ASSEMBLY LANGUAGE NOTATION

Assembly language notation is another type of notation to represent machine instructions and
programs. A generic instruction to transfer content of a memory location LOC to processor
register R1 can be specified by the statement
Load R1, LOC
The contents of LOC are unchanged by the execution of the instruction, but the old contents of
register R1 are overwritten. The name Load is appropriate for the instruction because the
contents read from a memory location are loaded into a processor register.
Example 2. Adding of two numbers contained in processor register R1 and R2 and placing their
sum in R3 can be specified by the assembly-language statement
Add R3, R2, R1
In this case, register R1 and R2 hold the source operands, while R3 is the destination.
The English words Load and Add are used to denote the required operations. In the assembly-
language instructions of actual (commercial) processors, such operations are defined by using
mnemonics, which are typically abbreviations of the words describing the operations. For
example, the operation Load may be written as LD, while the operation Store, which transfers a
word from a processor register to the memory, may be written as STR or ST. Assembly
languages for different processors often use different mnemonics for a given operation.

What is Assembly Language?

Each personal computer has a microprocessor that manages the computer's arithmetical, logical,
and control activities.
Each family of processors has its own set of instructions for handling various operations such as
getting input from keyboard, displaying information on screen and performing various other
jobs. These set of instructions are called 'machine language instructions'.
A processor understands only machine language instructions, which are strings of 1's and 0's.
However, machine language is too obscure and complex for using in software development. So,
the low-level assembly language is designed for a specific family of processors that represents
various instructions in symbolic code and a more understandable form.
Advantages of Assembly Language
Having an understanding of assembly language makes one aware of −
 How programs interface with OS, processor, and BIOS;
 How data is represented in memory and other external devices;
 How the processor accesses and executes instruction;
 How instructions access and process data;
 How a program accesses external devices.
Other advantages of using assembly language are −
 It requires less memory and execution time;
 It allows hardware-specific complex jobs in an easier way;
 It is suitable for time-critical jobs;
 It is most suitable for writing interrupt service routines and other memory resident
programs.
Basic Features of PC Hardware
The main internal hardware of a PC consists of processor, memory, and registers. Registers are
processor components that hold data and address. 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 related bits makes a byte on most of the modern computers.
So, 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 the following data sizes −
 Word: a 2-byte data item
 Doubleword: a 4-byte (32 bit) data item
 Quadword: an 8-byte (64 bit) data item
 Paragraph: a 16-byte (128 bit) area
  Kilobyte: 1024 bytes
 Megabyte: 1,048,576 bytes

Addressing Data in Memory

The process through which the processor controls the execution of instructions is referred as
the fetch-decode-execute cycle or the execution cycle. It consists of three continuous steps −
 Fetching the instruction from memory
 Decoding or identifying the instruction
 Executing the instruction
The processor may access one or more bytes of memory at a time. Let us consider a hexadecimal
number 0725H. This number will require two bytes of memory. The high-order byte or most
significant byte is 07 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 0725H from register to memory, it will transfer 25 first to the lower memory address and
07 to the next memory address.

x: memory address
When the processor gets the numeric data from memory to register, it again reverses the bytes.
There are two kinds of memory addresses −
 Absolute address - a direct reference of specific location.
 Segment address (or offset) - starting address of a memory segment with the offset value.

Local Environment Setup

Assembly language is dependent upon the instruction set and the architecture of the processor. In
this tutorial, we focus on Intel-32 processors like Pentium. To follow this tutorial, you will need
−
 An IBM PC or any equivalent compatible computer
 A copy of Linux operating system
 A copy of NASM assembler program
There are many good assembler programs, such as −
 Microsoft Assembler (MASM)
 Borland Turbo Assembler (TASM)
 The GNU assembler (GAS)
We will use the NASM assembler, as it is −
 Free. You can download it from various web sources.
 Well documented and you will get lots of information on net.
 Could be used on both Linux and Windows.

An assembly program can be divided into three sections −

 The data section,
 The bss section, and
 The text section.
The data Section
The data section is used for declaring initialized data or constants. This data does not change at
runtime. You can declare various constant values, file names, or buffer size, etc., in this section.
The syntax for declaring data section is −
section.data
The bss Section
The bss section is used for declaring variables. The syntax for declaring bss section is −
section.bss
Comments
Assembly language comment begins with a semicolon (;). It may contain any printable character
including blank. It can appear on a line by itself, like −
; This program displays a message on screen
or, on the same line along with an instruction, like −
add eax, ebx ; adds ebx to eax
Assembly Language Statements
Assembly language programs consist of three types of statements −
 Executable instructions or instructions,
 Assembler directives or pseudo-ops, and
 Macros.
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.
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.
Macros are basically a text substitution mechanism.
Syntax of Assembly Language Statements
Assembly language statements are entered one statement per line. Each statement follows the
following format −
[label] mnemonic [operands] [;comment]
The fields in the square brackets are optional. 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.
Following are some examples of typical assembly language statements −
INC COUNT ; Increment the memory variable COUNT

MOV TOTAL, 48 ; Transfer the value 48 in the

; memory variable TOTAL
ADD AH, BH ; Add the content of the
; BH register into the AH register

