Part 4: Assembly language modes and code practices.

Part 4: Assembly language modes and code practices.

Assembly language program practice

In the previous article we have share many basic resources and other compile program with Assembly language. Do and keep practice with assembly helps to gather better knowledge on the language.


section .data                           ;This statement for Data segment

   userMsg db 'Please enter a number: ' ; This is for the user to enter a number

   lenUserMsg equ $-userMsg             ; This is the length of the message

   dispMsg db 'You have enter: '

   lenDispMsg equ $-dispMsg                

section .bss           ;This is Uninitialized data

   num resb 5


section .text          ;This Code Segment

   global _start


_start:                ;This is for User prompt

   mov eax, 4

   mov ebx, 1

   mov ecx, userMsg

   mov edx, lenUserMsg

   int 80h   ;Read and store the user input

   mov eax, 3

   mov ebx, 2

   mov ecx, num 

   mov edx, 5          ;5 bytes (numeric, 1 for sign) of that information

   int 80h             ;Output the message 'The enter number is: '

   mov eax, 4

   mov ebx, 1

   mov ecx, dispMsg

   mov edx, lenDispMsg

   int 80h    ;Output the number enter

   mov eax, 4

   mov ebx, 1

   mov ecx, num

   mov edx, 5

   int 80h    ; Exit code

   mov eax, 1

   mov ebx, 0

   int 80h


Please enter a number:


You have enter:1234

Most assembly language instructions require operands to be processed. An operand address provides the location, where the data to be processed is stored. Some instructions do not require an operand, whereas some other instructions may require one, two, or three operands.  Generally, in assembly language an instruction needs two operands, the first operand is the destination, where data in a register or memory location and the second operand is the source. Source contains either the data to be delivered that is immediate addressing or the address that means in register / memory of the data. The source data remains unchanged after the operation.

The three basic modes of addressing are:

  1. Register addressing
  2. Immediate addressing
  3. Memory addressing

Register Addressing:

A register which  contains the operand. In this addressing mode this is the first concern. In order to depending upon the instruction, the register may be the first operand, the second operand or both.

For example,

MOV DX, TAX_RATE   ; This is the Register in first operand

MOV COUNT, CX                 ; This is the Register in second operand

MOV EAX, EBX      ; This is Both the operands are in registers

As handing out data between registers does not comprise memory, it provides fastest processing of data.

Immediate Addressing:

In an immediate operand, it has a constant value or an expression. Therefore, when an instruction with two operands uses immediate addressing, the first operand may be a register or memory location, and the second operand is an immediate constant. The first operand defines the length of the data.

For example,

BYTE_VALUE  DB  150    ; This is A byte value is defined

WORD_VALUE  DW  300    ; This statement A word value is defined

ADD  BYTE_VALUE, 65    ; This statement An immediate operand 65 is added

MOV  AX, 45H           ; This statement is Immediate constant 45H is transferred to AX

Direct Memory Addressing:

This is used, when operands are specified in memory addressing mode. In direct access to main memory, usually to the data segment, it is needed. This is a  way of addressing where results in slower processing of data. In order to locate the precise location of data in memory, we need the segment start address, which is typically found in the DS register and an offset value. Hence, this offset value is also called effective address. Again, In direct addressing mode, the offset value is definite directly as part of the instruction, indicated by the variable name. The assembler calculates the offset value and maintains a symbol table, which stores the offset values of all the variables used in the program. In direct memory addressing, one of the operands refers to a memory location and the other operand references a register.

For example,

ADD       BYTE_VALUE, DL               ; This statement is Adds the register in the memory location

MOV     BX, WORD_VALUE           ; This statement is Operand from the memory is added to register

Direct-Offset Addressing

This addressing mode uses the arithmetic operators to modify an address. For instance,  following definitions define tables of data;

A_BYTE_TABLE DB  14, 15, 22, 45      ; This statement is Tables of bytes

A_WORD_TABLE DW  134, 345, 564, 123  ; This statement is Tables of words

The below operations access data from the tables in the memory into registers:

MOV CL, A_BYTE_TABLE [2]         ; This is used to Gets the 3rd element of the BYTE_TABLE

MOV CL, A_BYTE_TABLE + 2         ; This is used, Gets the 3rd element of the BYTE_TABLE

MOV CX, A_WORD_TABLE[3]      ; This is used, Gets the 4th element of the WORD_TABLE

MOV CX, A_WORD_TABLE + 3     ; This is used , Gets the 4th element of the WORD_TABLE

Indirect Memory Addressing

This addressing mode operates the computer’s capability of Segment like Offset addressing. Normally, the base registers EBX, EBP or BX, BP and the index registers DI, SI, coded within square brackets for memory references, are used for this purpose. Indirect addressing is  used for variables containing several elements like, arrays. Starting address of the array is stored in, say, the EBX register.

The following code snippet shows how to access different elements of the variable.

