ASSEMBLY LANGUAGE

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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 like getting input from keyboard, displaying information on screen and performing various other jobs. These set of instructions are called 'machine language instructions'.

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

An understanding of assembly language provides knowledge of:

• Interface of programs with OS, processor and BIOS;
• Representation of data in memory and other external devices;
• How 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 the processor, memory and the registers. The 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). A group of nine related bits makes a byte. Eight bits are used for data and the last one is used for parity. According to the rule of parity, number of bits that are on (1) in each byte should always be odd.

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 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., the low-order byte is stored in low memory address and high-order byte in high memory address. So if 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:

• An absolute address - a direct reference of specific location.
• The segment address (or offset) - starting address of a memory segment with the offset value.

Assembly - 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, like:

• 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.

Installing NASM

If you select "Development Tools" while installing Linux, you may get NASM installed along with the Linux operating system and you do not need to download and install it separately. For checking whether you already have NASM installed, take the following steps:

• Open a Linux terminal.
• Type whereis nasm and press ENTER.
• If it is already installed, then a line like, nasm: /usr/bin/nasm appears. Otherwise, you will see just nasm:, then you need to install NASM.

To install NASM take the following steps:

• Check The netwide assembler (NASM) website for the latest version.
• Download the Linux source archive nasm-X.XX. ta .gz, where X.XX is the NASM version number in the archive.
• Unpack the archive into a directory which creates a subdirectory nasm-X. XX.
• cd to nasm-X. XX and type ./configure . This shell script will find the best C compiler to use and set up Makefiles accordingly.
• Type make to build the nasm and ndisasm binaries.
• Type make install to install nasm and ndisasm in /usr/local/bin and to install the man pages.

This should install NASM on your system. Alternatively, you can use an RPM distribution for the Fedora Linux. This version is simpler to install, just double-click the RPM file.

Assembly - Basic Syntax

An assembly program can be divided into three sections:

• The data section
• The bss section
• 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

The text section

The text section is used for keeping the actual code. This section must begin with the declarationglobal _start, which tells the kernel where the program execution begins.

The syntax for declaring text section is:

section .text



   global _start



_start:

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
• 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 - 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.

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.

EXAMPLE:

The following example asks two digits from the user, stores the digits in the EAX and EBX register, respectively, adds the values, stores the result in a memory location 'res' and finally displays the result.
SYS_EXIT  equ 1
SYS_READ  equ 3
SYS_WRITE equ 4
STDIN     equ 0
STDOUT    equ 1

segment .data 

    msg1 db "Enter a digit ", 0xA,0xD 
    len1 equ $- msg1 

    msg2 db "Please enter a second digit", 0xA,0xD 
    len2 equ $- msg2 

    msg3 db "The sum is: "
    len3 equ $- msg3

segment .bss

    num1 resb 2 
    num2 resb 2 
    res resb 1    

section .text
    global _start    ;must be declared for using gcc
_start:    ;tell linker entry point
    mov eax, SYS_WRITE         
    mov ebx, STDOUT         
    mov ecx, msg1         
    mov edx, len1 
    int 0x80                

    mov eax, SYS_READ 
    mov ebx, STDIN  
    mov ecx, num1 
    mov edx, 2
    int 0x80            

    mov eax, SYS_WRITE        
    mov ebx, STDOUT         
    mov ecx, msg2          
    mov edx, len2         
    int 0x80

    mov eax, SYS_READ  
    mov ebx, STDIN  
    mov ecx, num2 
    mov edx, 2
    int 0x80        

    mov eax, SYS_WRITE         
    mov ebx, STDOUT         
    mov ecx, msg3          
    mov edx, len3         
    int 0x80

    ; moving the first number to eax register and second number to ebx
    ; and subtracting ascii '0' to convert it into a decimal number
    mov eax, [number1]
    sub eax, '0'
    mov ebx, [number2]
    sub ebx, '0'

    ; add eax and ebx
    add eax, ebx
    ; add '0' to to convert the sum from decimal to ASCII
    add eax, '0'

    ; storing the sum in memory location res
    mov [res], eax

    ; print the sum 
    mov eax, SYS_WRITE        
    mov ebx, STDOUT
    mov ecx, res         
    mov edx, 1        
    int 0x80
exit:    
    mov eax, SYS_EXIT   
    xor ebx, ebx 
    int 0x80

When the above code is compiled and executed, it produces the following result:
Enter a digit:
3
Please enter a second digit:
4
The sum is:
7

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:

SN
Scenarios

1
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-order 8 bits are stored in AL




2
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.

The resultant product is a double word, which will need two registers. The high-order (leftmost) portion gets stored in DX and the lower-order (rightmost) portion gets stored in AX.




3
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 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.

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