AND MASK1, 128 ; Perform AND operation on the

; variable MASK1 and 128

ADD MARKS, 10 ; Add 10 to the variable MARKS

MOV AL, 10 ; Transfer the value 10 to the AL register

Assembly - Memory Segments

We have already discussed the three sections of an assembly program. These sections represent
various memory segments as well.
Memory Segments
A segmented memory model divides the system memory into groups of independent segments
referenced by pointers located in the segment registers. Each segment is used to contain a
specific type of data. One segment is used to contain instruction codes, another segment stores
the data elements, and a third segment keeps the program stack.
In the light of the above discussion, we can specify various memory segments as −
 Data segment − It is represented by .data section and the .bss. The .data section is used
to declare the memory region, where data elements are stored for the program. This
section cannot be expanded after the data elements are declared, and it remains static
throughout the program.
The .bss section is also a static memory section that contains buffers for data to be declared later
in the program. This buffer memory is zero-filled.
 Code segment − It is represented by .text section. This defines an area in memory that
stores the instruction codes. This is also a fixed area.
 Stack − This segment contains data values passed to functions and procedures within the
program.
Assembly - Registers
Processor operations mostly involve processing data. This data can be stored in memory and
accessed from thereon. However, reading data from and storing data into memory slows down
the processor, as it involves complicated processes of sending the data request across the control
bus and into the memory storage unit and getting the data through the same channel.
To speed up the processor operations, the processor includes some internal memory storage
locations, called registers.
The registers store data elements for processing without having to access the memory. A limited
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 −
 General registers,
 Control registers, and
 Segment registers.
The general registers are further divided into the following groups −
 Data registers,
 Pointer registers, and
 Index registers.

Data Registers
Four 32-bit data registers are used for arithmetic, logical, and other operations. These 32-bit
registers can be used in three ways −
 As complete 32-bit data registers: EAX, EBX, ECX, EDX.
 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.
Some of these data registers have specific use in arithmetical operations.
AX is the primary accumulator; it is used in input/output and most arithmetic instructions. For
example, 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. It 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 (as CS:IP) 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.
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 −
 Source Index (SI) − It is used as source index for string operations.
 Destination Index (DI) − It is used as destination index for string operations.

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:
The common flag bits are:
 Overflow Flag (OF) − It indicates the overflow of a high-order bit (leftmost bit) of data
after a signed arithmetic operation.
 Direction Flag (DF) − It determines 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) − It determines 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) − It 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) − It 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) − It 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) − It contains 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) − It indicates 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) − It contains 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.
The following table indicates the position of flag bits in the 16-bit Flags register:

Flag: O D I T S Z A P C

Bit no: 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Segment Registers
Segments are specific areas defined in a program for containing data, code and stack. There are
three main segments −
 Code Segment − It contains all the instructions to be executed. A 16-bit Code Segment
register or CS register stores the starting address of the code segment.
 Data Segment − It contains data, constants and work areas. A 16-bit Data Segment
register or DS register stores the starting address of the data segment.
 Stack Segment − It contains 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.
In assembly programming, a program needs to access the memory locations. All memory
locations within a segment are relative to the starting address of the segment. A segment begins
in an address evenly divisible by 16 or hexadecimal 10. So, the rightmost hex digit in all such
memory addresses is 0, which is not generally stored in the segment registers.
The segment registers stores the starting addresses of a segment. To get the exact location of data
or instruction within a segment, an offset value (or displacement) is required. To reference any
memory location in a segment, the processor combines the segment address in the segment
register with the offset value of the location.
Assembly - System Calls
System calls are APIs for the interface between the user space and the kernel space. We have
already used the system calls. sys_write and sys_exit, for writing into the screen and exiting from
the program, respectively.
Linux System Calls
You can make use of Linux system calls in your assembly programs. You need to take the
following steps for using Linux system calls in your program −
 Put the system call number in the EAX register.
 Store the arguments to the system call in the registers EBX, ECX, etc.
 Call the relevant interrupt (80h).
 The result is usually returned in the EAX register.
There are six registers that store the arguments of the system call used. These are the EBX, ECX,
EDX, ESI, EDI, and EBP. These registers take the consecutive arguments, starting with the EBX
register. If there are more than six arguments, then the memory location of the first argument is
stored in the EBX register.
The following code snippet shows the use of the system call sys_exit −
mov eax,1 ; system call number (sys_exit)
int 0x80 ; call kernel
The following code snippet shows the use of the system call sys_write −
mov edx,4 ; 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
All the syscalls are listed in /usr/include/asm/unistd.h, together with their numbers (the value to
put in EAX before you call int 80h).
The following table shows some of the system calls used in this tutorial −

%eax Name %ebx %ecx %edx %esx %edi

1 sys_exit int - - - -

2 sys_fork struct pt_regs - - - -

3 sys_read unsigned int char * size_t - -

4 sys_write unsigned int const char * size_t - -

5 sys_open const char * int int - -

6 sys_close unsigned int - - - -

Assembly - Addressing Modes

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.
When an instruction requires two operands, the first operand is generally the destination, which
contains data in a register or memory location and the second operand is the source. Source
contains either the data to be delivered (immediate addressing) or the address (in register or
memory) of the data. Generally, the source data remains unaltered after the operation.
The three basic modes of addressing are −
 Register addressing
 Immediate addressing
 Memory addressing
Register Addressing
In this addressing mode, a register contains the operand. Depending upon the instruction, the
register may be the first operand, the second operand or both.
For example,
MOV DX, TAX_RATE ; Register in first operand
MOV COUNT, CX ; Register in second operand
MOV EAX, EBX ; Both the operands are in registers
As processing data between registers does not involve memory, it provides fastest processing of
data.
Immediate Addressing
An immediate operand has a constant value or an expression. 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 ; A byte value is defined
WORD_VALUE DW 300 ; A word value is defined
ADD BYTE_VALUE, 65 ; An immediate operand 65 is added
MOV AX, 45H ; Immediate constant 45H is transferred to AX