MINE_TABLE TIMES 10 DW 0  ; This is used to Allocates 10 words (2 bytes) each initialized to 0

MOV EBX, [MY_TABLE]     ; This is used to Effective Address of MINE_TABLE in EBX

MOV [EBX], 110          ; MINE _TABLE[0] = 110

ADD EBX, 2              ; EBX = EBX +2

MOV [EBX], 123          ; MINE _TABLE[1] = 123

The MOV Instruction

MOV instruction that is used for moving data from one storage space to another. The MOV instruction takes two operands.

  • Syntax

The syntax of the MOV instruction is −

  • MOV destination, source

The MOV instruction may have one of the following five forms, example statements are given below:


MOV  register, register

MOV  register, immediate

MOV  memory, immediate

MOV  register, memory

MOV  memory, register

Here, Both of the operands in MOV operation should be of same size so the value of source operand remains unchanged. The MOV instruction reasons ambiguity at times. For example, look at the statements:

MOV  EBX, [MINE_TABLE]  ; Effective Address of MINE_TABLE in EBX

MOV  [EBX], 110                      ; MY_TABLE[0] = 110

It is not clear. Here, either you want to move a byte equivalent or you want a word equivalent of the number 110. In such cases, it is wise to use a type specifier. Here is a table shows some of the common type of specifiers:

Type Specifier    Bytes addressed

BYTE      1

WORD   2

DWORD                4

QWORD               8

TBYTE    10


There is a program which illustrates some of the concepts discussed in the post. It stores a name ‘ATM SAMSUZZAMAN’ in the data section of the memory, then changes its value to another name ‘Humayon Faridi’ programmatically and displays both the names.

section .text

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

_start:             ; This tell linker entry point


   ;writing the name 'ATM SAMSUZZAMAN'

   mov    edx,9       ;This is for message length

   mov    ecx, name   ; This is for message to write

   mov    ebx,1       ; This is for a file descriptor (stdout)

   mov    eax,4       ; This is system call number (sys_write)

   int       0x80        ; This call kernel            

   mov    [name],  dword ' Humayon Faridi '    ; Changed the name to Humayon Faridi ;writing the name ‘Humayon Faridi’

   mov    edx,8       ;This for message length

   mov    ecx,name    ;This is for a message to write

   mov    ebx,1       ;This is the file descriptor (stdout)

   mov    eax,4       ;This is the system call number (sys_write)

   int       0x80        ;this call kernel               

   mov    eax,1       ;This is system call number (sys_exit)

   int       0x80        ;This call kernel 

section .data


When the above code is compiled and executed, it produces the following result as:


Part 4: Assembly language modes and code practices.

Part 3: Memory segment and assembly language code practices

Memory Arrangement or Memory Segments

A segmented memory model splits the system memory into clusters or set of autonomous segments. Each independent segments referenced by pointers located in the segment registers. This is used to contain a specific type of data. One segment is used to hold instruction codes, another segment stores the data elements, and a third segment preserves the program stack. Though there are various memory segments such as

  • Data segment :

This segment is represented by .data section and the .bss. The .data section is used to declare the memory section, where data elements are stored for the program. A section cannot be extended after the data elements are declared, and it remainders static all over the program.

  • .bss section

This segment .bss section is also a static or not change memory section. This section comprises buffers for data to be declared later in the program. This buffer memory is zero-filled.

  • Code segment

This is represented by .text section. This defines an area in memory that stores the instruction codes. This is also  static or unchangeable or a fixed area.

  • Stack

This segment that is stack contains data values passed to functions and procedures within the program. In order to speed up the processor operations, the processor includes some internal memory storage locations, called registers. Registers store data elements for processing without taking to access the memory. A partial number of registers are built into the processor chip.

Processor Registers

There are ten 32-bit and six 16-bit processor registers in IA-32 architecture. The registers are grouped into three categories, for example

  1. General registers,
  2. Control registers,
  3. Segment registers.

The general registers are further divided into the following categorise:

  1. Data registers,
  2. Pointer registers, and
  3. Index registers.
  4. Data Registers

There are four 32-bit data registers which are used for arithmetic, logical, and other operations. These 32-bit registers can be used in three ways, such as complete 32-bit data registers: EAX, EBX, ECX, EDX. Here, Lower halves of the 32-bit registers can be used as four 16-bit data registers: AX, BX, CX and DX. Lower and higher halves of the above-mentioned four 16-bit registers can be used as eight 8-bit data registers: AH, AL, BH, BL, CH, CL, DH, and DL.

Data Registers

Some of these data registers have specific use in arithmetical operations.

AX  is the primary accumulator where this is used in input/output and most arithmetic instructions. For instance , in multiplication operation, one operand is stored in EAX or AX or AL register according to the size of the operand.

BX – is known as the base register, as it could be used in indexed addressing.

CX- is known as the count register, as the ECX, CX registers store the loop count in iterative operations.

DX- is known as the data register. This register is also used in input/output operations. It is also used with AX register along with DX for multiply and divide operations involving large values.

