int main() {

int count = 65535, count2=10000;

while(count>0) { count2 = 10000;

while(count2>0) { count2--;

} count--;

}

printf("Final result is: %d", count);

return 0;

}

Unsing ML command

Type “ML /Fl count.asm” to complete compilation and linking in a single step

Note that it is case sensitive

Check the size of two executable files

Timing Command

time is a simple command or give resource usage in Unix/Linux.

Syntax:

time executable file

Output:

Real: Wall clock time

User: User CPU time

Sys: System CPU time

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Summary of file size and execution

time

Program Size Time

hello.asm 911 bytes

hello.c 82 bytes

hello.exe 546 bytes

hello_c.exe 8873 bytes HelloWorld.exe 155748 bytes

count.asm 1283 bytes

count.c 245 bytes

count.exe 568 bytes 0.751sec

Count_c.exe 8873 bytes 1.923sec