Direct Memory Addressing

When operands are specified in memory addressing mode, direct access to main memory, usually
to the data segment, is required. This way of addressing results in slower processing of data. To
locate the exact location of data in memory, we need the segment start address, which is typically
found in the DS register and an offset value. This offset value is also called effective address.
In direct addressing mode, the offset value is specified directly as part of the instruction, usually
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 ; Adds the register in the memory location
MOV BX, WORD_VALUE ; Operand from the memory is added to register
Direct-Offset Addressing
This addressing mode uses the arithmetic operators to modify an address. For example, look at
the following definitions that define tables of data −
BYTE_TABLE DB 14, 15, 22, 45 ; Tables of bytes
WORD_TABLE DW 134, 345, 564, 123 ; Tables of words
The following operations access data from the tables in the memory into registers −
MOV CL, BYTE_TABLE[2] ; Gets the 3rd element of the BYTE_TABLE
MOV CL, BYTE_TABLE + 2 ; Gets the 3rd element of the BYTE_TABLE
MOV CX, WORD_TABLE[3] ; Gets the 4th element of the WORD_TABLE
MOV CX, WORD_TABLE + 3 ; Gets the 4th element of the WORD_TABLE
Indirect Memory Addressing
This addressing mode utilizes the computer's ability of Segment: Offset addressing. Generally,
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 generally 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.
MY_TABLE TIMES 10 DW 0 ; Allocates 10 words (2 bytes) each initialized to 0
MOV EBX, [MY_TABLE] ; Effective Address of MY_TABLE in EBX
MOV [EBX], 110 ; MY_TABLE[0] = 110
ADD EBX, 2 ; EBX = EBX +2
MOV [EBX], 123 ; MY_TABLE[1] = 123
The MOV Instruction
We have already used the 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 −
MOV register, register
MOV register, immediate
MOV memory, immediate
MOV register, memory
MOV memory, register
Please note that −
 Both the operands in MOV operation should be of same size
 The value of source operand remains unchanged
The MOV instruction causes ambiguity at times. For example, look at the statements −
MOV EBX, [MY_TABLE] ; Effective Address of MY_TABLE in EBX
MOV [EBX], 110 ; MY_TABLE[0] = 110
It is not clear whether you want to move a byte equivalent or word equivalent of the number 110.
In such cases, it is wise to use a type specifier.
Following table shows some of the common type specifiers −

Type Specifier Bytes addressed

BYTE 1

WORD 2

DWORD 4

QWORD 8

TBYTE 10

Allocating Storage Space for Initialized Data

The syntax for storage allocation statement for initialized data is −
[variable-name] define-directive initial-value [,initial-value]...
Where, variable-name is the identifier for each storage space. The assembler associates an
offset value for each variable name defined in the data segment.
There are five basic forms of the define directive −

Directive Purpose Storage Space

DB Define Byte allocates 1 byte

DW Define Word allocates 2 bytes

DD Define Doubleword allocates 4 bytes

DQ Define Quadword allocates 8 bytes

DT Define Ten Bytes allocates 10 bytes

Following are some examples of using define directives −

choice DB 'y'
number DW 12345
neg_number DW -12345
big_number DQ 123456789
real_number1 DD 1.234
real_number2 DQ 123.456
Please note that −
 Each byte of character is stored as its ASCII value in hexadecimal.
 Each decimal value is automatically converted to its 16-bit binary equivalent and
stored as a hexadecimal number.
 Processor uses the little-endian byte ordering.
 Negative numbers are converted to its 2's complement representation.
 Short and long floating-point numbers are represented using 32 or 64 bits,
respectively.
Allocating Storage Space for Uninitialized Data
The reserve directives are used for reserving space for uninitialized data. The reserve directives
take a single operand that specifies the number of units of space to be reserved. Each define
directive has a related reserve directive.
There are five basic forms of the reserve directive −

Directive Purpose

RESB Reserve a Byte

RESW Reserve a Word

RESD Reserve a Doubleword

RESQ Reserve a Quadword

REST Reserve a Ten Bytes

Multiple Definitions
You can have multiple data definition statements in a program. For example −
choice DB 'Y' ;ASCII of y = 79H
number1 DW 12345 ;12345D = 3039H
number2 DD 12345679 ;123456789D = 75BCD15H
The assembler allocates contiguous memory for multiple variable definitions.
Multiple Initializations
The TIMES directive allows multiple initializations to the same value. For example, an array
named marks of size 9 can be defined and initialized to zero using the following statement −
marks TIMES 9 DW 0
The TIMES directive is useful in defining arrays and tables.

