Part 10: Procedures and Recursion used in Assembly  Language

Part 10: Procedures and Recursion used in Assembly  Language

Procedures and Recursion used in Assembly  Language:

By nature in Assembly there is an inclination towards to be big size of Assembly program.  Also there is some way to keep remain in smaller in code size by subroutines or procedures. The article is concerned with the procedures and its various kind of implementation with example.

Procedures in Assembly

Assembly language programs intend to be large in size. Procedures or subroutines are very vital in assembly language which are identified by a name. Following this name, the body of the procedure is defined which accomplishes a well-defined job. End of the procedure is indicated by a return statement.



   procedure body



The procedure is called from another function by using the CALL instruction. The CALL instruction should have the name of the called procedure as an argument as shown below:

CALL proc_name

The called procedure returns the control to the calling procedure by using the RET instruction.

Example:  Write a very simple procedure named sum that adds the variables stored in the ECX and EDX register and returns the sum in the EAX register.

section .text

   global _start        ;must be declared for using gcc               

_start:                   ;tell linker entry point

   mov    ecx,'4'

   sub     ecx, '0'               

   mov    edx, '5'

   sub     edx, '0'               

   call    sum          ;call sum procedure

   mov    [res], eax

   mov    ecx, msg              

   mov    edx, len

   mov    ebx,1             ;file descriptor (stdout)

   mov    eax,4             ;system call number (sys_write)

   int       0x80               ;call kernel               

   mov    ecx, res

   mov    edx, 1

   mov    ebx, 1            ;file descriptor (stdout)

   mov    eax, 4            ;system call number (sys_write)

   int       0x80               ;call kernel               

   mov    eax,1             ;system call number (sys_exit)

   int       0x80               ;call kernel


   mov     eax, ecx

   add     eax, edx

   add     eax, '0'


section .data

msg db "The sum is:", 0xA,0xD

len equ $- msg  

segment .bss

res resb 1


The sum is:

Stacks Data Structure

A stack is an array-like data structure in the memory in which data can be stored and removed from a location called the ‘top’ of the stack. The data desires to be stored is ‘pushed’ into the stack and data to be retrieved is ‘popped’ out from the stack in LIFO structure. So, the data stored first is retrieved last. Assembly language provides two instructions for stack operations: PUSH and POP. These instructions have syntaxes like:

PUSH    operand

POP     address/register

The memory space used in the stack segment is used for implementing stack. The registers SS and ESP (or SP) are used for implementing the stack. The top of the stack, which points to the last data item injected into the stack is pointed to by the SS: ESP register, where the SS register points to the opening of the stack segment and the SP (or ESP) gives the offset into the stack segment. The stack implementation has the following characteristics:

Only words or doublewords could be saved into the stack, not a byte. The stack grows in the reverse direction. For example, toward the lower memory address. The top of the stack points to the last item inserted in the stack; it points to the lower byte of the last word inserted. For storing the values of the registers in the stack before using them for some use; it can be done in following way:

; Save the AX and BX registers in the stack



; Use the registers for other purpose






; Restore the original values

POP       BX

POP       AX



The following program displays the entire ASCII character set. The main program calls a procedure named display, which displays the ASCII character set.

section .text

   global _start        ;must be declared for using gcc            

_start:                   ;tell linker entry point

   call    display

   mov    eax,1             ;system call number (sys_exit)

   int       0x80               ;call kernel


   mov    ecx, 256               


   push    ecx

   mov     eax, 4

   mov     ebx, 1

   mov     ecx, achar

   mov     edx, 1

   int     80h         

   pop     ecx       

   mov    dx, [achar]

   cmp    byte [achar], 0dh

   inc       byte [achar]

   loop    next


section .data

achar db '0'


0123456789:;<=>[email protected]BCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}

Recursion in Assembly

A recursive procedure is one that calls itself. There are two types of recursion:

  • direct and
  • indirect

In direct recursion, the procedure calls itself and in indirect recursion, the first procedure calls a second procedure, which in turn calls the first procedure. Recursion could be observed in numerous mathematical algorithms. For instance, suppose the case of calculating the factorial of a number. Factorial of a number is given by the equation:

Fact (n) = n * fact (n-1) for n > 0

For instance: factorial of 5 is 1 x 2 x 3 x 4 x 5 = 5 x factorial of 4 and this can be a good instance of showing a recursive procedure. Every recursive algorithm must have an ending condition. In the case of factorial algorithm, the end condition is reached when n is 0.