Pointer Registers

The pointer registers are 32-bit EIP, ESP, and EBP registers and corresponding 16-bit right portions IP, SP, and BP. There are three categories of pointer registers:

Instruction Pointer (IP):

The 16-bit IP register stores the offset address of the next instruction to be executed. IP in association with the CS register gives the complete address of the current instruction in the code segment.

Stack Pointer (SP):

The 16-bit SP register provides the offset value within the program stack. SP in association with the SS register (SS:SP) refers to be current position of data or address within the program stack.

Base Pointer (BP):

The 16-bit BP register mainly helps in referencing the parameter variables passed to a subroutine. The address in SS register is combined with the offset in BP to get the location of the parameter. BP can also be combined with DI and SI as base register for special addressing.

Pointer Registers

Index Registers

The 32-bit index registers, ESI and EDI, and their 16-bit rightmost portions. SI and DI, are used for indexed addressing and sometimes used in addition and subtraction. There are two sets of index pointers:

  1. Source Index (SI) − It is used as source index for string operations.
  2. Destination Index (DI) − It is used as destination index for string operations.

Index Registers

Control Registers

The 32-bit instruction pointer register and the 32-bit flags register combined are considered as the control registers. Many instructions involve comparisons and mathematical calculations and change the status of the flags and some other conditional instructions test the value of these status flags to take the control flow to other location. The common flag bits are:

Overflow Flag (OF): This flag is indicating the overflow of a high-order bit (leftmost bit) of data after a signed arithmetic operation.

Direction Flag (DF): This flag is   determining left or right direction for moving or comparing string data. When the DF value is 0, the string operation takes left-to-right direction and when the value is set to 1, the string operation takes right-to-left direction.

Interrupt Flag (IF): This flag is   governs whether the external interrupts like keyboard entry, etc., are to be ignored or processed. It disables the external interrupt when the value is 0 and enables interrupts when set to 1.

Trap Flag (TF) ): This flag is  allows setting the operation of the processor in single-step mode. The DEBUG program we used sets the trap flag, so we could step through the execution one instruction at a time

Sign Flag (SF) ): This flag is   shows the sign of the result of an arithmetic operation. This flag is set according to the sign of a data item following the arithmetic operation. The sign is indicated by the high-order of leftmost bit. A positive result clears the value of SF to 0 and negative result sets it to 1.

Zero Flag (ZF) ): This flag is   indicates the result of an arithmetic or comparison operation. A nonzero result clears the zero flag to 0, and a zero result sets it to 1.

Auxiliary Carry Flag (AF): This flag is containing the carry from bit 3 to bit 4 following an arithmetic operation; used for specialized arithmetic. The AF is set when a 1-byte arithmetic operation causes a carry from bit 3 into bit 4.

Parity Flag (PF): This flag is indicating the total number of 1-bits in the result obtained from an arithmetic operation. An even number of 1-bits clears the parity flag to 0 and an odd number of 1-bits sets the parity flag to 1.

Carry Flag (CF): This flag is containing the carry of 0 or 1 from a high-order bit (leftmost) after an arithmetic operation. It also stores the contents of last bit of a shift or rotate operation.

Segment Registers

Segments are specific areas defined in a program for containing data, code and stack. There are three main segments:

  1. Code Segment: This flag is containing all the instructions to be executed. A 16-bit Code Segment register or CS register stores the starting address of the code segment.
  2. Data Segment: This flag is contains data, constants and work areas. A 16-bit Data Segment register or DS register stores the starting address of the data segment.
  3. Stack Segment:  This flag is containing data and return addresses of procedures or subroutines. It is implemented as a ‘stack’ data structure. The Stack Segment register or SS register stores the starting address of the stack.

Apart from the DS, CS and SS registers, there are other extra segment registers – ES (extra segment), FS and GS, which provide additional segments for storing data. These are combines the segment address in the segment register with the offset value of the location. Look at the following simple program to understand the use of registers.

The use of registers in assembly programming. This program displays 7 stars on the screen with a message:

section .text

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


_start:            ; This tell linker entry point

   mov    edx,len  ; This is  a message length

   mov    ecx,msg  ; This is a message to write

   mov    ebx,1    ; This is  a file descriptor (stdout)

   mov    eax,4    ; This is system call number (sys_write)

   int       0x80     ; This is call kernel


   mov    edx,7    ; This is message length

   mov    ecx,s2   ; This is message to write

   mov    ebx,1    ; This is a file descriptor (stdout)

   mov    eax,4    ; This is system call number (sys_write)

   int       0x80     ; This is call kernel


   mov    eax,1    ; This is system call number (sys_exit)

   int       0x80     ; This is call kernel
   section .data

    msg db 'Displaying 7 stars',0xa ;a message

    len equ $ - msg  ;length of message

    s2 times 7 db '*'


Displaying 7 stars


Part 4: Assembly language modes and code practices.

Part 2: Assembly language Environment Setup and Run a Program

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 –

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 “”

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


global _start



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.

  1. 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.

  1. 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


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


Hello, world!