Assembly - Constants
There are several directives provided by NASM that define constants. We have already used the
EQU directive in previous chapters. We will particularly discuss three directives −
  EQU
 %assign
 %define
The EQU Directive
The EQU directive is used for defining constants. The syntax of the EQU directive is as follows
−
CONSTANT_NAME EQU expression
For example,
TOTAL_STUDENTS equ 50
You can then use this constant value in your code, like −
mov ecx, TOTAL_STUDENTS
cmp eax, TOTAL_STUDENTS
The operand of an EQU statement can be an expression −
LENGTH equ 20
WIDTH equ 10
AREA equ length * width
Above code segment would define AREA as 200.
The %assign Directive
The %assign directive can be used to define numeric constants like the EQU directive. This
directive allows redefinition. For example, you may define the constant TOTAL as −
%assign TOTAL 10
Later in the code, you can redefine it as −
%assign TOTAL 20
This directive is case-sensitive.
The %define Directive
The %define directive allows defining both numeric and string constants. This directive is
similar to the #define in C. For example, you may define the constant PTR as −
%define PTR [EBP+4]
The above code replaces PTR by [EBP+4].
This directive also allows redefinition and it is case-sensitive.
Assembly - Arithmetic Instructions
The INC Instruction
The INC instruction is used for incrementing an operand by one. It works on a single operand
that can be either in a register or in memory.
Syntax
The INC instruction has the following syntax −
INC destination
The operand destination could be an 8-bit, 16-bit or 32-bit operand.
Example
INC EBX ; Increments 32-bit register
INC DL ; Increments 8-bit register
INC [count] ; Increments the count variable
The DEC Instruction
The DEC instruction is used for decrementing an operand by one. It works on a single operand
that can be either in a register or in memory.
Syntax
The DEC instruction has the following syntax −
DEC destination
The operand destination could be an 8-bit, 16-bit or 32-bit operand.
Example
segment .data
count dw 0
value db 15

segment .text
inc [count]
 dec [value]

mov ebx, count

inc word [ebx]

mov esi, value

dec byte [esi]
The ADD and SUB Instructions
The ADD and SUB instructions are used for performing simple addition/subtraction of binary
data in byte, word and doubleword size, i.e., for adding or subtracting 8-bit, 16-bit or 32-bit
operands, respectively.
Syntax
The ADD and SUB instructions have the following syntax −
ADD/SUB destination, source
The ADD/SUB instruction can take place between −
 Register to register
 Memory to register
 Register to memory
 Register to constant data
 Memory to constant data
However, like other instructions, memory-to-memory operations are not possible using
ADD/SUB instructions. An ADD or SUB operation sets or clears the overflow and carry flags.
The MUL/IMUL Instruction
There are two instructions for multiplying binary data. The MUL (Multiply) instruction handles
unsigned data and the IMUL (Integer Multiply) handles signed data. Both instructions affect the
Carry and Overflow flag.
Syntax
The syntax for the MUL/IMUL instructions is as follows −
MUL/IMUL multiplier
Multiplicand in both cases will be in an accumulator, depending upon the size of the
multiplicand and the multiplier and the generated product is also stored in two registers
depending upon the size of the operands. Following section explains MUL instructions with
three different cases −

Sr.No. Scenarios

When two bytes are multiplied −

The multiplicand is in the AL register, and the multiplier is a byte in the memory or in another
register. The product is in AX. High-order 8 bits of the product is stored in AH and the low-
1 order 8 bits are stored in AL.

When two one-word values are multiplied −

The multiplicand should be in the AX register, and the multiplier is a word in memory or
another register. For example, for an instruction like MUL DX, you must store the multiplier in
DX and the multiplicand in AX.
2
The resultant product is a doubleword, which will need two registers. The high-order
(leftmost) portion gets stored in DX and the lower-order (rightmost) portion gets stored in AX.

When two doubleword values are multiplied −

When two doubleword values are multiplied, the multiplicand should be in EAX and the
multiplier is a doubleword value stored in memory or in another register. The product
3 generated is stored in the EDX:EAX registers, i.e., the high order 32 bits gets stored in the
EDX register and the low order 32-bits are stored in the EAX register.

Example
MOV AL, 10
MOV DL, 25
MUL DL
...
MOV DL, 0FFH ; DL= -1
MOV AL, 0BEH ; AL = -66
IMUL DL
The DIV/IDIV Instructions
The division operation generates two elements - a quotient and a remainder. In case of
multiplication, overflow does not occur because double-length registers are used to keep the
product. However, in case of division, overflow may occur. The processor generates an interrupt
if overflow occurs.
The DIV (Divide) instruction is used for unsigned data and the IDIV (Integer Divide) is used for
signed data.
Syntax
The format for the DIV/IDIV instruction −
DIV/IDIV divisor
The dividend is in an accumulator. Both the instructions can work with 8-bit, 16-bit or 32-bit
operands. The operation affects all six status flags. Following section explains three cases of
division with different operand size −

Sr.No. Scenarios

When the divisor is 1 byte −

The dividend is assumed to be in the AX register (16 bits). After division, the quotient goes to
the AL register and the remainder goes to the AH register.

2 When the divisor is 1 word −

The dividend is assumed to be 32 bits long and in the DX:AX registers. The high-order 16 bits
 are in DX and the low-order 16 bits are in AX. After division, the 16-bit quotient goes to the
AX register and the 16-bit remainder goes to the DX register.

When the divisor is doubleword −

The dividend is assumed to be 64 bits long and in the EDX:EAX registers. The high-order 32
bits are in EDX and the low-order 32 bits are in EAX. After division, the 32-bit quotient goes
to the EAX register and the 32-bit remainder goes to the EDX register.