Example: The following program shows how factorial n is implemented in assembly language. To keep the program simple, we will calculate factorial 3.

section .text

   global _start         ;must be declared for using gcc           

_start:                  ;tell linker entry point

   mov bx, 3             ;for calculating factorial 3

   call  proc_fact

   add   ax, 30h

   mov  [fact], ax   

   mov      edx,len        ;message length

   mov      ecx,msg        ;message to write

   mov      ebx,1          ;file descriptor (stdout)

   mov      eax,4          ;system call number (sys_write)

   int         0x80           ;call kernel

   mov   edx,1            ;message length

   mov      ecx,fact       ;message to write

   mov      ebx,1          ;file descriptor (stdout)

   mov      eax,4          ;system call number (sys_write)

   int         0x80           ;call kernel   

   mov      eax,1          ;system call number (sys_exit)

   int         0x80           ;call kernel 


   cmp   bl, 1

   jg    do_calculation

   mov   ax, 1



   dec   bl

   call  proc_fact

   inc   bl

   mul   bl        ;ax = al * bl


section .data

msg db 'Factorial 3 is:',0xa           

len equ $ - msg

section .bss

fact resb 1


Factorial 3 is:


Part 10: Procedures and Recursion used in Assembly  Language

Part 8: Conditions and Loop uses in Assembly Language

Conditions and Loop with code in Assembly

Conditional run in assembly language is accompanied by many  looping and branching instructions. Firstly we discuss here with conditions in assembly then the loops in Assembly.

Conditions in Assembly

Conditional execution in assembly language is accomplished by several looping and branching instructions. These instructions can change the flow of control in a program. Conditional execution is observed in two scenarios:

  1. Unconditional jump

This is performed by the JMP instruction. Conditional execution often involves a transfer of control to the address of an instruction that does not follow the currently executing instruction. Transfer of control may be forward, to execute a new set of instructions or backward, to re-execute the same steps.

 2. Conditional jump

This is performed by a set of jump instructions j<condition> depending upon the condition. The conditional instructions transfer the control by breaking the sequential flow and they do it by changing the offset value in IP.

Unconditional Jump

This is performed by the JMP instruction. Conditional execution often involves a transfer of control to the address of an instruction that does not follow the currently executing instruction. Transfer of control may be forward, to execute a new set of instructions or backward, to re-execute the same steps.


The JMP instruction provides a label name where the flow of control is transferred immediately. The syntax of the JMP instruction is:


MOV  AX, 00    ; Initializing AX to 0

MOV  BX, 00    ; Initializing BX to 0

MOV  CX, 01    ; Initializing CX to 1


ADD  AX, 01    ; Increment AX

ADD  BX, AX    ; Add AX to BX

SHL  CX, 1     ; shift left CX, this in turn doubles the CX value

JMP  L20       ; repeats the statements


Conditional Jump

If some specified condition is satisfied in conditional jump, the control flow is transferred to a target instruction. There are numerous conditional jump instructions depending upon the condition and data. The syntax for the J<condition> set of instructions.


CMP      AL, BL

JE       EQUAL

CMP      AL, BH

JE       EQUAL

CMP      AL, CL

JE       EQUAL


EQUAL: ...


