Environment Setup

Assembly language is dependent upon the instruction set and the architecture of the processor. There are many good assembler programs, for example

1. NASM: It is an operating system independent assembler. One of the two widely used Linux assemblers and the other GNU
2. The GNU assembler (GAS): The syntax differs significantly in many ways from
3. MASM (Microsoft Assembler): MASM syntax is a standard. So, almost always understood by other x86 assemblers TASM, CHASM, A386, etc. The syntax has some significant defects that makes coding error hence many of them are rectified in NASM.
4. Borland Turbo Assembler (TASM)

Installing NASM

It could be used on both Linux and Windows, can download from various web sources. All are well documented. While installing Linux, if “Development Tools” is chacked, NASM installed along with the Linux operating system. For checking have NASM installed, take the following steps

• Open a Linux terminal.
• Type where is nasm and press ENTER.

We use a online compiler of assymbly language in this blog. Go to this link – https://www.jdoodle.com/compile-assembler-nasm-online/

Basic of NASM assembler

Character Set: Letters a..z; A..Z; ()

Digits: 0.9

Special Chars: ? _ @ $ . ~

• NASM is case-sensitive with respect to labels and variables
• It is not case-sensitive with respect to keywords, mnemonics, register names, directives, etc.
• Special Characters.

Write a basic assembly program

Generally, an assembly program can be divided into three sections, such as

1. The data section,
2. The bss section, and
3. The text section.

The data Section

The section is used for declaring  data or constants which are not modify at runtime. Various constant values, file names, or buffer size, etc. are declare in this section.

The syntax for declaring data section is “section.data”

The bss Section

The section is used for declaring variables. The syntax for declaring bss section is “section.bss”

The text section

This section must be begun with the declaration global _start, which tells the kernel where the program execution begins. The section is used for care the actual code.

The syntax for declaring text section is

section.text

global _start

_start:

Comments

AL comment begins with a semicolon (;). It may contain any printable character including blank. It can appear on a line by itself, like below

; This program displays a message on screen

or, on the same line along with an instruction, like

add eax, ebx     ; this statement state as adds ebx to eax

Assembly Language Statements

Assembly language programs consist of three types of statements, for example

1. Executable instructions or instructions:

The executable instructions or simply instructions tell the processor what to do. Each instruction consists of an operation code (opcode). Each executable instruction generates one machine language instruction.

2. Assembler directives or pseudo-ops

The assembler directives or pseudo-ops tell the assembler about the various aspects of the assembly process. These are non-executable and do not generate machine language instructions.

3. macros

In AL macros are basically a text substitution mechanism.

Syntax of Assembly Language(AL) Statements

Assembly language statements are entered one statement per line. Each statement follows the following format:

[label]   mnemonic   [operands]   [;comment]

A basic instruction has two parts, the first one is the name of the instruction (or the mnemonic), which is to be executed, and the second are the operands or the parameters of the command. Here the fields in the square brackets are optional.

Following are some examples of typical assembly language statements :

INC COUNT        ; this statement state as Increment the memory variable COUNT

MOV TOTAL, 48    ; this statement state as Transfer the value 48 in the ; memory variable TOTAL

ADD AH, BH       ; this statement state as Add the content of the BH register into the AH register

AND MASK1, 128   ; this statement state as Perform AND operation on the  variable MASK1 and 128

ADD MARKS, 10    ; this statement state as Add 10 to the variable MARKS

MOV AL, 10       ; this statement state as Transfer the value 10 to the AL register

The Hello World Program in Assembly Language

The following assembly language code displays the string ‘Hello World’ on the screen −

section .text

global _start     ;This is must be declared for linker (ld)

_start:              ; this statement tells linker entry point

mov  edx,len     ; this state as message length

mov  ecx,msg     ; this statement state as message to write

mov  ebx,1       ; this statement state as file descriptor (stdout)

mov  eax,4       ; this statement state as system call number (sys_write)

int     0x80        ;call kernel

mov  eax,1       ; this statement state as system call number (sys_exit)

int     0x80        ;call kernel

section .data

msg db 'Hello, world!', 0xa  ;string to be printed

len equ $ - msg     ;length of the string

OUTPUT:

Hello, world!

Now Compiling and Linking an AL Program in NASM. Make sure you have set the path of nasm and ld binaries in your PATH environment variable. Now, take the following steps for compiling and linking the above program:

• Type the above code using a text editor and save it as hello.asm.
• Make sure that you are in the same directory as where you saved hello.asm.
• To assemble the program, type nasm -f elf hello.asm
• If there is any error, you will be prompted about that at this stage. Otherwise, an object file of your program named hello.o will be created.
• To link the object file and create an executable file named hello, type ld -m elf_i386 -s -o hello hello.o
• Execute the program by typing ./hello
• If you have done everything correctly, it will display ‘Hello, world!’ on the screen.

if replace the section keyword with segment by follow code:

segment .text

global _start     ;This is must be declared for linker (ld)

_start:              ; this statement tells linker entry point

mov  edx,len     ; this state as message length

mov  ecx,msg     ; this statement state as message to write

mov  ebx,1       ; this statement state as file descriptor (stdout)

mov  eax,4       ; this statement state as system call number (sys_write)

int     0x80        ;call kernel

mov  eax,1       ; this statement state as system call number (sys_exit)

int     0x80        ;call kernel

segment .data

msg db 'Hello, world!', 0xa  ;string to be printed

len equ $ - msg     ;length of the string

OUTPUT

Hello, world!