Assembly - Logical Instructions

The processor instruction set provides the instructions AND, OR, XOR, TEST, and NOT
Boolean logic, which tests, sets, and clears the bits according to the need of the program.
The format for these instructions −

Sr.No. Instruction Format

1 AND AND operand1, operand2

2 OR OR operand1, operand2

3 XOR XOR operand1, operand2

4 TEST TEST operand1, operand2

5 NOT NOT operand1

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 (constant) value. However, 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 supporting logical expressions by performing bitwise AND
operation. The bitwise AND 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 -> 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
Let's take up another example. If you want to check whether a given number is odd or even, a
simple test would be to check the least significant bit of the number. If this is 1, the number is
odd, else the number is even.
Assuming the number is in AL register, we can write −
AND AL, 01H ; ANDing with 0000 0001
JZ EVEN_NUMBER
The OR Instruction
The OR instruction is used for supporting logical expression by performing bitwise OR
operation. The bitwise OR operator 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 XOR Instruction
The XOR instruction implements 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.
XOR EAX, EAX
The TEST Instruction
The TEST instruction works same as 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.
TEST AL, 01H
JZ EVEN_NUMBER
The NOT Instruction
The NOT instruction implements the bitwise NOT operation. NOT 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
Assembly - Conditions
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 −

Sr.No. Conditional Instructions

Unconditional jump
This is performed by the JMP instruction. Conditional execution often involves a transfer of
1 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.

Conditional jump

2 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.

Let us discuss the CMP instruction before discussing the conditional instructions.
CMP Instruction
The CMP instruction compares two operands. It is generally used in conditional execution. This
instruction basically subtracts one operand from the other for comparing whether the operands
are equal or not. It does not disturb the destination or source operands. It is used along with the
conditional jump instruction for decision making.
Syntax
CMP destination, source
CMP compares two numeric data fields. The destination operand could be either in register or in
memory. The source operand could be a constant (immediate) data, register or memory.
Example
CMP DX, 00 ; Compare the DX value with zero
JE L7 ; If yes, then jump to label L7
.
.
L7: ...
CMP is often used for comparing whether a counter value has reached the number of times a
loop needs to be run. Consider the following typical condition −
INC EDX
CMP EDX, 10 ; Compares whether the counter has reached 10
JLE LP1 ; If it is less than or equal to 10, then jump to LP1
Unconditional Jump
As mentioned earlier, 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.
Syntax
The JMP instruction provides a label name where the flow of control is transferred immediately.
The syntax of the JMP instruction is −
JMP label
Example
The following code snippet illustrates the JMP instruction −
MOV AX, 00 ; Initializing AX to 0
MOV BX, 00 ; Initializing BX to 0
MOV CX, 01 ; Initializing CX to 1
L20:
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.
Following are the conditional jump instructions used on signed data used for arithmetic
operations −

Instruction Description Flags tested

JE/JZ Jump Equal or Jump Zero ZF

JNE/JNZ Jump not Equal or Jump Not Zero ZF

JG/JNLE Jump Greater or Jump Not Less/Equal OF, SF, ZF

JGE/JNL Jump Greater/Equal or Jump Not Less OF, SF

JL/JNGE Jump Less or Jump Not Greater/Equal OF, SF

JLE/JNG Jump Less/Equal or Jump Not Greater OF, SF, ZF

Following are the conditional jump instructions used on unsigned data used for logical
operations −

Instruction Description Flags tested

JE/JZ Jump Equal or Jump Zero ZF

JNE/JNZ Jump not Equal or Jump Not Zero ZF

JA/JNBE Jump Above or Jump Not Below/Equal CF, ZF

JAE/JNB Jump Above/Equal or Jump Not Below CF

JB/JNAE Jump Below or Jump Not Above/Equal CF

JBE/JNA Jump Below/Equal or Jump Not Above AF, CF

The following conditional jump instructions have special uses and check the value of flags −