The following program displays the largest of three variables. The variables are double-digit variables. The three variables num1, num2 and num3 have values 47, 22 and 31, respectively:

section .text

   global _start         ;must be declared for using gcc

_start:                    ;tell linker entry point

   mov   ecx, [num1]

   cmp   ecx, [num2]

   jg    check_third_num
   mov   ecx, [num2]


   cmp   ecx, [num3]

   jg    _exit

   mov   ecx, [num3]  


   mov   [largest], ecx

   mov   ecx,msg

   mov   edx, len

   mov   ebx,1     ;file descriptor (stdout)

   mov   eax,4      ;system call number (sys_write)

   int   0x80           ;call kernel               

   mov   ecx,largest

   mov   edx, 2

   mov   ebx,1     ;file descriptor (stdout)

   mov   eax,4      ;system call number (sys_write)

   int   0x80           ;call kernel   

   mov   eax, 1

   int   80h

section .data

   msg db "The largest digit is: ", 0xA,0xD

   len equ $- msg

   num1 dd '47'

   num2 dd '22'

   num3 dd '31'

segment .bss

   largest resb 2



The largest digit is:


Loop uses in Assembly

The JMP instruction can be used for implementing loops. For example, the following code snippet can be used for executing the loop-body 10 times.

MOV     CL, 10



DEC        CL

JNZ         L1

The processor instruction set, however, includes a group of loop instructions for implementing iteration. The basic LOOP instruction has the following syntax :

LOOP     label

Where, label is the target label that identifies the target instruction as in the jump instructions. The LOOP instruction assumes that the ECX register contains the loop count. When the loop instruction is executed, the ECX register is decremented and the control jumps to the target label, until the ECX register value, i.e., the counter reaches the value zero. The above code snippet could be written as:

mov ECX,10


<loop body>

loop l1


The following program prints the number 1 to 9 on the screen:

section .text

   global _start        ;must be declared for using gcc               

_start:                   ;tell linker entry point

   mov ecx,10
   mov eax, '1'               


   mov [num], eax

   mov eax, 4

   mov ebx, 1

   push ecx               

   mov ecx, num       

   mov edx, 1       

   int 0x80               

   mov eax, [num]

   sub eax, '0'

   inc eax

   add eax, '0'

   pop ecx

   loop l1               

   mov eax,1             ;system call number (sys_exit)

   int 0x80              ;call kernel

section .bss

num resb 1




Part 10: Procedures and Recursion used in Assembly  Language

Part 7: Logical Instructions used in Assembly Language

Assembly Logical Instructions

The processor instruction set offers the instructions Boolean logic namely AND, OR, XOR, TEST, and NOT. This is tests, sets, and clears the bits according to the need of the program.

The format for these instructions:



AND AND operand1, operand2
OR OR operand1, operand2
XOR XOR operand1, operand2
TEST TEST operand1, operand2

The first operand in all the cases could be either in register or in memory. The second operand could be either in register/memory or an immediate or constant value. Although, memory-to-memory operations are not possible. These instructions compare or match bits of the operands and set the CF, OF, PF, SF and ZF flags.

The AND Instruction

The AND instruction is used for logical expressions bitwise AND operation. The operation returns 1, if the matching bits from both the operands are 1, otherwise it returns 0. For example:

Operand1: 0101
Operand2: 0011
After AND Operation -> Operand1:0001

The AND operation can be used for clearing one or more bits. For example, say the BL register contains 0011 1010. If you need to clear the high-order bits to zero, you AND it with 0FH.

AND BL, 0FH ; This sets BL to 0000 1010


section .text
global _start ;must be declared for using gcc

_start: ;tell linker entry point
mov ax, 8h ;getting 8 in the ax
and ax, 1 ;and ax with 1
jz evnn
mov eax, 4 ;system call number (sys_write)
mov ebx, 1 ;file descriptor (stdout)
mov ecx, odd_msg ;message to write
mov edx, len2 ;length of message
int 0x80 ;call kernel
jmp outprog


mov ah, 09h
mov eax, 4 ;system call number (sys_write)
mov ebx, 1 ;file descriptor (stdout)
mov ecx, even_msg ;message to write
mov edx, len1 ;length of message
int 0x80 ;call kernel


mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel

section .data
even_msg db 'Even Number!' ;message showing even number
len1 equ $ - even_msg

odd_msg db 'Odd Number!' ;message showing odd number
len2 equ $ - odd_msg


Even Number!
Change the value in the ax register with an odd digit, such as:

mov ax, 9h ; getting 9 in the ax
The program would display:

Odd Number!

