Assembly Language Programming

Assembly Language

•   A low-level language for programming computers.
•  It implements a symbolic representation of the numeric machine codes and other constants needed to program a particular CPU architecture.
•   This representation is usually defined by the hardware manufacturer. 

Assembler

• A utility program that is used to translate assembly language statements into the target computer's machine code.
• Eg: MASM, TASM etc.
• With MASM (Microsoft Macro Assembler), you can create x86 assembly language programs that run under MS-DOS, Windows, or Windows NT ; or all three, in some cases.
    

Assembly Language Program Format

• There are two types of x86 assembly language programs
•                  exe-format and com-format. 

• .EXE programs consist of separate code, data and stack segments, whereas .COM programs consist of one segment that contains code, data and the stack.

General Structure of an Exe-Format Program

An exe-format program must contain a code segment and a stack segment. It may contain a data segment or an extra segment.
                                    

<stack_name> SEGMENT STACK

<stack declaration>

<stack_name> ENDS

<data_seg_name> SEGMENT

<data declaration>

<data_seg_name> ENDS

<code_seg_name> SEGMENT

ASSUME SS:stack_name, DS:data_seg_name, CS:code_seg_name

<label>:     ;starting address of the program

                  . . .

<program instructions>

. . .

;terminate the program

MOV AH, 0

INT 21h

<code_seg_name> ENDS

END <label>

 

Example:

                          

SSEG  SEGMENT STACK

            DW 80h dup (0)

SSEG  ENDS

DSEG SEGMENT

                        message DB ‘ICT 3406’, ‘$’

DSEG ENDS

CSEG  SEGMENT

ASSUME SS:SSEG, DS:DSEG, CS:CSEG

BEGIN:    MOV DX, OFFSET message

                  MOV AH, 09H

                              INT 21H

MOV AH, 0

INT 21h

CSEG ENDS

END BEGIN

General Structure of a Com-Format Program

A com-format program contains only the code segment and therefore all segment registers (CS, DS, SS and ES) must be initialize to the address of the code segment using the ASSUME directive.

ASSUME CS:CODE, DS:CODE, SS:CODE, ES:CODE

The first 256-byte (100h) data area of the code segment called the Program Segment Prefix (PSP) is used to store some important information about the program. Therefore the location counter must be set to 100H by the directive:

ORG   100H

This statement must appear at the beginning of every com-format program before the program entry point. It places the first instruction at offset 100h in the code segment.

                    

<code_seg_name> SEGMENT

ASSUME SS: code_seg_name, DS: code_seg_name, CS:code_seg_name

            ORG 100H

<label>:     JMP MAIN

<data declaration>

. . .

MAIN PROC

<program instructions>

. . .

;terminate the program

MOV AH, 0

INT 21h

MAIN ENDP

<code_seg_name> ENDS

END <label>

Example:

                 

CODE SEGMENT

ASSUME SS:CODE, DS:CODE, CS:CODE

            ORG 100H

BEGIN: JMP MAIN

                           message DB ‘ICT 3406’, ‘$’

MAIN PROC

               MOV DX, OFFSET message

               MOV AH, 09H

                           INT 21H

                           MOV AH, 0

                           INT 21h

MAIN ENDP

CODE ENDS

END BEGIN