Instruction Description Flags tested

 JXCZ Jump if CX is Zero none

JC Jump If Carry CF

JNC Jump If No Carry CF

JO Jump If Overflow OF

JNO Jump If No Overflow OF

JP/JPE Jump Parity or Jump Parity Even PF

JNP/JPO Jump No Parity or Jump Parity Odd PF

JS Jump Sign (negative value) SF

JNS Jump No Sign (positive value) SF

The syntax for the J<condition> set of instructions −

Example,
CMP AL, BL
JE EQUAL
CMP AL, BH
JE EQUAL
CMP AL, CL
JE EQUAL
NON_EQUAL: ...
EQUAL: ...
Assembly - Loops
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
L1:
<LOOP-BODY>
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
l1:
<loop body>
loop l1
Assembly - Numbers
Numerical data is generally represented in binary system. Arithmetic instructions operate on
binary data. When numbers are displayed on screen or entered from keyboard, they are in ASCII
form.
ASCII Representation
In ASCII representation, decimal numbers are stored as string of ASCII characters. For example,
the decimal value 1234 is stored as −
31 32 33 34H
Where, 31H is ASCII value for 1, 32H is ASCII value for 2, and so on. There are four
instructions for processing numbers in ASCII representation −
 AAA − ASCII Adjust After Addition
 AAS − ASCII Adjust After Subtraction
 AAM − ASCII Adjust After Multiplication
 AAD − ASCII Adjust Before Division
These instructions do not take any operands and assume the required operand to be in the AL
register.
BCD Representation
There are two types of BCD representation −
 Unpacked BCD representation
 Packed BCD representation
In unpacked BCD representation, each byte stores the binary equivalent of a decimal digit. For
example, the number 1234 is stored as −
01 02 03 04H
There are two instructions for processing these numbers −
 AAM − ASCII Adjust After Multiplication
 AAD − ASCII Adjust Before Division
The four ASCII adjust instructions, AAA, AAS, AAM, and AAD, can also be used with
unpacked BCD representation. In packed BCD representation, each digit is stored using four bits.
Two decimal digits are packed into a byte. For example, the number 1234 is stored as −
12 34H
There are two instructions for processing these numbers −
 DAA − Decimal Adjust After Addition
 DAS − decimal Adjust After Subtraction
There is no support for multiplication and division in packed BCD representation.
Assembly - Strings
We have already used variable length strings in our previous examples. The variable length
strings can have as many characters as required. Generally, we specify the length of the string by
either of the two ways −
 Explicitly storing string length
 Using a sentinel character
We can store the string length explicitly by using the $ location counter symbol that represents
the current value of the location counter. In the following example −
msg db 'Hello, world!',0xa ;our dear string
len equ $ - msg ;length of our dear string
$ points to the byte after the last character of the string variable msg. Therefore, $-msg gives the
length of the string. We can also write
msg db 'Hello, world!',0xa ;our dear string
len equ 13 ;length of our dear string
Alternatively, you can store strings with a trailing sentinel character to delimit a string instead of
storing the string length explicitly. The sentinel character should be a special character that does
not appear within a string.
For example −
message DB 'I am loving it!', 0
String Instructions
Each string instruction may require a source operand, a destination operand or both. For 32-bit
segments, string instructions use ESI and EDI registers to point to the source and destination
operands, respectively.
For 16-bit segments, however, the SI and the DI registers are used to point to the source and
destination, respectively.
There are five basic instructions for processing strings. They are −
 MOVS − This instruction moves 1 Byte, Word or Doubleword of data from memory
location to another.
 LODS − This instruction loads from memory. If the operand is of one byte, it is loaded
into the AL register, if the operand is one word, it is loaded into the AX register and a
doubleword is loaded into the EAX register.
 STOS − This instruction stores data from register (AL, AX, or EAX) to memory.
 CMPS − This instruction compares two data items in memory. Data could be of a byte
size, word or doubleword.
 SCAS − This instruction compares the contents of a register (AL, AX or EAX) with the
contents of an item in memory.
Each of the above instruction has a byte, word, and doubleword version, and string instructions
can be repeated by using a repetition prefix.
These instructions use the ES:DI and DS:SI pair of registers, where DI and SI registers contain
valid offset addresses that refers to bytes stored in memory. SI is normally associated with DS
(data segment) and DI is always associated with ES (extra segment).
The DS:SI (or ESI) and ES:DI (or EDI) registers point to the source and destination operands,
respectively. The source operand is assumed to be at DS:SI (or ESI) and the destination operand
at ES:DI (or EDI) in memory.
For 16-bit addresses, the SI and DI registers are used, and for 32-bit addresses, the ESI and EDI
registers are used.
The following table provides various versions of string instructions and the assumed space of the
operands.

Double
Basic Instruction Operands at Byte Operation Word Operation word
Operation

MOVS ES:DI, DS:SI MOVSB MOVSW MOVSD

LODS AX, DS:SI LODSB LODSW LODSD

STOS ES:DI, AX STOSB STOSW STOSD

CMPS DS:SI, ES: DI CMPSB CMPSW CMPSD

SCAS ES:DI, AX SCASB SCASW SCASD

Repetition Prefixes
The REP prefix, when set before a string instruction, for example - REP MOVSB, causes
repetition of the instruction based on a counter placed at the CX register. REP executes the
instruction, decreases CX by 1, and checks whether CX is zero. It repeats the instruction
processing until CX is zero.
The Direction Flag (DF) determines the direction of the operation.
 Use CLD (Clear Direction Flag, DF = 0) to make the operation left to right.
 Use STD (Set Direction Flag, DF = 1) to make the operation right to left.
The REP prefix also has the following variations:
 REP: It is the unconditional repeat. It repeats the operation until CX is zero.
 REPE or REPZ: It is conditional repeat. It repeats the operation while the zero flag
indicates equal/zero. It stops when the ZF indicates not equal/zero or when CX is zero.
  REPNE or REPNZ: It is also conditional repeat. It repeats the operation while the zero
flag indicates not equal/zero. It stops when the ZF indicates equal/zero or when CX is
decremented to zero.