Similarly to clear the entire register you can AND it with 00H.

The OR Instruction

The OR instruction is used for performing bitwise OR operation. The operations returns 1, if the matching bits from either or both operands are one. It returns 0, if both the bits are zero.

For example,

Operand1: 0101
Operand2: 0011
After OR -> Operand1: 0111

The OR operation can be used for setting one or more bits. For example, let us assume the AL register contains 0011 1010, you need to set the four low-order bits, you can OR it with a value 0000 1111, i.e., FH.

OR BL, 0FH ; This sets BL to 0011 1111

The following example demonstrates the OR instruction. Let us store the value 5 and 3 in the AL and the BL registers, respectively, then the instruction,

should store 7 in the AL register:

section .text
global _start ;must be declared for using gcc

_start: ;tell linker entry point
mov al, 5 ;getting 5 in the al
mov bl, 3 ;getting 3 in the bl
or al, bl ;or al and bl registers, result should be 7
add al, byte '0' ;converting decimal to ascii

mov [result], al
mov eax, 4
mov ebx, 1
mov ecx, result
mov edx, 1
int 0x80

mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel

section .bss
result resb 1



The XOR Instruction

The XOR instruction implements from the bitwise XOR operation. The XOR operation sets the resultant bit to 1, if and only if the bits from the operands are different. If the bits from the operands are same (both 0 or both 1), the resultant bit is cleared to 0.

For example,

Operand1: 0101
Operand2: 0011
After XOR -> Operand1: 0110

XORing an operand with itself changes the operand to 0. This is used to clear a register.


The TEST Instruction

The TEST instruction works similar with the AND operation, but unlike AND instruction, it does not change the first operand. So, if we need to check whether a number in a register is even or odd, we can also do this using the TEST instruction without changing the original number.

The NOT Instruction

The NOT instruction is implementing the bitwise NOT operation. The operation reverses the bits in an operand. The operand could be either in a register or in the memory.

For example,

Operand1: 0101 0011
After NOT -> Operand1: 1010 1100

Part 10: Procedures and Recursion used in Assembly  Language

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 10: Procedures and Recursion used in Assembly  Language

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 10: Procedures and Recursion used in Assembly  Language

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!

Part 10: Procedures and Recursion used in Assembly  Language

Part 1: Assembly Language with Its system properties

What is Assembly Language?

Assembly Language (AL) is one line of code translates to one machine instruction. Every computer has a microprocessor that achieves the computer’s arithmetical, logical, and control actions. ALs are NOT machine-independent that is each different machine or processor has a different machine languages. Any particular machine can have more than one assembly language. This is a low-level language that represents various instructions in symbolic code and a clearer form. A processor understands only machine language instructions, which are strings of 1’s and 0’s. Each set of processors has own instructions set for handling numerous operations such as getting input from keyboard, displaying information on screen and performing many works where each set of instructions are named ‘machine language instructions’.

Assembly language makes one aware of:

  • How OS, processor, and BIOS programs interface mutually;
  • How data is represented in memory with other devices;
  • How the processor accesses and executes instruction;
  • How each instruction access to process data;
  • How a program accesses external device

Assembly Language Advantages:

  • The language requires less memory and execution time;
  • It allows hardware-specific complex jobs in an easier way;
  • It is suitable for time-critical trades;
  • It is most suitable for writing interrupt service routines along other memory resident programs.

A computer Hardware Basic Features

The main internal hardware of a PC consists of processor, memory, and registers. Registers are processor components where data and address are hold. To execute a program, the system copies it from the external device into the internal memory. The processor executes the program instructions.