Assembly - Arrays
We have already discussed that the data definition directives to the assembler are used for
allocating storage for variables. The variable could also be initialized with some specific value.
The initialized value could be specified in hexadecimal, decimal or binary form.
For example, we can define a word variable 'months' in either of the following way −
MONTHS DW 12
MONTHS DW 0CH
MONTHS DW 0110B
The data definition directives can also be used for defining a one-dimensional array. Let us
define a one-dimensional array of numbers.
NUMBERS DW 34, 45, 56, 67, 75, 89
The above definition declares an array of six words each initialized with the numbers 34, 45, 56,
67, 75, 89. This allocates 2x6 = 12 bytes of consecutive memory space. The symbolic address of
the first number will be NUMBERS and that of the second number will be NUMBERS + 2 and
so on.
Let us take up another example. You can define an array named inventory of size 8, and initialize
all the values with zero, as −
INVENTORY DW 0
DW 0
DW 0
DW 0
DW 0
DW 0
DW 0
DW 0
Which can be abbreviated as −
INVENTORY DW 0, 0 , 0 , 0 , 0 , 0 , 0 , 0
The TIMES directive can also be used for multiple initializations to the same value. Using
TIMES, the INVENTORY array can be defined as:
INVENTORY TIMES 8 DW 0
Assembly - Procedures
Procedures or subroutines are very important in assembly language, as the assembly language
programs tend to be large in size. Procedures are identified by a name. Following this name, the
body of the procedure is described which performs a well-defined job. End of the procedure is
indicated by a return statement.
Syntax
Following is the syntax to define a procedure −
proc_name:
procedure body
...
ret
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.
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 that needs to be stored is 'pushed' into the
stack and data to be retrieved is 'popped' out from the stack. Stack is a LIFO data structure, i.e.,
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 reserved 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 inserted into the stack is pointed to by the SS:ESP register, where the SS register
points to the beginning 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, i.e., 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.
As we discussed about 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
PUSH AX
PUSH BX

; Use the registers for other purpose

MOV AX, VALUE1
MOV BX, VALUE2
...
MOV VALUE1, AX
MOV VALUE2, BX

; Restore the original values

POP BX
POP AX
Example
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

display:
mov ecx, 256

next:
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
ret

section .data
achar db '0'
When the above code is compiled and executed, it produces the following result −
0123456789:;<=>?
@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}
...
...

Assembly - Recursion
A recursive procedure is one that calls itself. There are two kind 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 example, consider 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 example: factorial of 5 is 1 x 2 x 3 x 4 x 5 = 5 x factorial of 4 and this can be a good example
of showing a recursive procedure. Every recursive algorithm must have an ending condition, i.e.,
the recursive calling of the program should be stopped when a condition is fulfilled. In the case
of factorial algorithm, the end condition is reached when n is 0.
Assembly - Macros
Writing a macro is another way of ensuring modular programming in assembly language.
 A macro is a sequence of instructions, assigned by a name and could be used anywhere in
the program.
 In NASM, macros are defined with %macro and %endmacro directives.
 The macro begins with the %macro directive and ends with the %endmacro directive.
The Syntax for macro definition −
%macro macro_name number_of_params
<macro body>
%endmacro
Where, number_of_params specifies the number parameters, macro_name specifies the name of
the macro.
The macro is invoked by using the macro name along with the necessary parameters. When you
need to use some sequence of instructions many times in a program, you can put those
instructions in a macro and use it instead of writing the instructions all the time.
For example, a very common need for programs is to write a string of characters in the screen.
For displaying a string of characters, you need the following sequence of instructions −
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
In the above example of displaying a character string, the registers EAX, EBX, ECX and EDX
have been used by the INT 80H function call. So, each time you need to display on screen, you
need to save these registers on the stack, invoke INT 80H and then restore the original value of
the registers from the stack. So, it could be useful to write two macros for saving and restoring
data.
We have observed that, some instructions like IMUL, IDIV, INT, etc., need some of the
information to be stored in some particular registers and even return values in some specific
register(s). If the program was already using those registers for keeping important data, then the
existing data from these registers should be saved in the stack and restored after the instruction is
executed.
Assembly - File Management
The system considers any input or output data as stream of bytes. There are three standard file
streams −
 Standard input (stdin),
 Standard output (stdout), and
 Standard error (stderr).
File Descriptor
A file descriptor is a 16-bit integer assigned to a file as a file id. When a new file is created or an
existing file is opened, the file descriptor is used for accessing the file.
File descriptor of the standard file streams - stdin, stdout and stderr are 0, 1 and 2, respectively.
File Pointer
A file pointer specifies the location for a subsequent read/write operation in the file in terms of
bytes. Each file is considered as a sequence of bytes. Each open file is associated with a file
pointer that specifies an offset in bytes, relative to the beginning of the file. When a file is
opened, the file pointer is set to zero.
File Handling System Calls
The following table briefly describes the system calls related to file handling −

%eax Name %ebx %ecx %edx

2 sys_fork struct pt_regs - -

3 sys_read unsigned int char * size_t

4 sys_write unsigned int const char * size_t

5 sys_open const char * int int

6 sys_close unsigned int - -

8 sys_creat const char * int -

19 sys_lseek unsigned int off_t unsigned int

The steps required for using the system calls are same, as we discussed earlier −
 Put the system call number in the EAX register.
 Store the arguments to the system call in the registers EBX, ECX, etc.
 Call the relevant interrupt (80h).
 The result is usually returned in the EAX register.
Creating and Opening a File
For creating and opening a file, perform the following tasks −
 Put the system call sys_creat() number 8, in the EAX register.
 Put the filename in the EBX register.
 Put the file permissions in the ECX register.
The system call returns the file descriptor of the created file in the EAX register, in case of error,
the error code is in the EAX register.
Opening an Existing File
For opening an existing file, perform the following tasks −
 Put the system call sys_open() number 5, in the EAX register.
 Put the filename in the EBX register.
 Put the file access mode in the ECX register.
 Put the file permissions in the EDX register.
The system call returns the file descriptor of the created file in the EAX register, in case of error,
the error code is in the EAX register.
Among the file access modes, most commonly used are: read-only (0), write-only (1), and read-
write (2).
Reading from a File
For reading from a file, perform the following tasks −
 Put the system call sys_read() number 3, in the EAX register.
 Put the file descriptor in the EBX register.
 Put the pointer to the input buffer in the ECX register.
 Put the buffer size, i.e., the number of bytes to read, in the EDX register.
The system call returns the number of bytes read in the EAX register, in case of error, the error
code is in the EAX register.
Writing to a File
For writing to a file, perform the following tasks −
 Put the system call sys_write() number 4, in the EAX register.
 Put the file descriptor in the EBX register.
 Put the pointer to the output buffer in the ECX register.
 Put the buffer size, i.e., the number of bytes to write, in the EDX register.
The system call returns the actual number of bytes written in the EAX register, in case of error,
the error code is in the EAX register.
Closing a File
For closing a file, perform the following tasks −
 Put the system call sys_close() number 6, in the EAX register.
  Put the file descriptor in the EBX register.
The system call returns, in case of error, the error code in the EAX register.
Updating a File
For updating a file, perform the following tasks −
 Put the system call sys_lseek () number 19, in the EAX register.
 Put the file descriptor in the EBX register.
 Put the offset value in the ECX register.
 Put the reference position for the offset in the EDX register.
The reference position could be:
 Beginning of file - value 0
 Current position - value 1
 End of file - value 2
The system call returns, in case of error, the error code in the EAX register.
Example
The following program creates and opens a file named myfile.txt, and writes a text 'Welcome to
Tutorials Point' in this file. Next, the program reads from the file and stores the data into a buffer
named info. Lastly, it displays the text as stored in info.
section .text
global _start ;must be declared for using gcc

_start: ;tell linker entry point

;create the file
mov eax, 8
mov ebx, file_name
mov ecx, 0777 ;read, write and execute by all
int 0x80 ;call kernel

mov [fd_out], eax

; write into the file
mov edx,len ;number of bytes
mov ecx, msg ;message to write
mov ebx, [fd_out] ;file descriptor
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel

; close the file

mov eax, 6
mov ebx, [fd_out]

; write the message indicating end of file write

mov eax, 4
mov ebx, 1
mov ecx, msg_done
mov edx, len_done
int 0x80

;open the file for reading

mov eax, 5
mov ebx, file_name
mov ecx, 0 ;for read only access
mov edx, 0777 ;read, write and execute by all
int 0x80

mov [fd_in], eax

 ;read from file
mov eax, 3
mov ebx, [fd_in]
mov ecx, info
mov edx, 26
int 0x80

; close the file

mov eax, 6
mov ebx, [fd_in]
int 0x80

; print the info

mov eax, 4
mov ebx, 1
mov ecx, info
mov edx, 26
int 0x80

mov eax,1 ;system call number (sys_exit)

int 0x80 ;call kernel

section .data
file_name db 'myfile.txt'
msg db 'Welcome to Tutorials Point'
len equ $-msg
msg_done db 'Written to file', 0xa
len_done equ $-msg_done

section .bss
fd_out resb 1
fd_in resb 1
info resb 26
When the above code is compiled and executed, it produces the following result −
Written to file
Welcome to Tutorials Point

Assembly - Memory Management

The sys_brk() system call is provided by the kernel, to allocate memory without the need of
moving it later. This call allocates memory right behind the application image in the memory.
This system function allows you to set the highest available address in the data section.
This system call takes one parameter, which is the highest memory address needed to be set. This
value is stored in the EBX register.
In case of any error, sys_brk() returns -1 or returns the negative error code itself. The following
example demonstrates dynamic memory allocation.

Assembly - Introduction
What is Assembly Language?
Each personal computer has a microprocessor that manages the computer's arithmetical, logical,
and control activities.
Each family of processors has its own set of instructions for handling various operations such as
getting input from keyboard, displaying information on screen and performing various other
jobs. These set of instructions are called 'machine language instructions'.
A processor understands only machine language instructions, which are strings of 1's and 0's.
However, machine language is too obscure and complex for using in software development. So,
the low-level assembly language is designed for a specific family of processors that represents
various instructions in symbolic code and a more understandable form.
Advantages of Assembly Language
Having an understanding of assembly language makes one aware of −
 How programs interface with OS, processor, and BIOS;
 How data is represented in memory and other external devices;
 How the processor accesses and executes instruction;
 How instructions access and process data;
 How a program accesses external devices.
Other advantages of using assembly language are −
 It requires less memory and execution time;
 It allows hardware-specific complex jobs in an easier way;
 It is suitable for time-critical jobs;
 It is most suitable for writing interrupt service routines and other memory resident
programs.

Basic Features of PC Hardware

The main internal hardware of a PC consists of processor, memory, and registers. Registers are
processor components that hold data and address. 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 related bits makes a byte on most of the modern computers.
So, 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 the following data sizes −
 Word: a 2-byte data item
 Doubleword: a 4-byte (32 bit) data item
 Quadword: an 8-byte (64 bit) data item
 Paragraph: a 16-byte (128 bit) area
 Kilobyte: 1024 bytes
 Megabyte: 1,048,576 bytes
Binary Number System
Every number system uses positional notation, i.e., each position in which a digit is written has a
different positional value. Each position is power of the base, which is 2 for binary number
system, and these powers begin at 0 and increase by 1.
‘int’ means interrupt
Int 20h ; to terminate
Int 10h; to display screen