The fundamental unit of computer storage is a bit; it could be ON (1) or OFF (0) and a group of 8 bits and makes a byte on most of the modern computers.

The parity bit is used to make the number of bits in a byte odd. If the parity is even, the system assumes that there had been a parity error (though rare), which might have been caused due to hardware fault or electrical disturbance.

The processor supports data sizes:

  • Word: Contain a 2-byte data item
  • Doubleword: Consist of a 4-byte (32 bit) data item
  • Quadword: Contain an 8-byte (64 bit) data item
  • Paragraph: Have a 16-byte (128 bit) area
  • Kilobyte: Have 1024 bytes
  • Megabyte: Have 1,048,576 bytes

Binary Number System

Every number system uses positional notation. Each position is power of the base, which is 2 for binary number system, and these powers begin at 0 and increase by 1.
The positional values for an 8-bit binary number, with all bits are set ‘ON’

Bit value 1 1 1 1 1 1 1 1
Position value as a power of base 2 128 64 32 16 8 4 2 1
Bit number 7 6 5 4 3 2 1 0

The value of a binary number is based on the presence of 1 bits and their positional value. So, the value of a given binary number is:

1 + 2 + 4 + 8 +16 + 32 + 64 + 128 = 255

which is same as 28 – 1.

Hexadecimal Number System

Hexadecimal number system uses base 16 range from 0 to 15 each digit. The letters A through F is represented corresponding to decimal values 10 through 15.

Decimal number Binary representation Hexadecimal representation
0 0 0
1 1 1
2 10 2
3 11 3
4 100 4
5 101 5
6 110 6
7 111 7
8 1000 8
9 1001 9
10 1010 A
11 1011 B
12 1100 C
13 1101 D
14 1110 E
15 1111 F

To convert a binary number to its hexadecimal equivalent, break it into groups of 4 consecutive groups each, starting from the right, and write those groups over the corresponding digits of the hexadecimal number.

Example − Binary number 1000 1100 1101 0001 is equivalent to hexadecimal – 8CD1

To convert a hexadecimal number to binary, just write each hexadecimal digit into its 4-digit binary equivalent.

Example − Hexadecimal number FCD7 is equivalent to binary – 1111 1100 1101 0111

Binary Arithmetic

The following table illustrates four simple rules for binary addition −

(i) (ii) (iii) (iv)
0 1 1 1
+0 +0 +1 +1
=0 =1 =10 =11

Rules (iii) and (iv) show a carry of a 1-bit into the next left position.


Decimal Binary
60 00111100
+42 00101010
102 01100110

A negative binary value is expressed in two’s complement notation. According to this rule, to convert a binary number to its negative value is to reverse its bit values and add 1.


Number 53 00110101
Reverse the bits 11001010
Add 1 00000001
Number -53 11001011

To subtract one value from another, convert the number being subtracted to two’s complement format and add the numbers.


Subtract 22 from 33

Number 33 00100001
Number 22 00010110
Reverse the bits of 42 11101001
Add 1 00000001
Number -22 00010110
33 – 22 = 11 00001011

Overflow of the last 1 bit is lost.

Addressing Data in Memory

The processor which the controls process of execution of instructions is denoted as the fetch-decode-execute cycle or the execution cycle. It consists of three continuous steps:

  1. Fetching the instruction from memory
  2. Decoding or identifying the instruction
  3. Executing the instruction

The processor may access one or more bytes of memory at a time. Let us consider a hexadecimal number 0225H. This number will require two bytes of memory. The high-order byte or most significant byte is 02 and the low-order byte is 25. The processor stores data in reverse-byte sequence, i.e., a low-order byte is stored in a low memory address and a high-order byte in high memory address. So, if the processor brings the value 0225H from register to memory, it will transfer 25 first to the lower memory address and 02 to the next memory address.

There are two kinds of memory addresses:

  • Absolute address: It is a direct reference of specific location.
  • Segment address (or offset) : It is a starting address of a memory segment with the offset value.