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CHAPTER 8 Assembly Variables NASM provides various define directives for reserving storage space for variables. The define assembler directive is used for allocation of storage space. It can be used to reserve as well as initialize one or more bytes. 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 TUTORIALS POINT Simply Easy Learning
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 The following program shows use of the define directive: section .text global main ;must be declared for linker (gcc) main: ;tell linker entry point section .data choice DB 'y' mov edx,1 ;message length mov ecx,choice ;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 When the above code is compiled and executed, it produces following result: y 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 RESB RESW RESD RESQ Purpose Reserve a Byte Reserve a Word Reserve a Doubleword Reserve a Quadword REST Multiple Definitions Reserve a Ten Bytes 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. TUTORIALS POINT Simply Easy Learning
Page 1 and 2: Assembly Language Tutorial
Page 3 and 4: ABOUT THE TUTORIAL Assembly Program
Page 5 and 6: Table of Content Assembly Programmi
Page 7 and 8: The XOR Instruction ...............
Page 9 and 10: CHAPTER 1 Assembly Introduction Wha
Page 11 and 12: Main use of hexadecimal numbers in
Page 13 and 14: x: memory address When the processo
Page 15 and 16: Check The netwide assembler (NASM)
Page 17 and 18: ; This program displays a message o
Page 19 and 20: CHAPTER 4 Assembly Memory Segments
Page 21 and 22: CHAPTER 5 Assembly Registers Proces
Page 23 and 24: Control Registers The 32-bit instru
Page 25 and 26: CHAPTER 6 Assembly System Calls Sys
Page 27 and 28: Please enter a number: 1234 You hav
Page 29 and 30: Direct Memory Addressing When opera
Page 31: int 0x80 ;call kernel mov eax,1 ;sy
Page 35 and 36: CHAPTER 9 Assembly Constants There
Page 37 and 38: The above code replaces PTR by [EBP
Page 39 and 40: dec [value] mov ebx, count inc word
Page 41 and 42: int 0x80 When the above code is com
Page 43 and 44: global main ;must be declared for u
Page 45 and 46: mov [res], ax mov ecx,msg mov edx,
Page 47 and 48: AND AL, 01H ; ANDing with 0000 0001
Page 49 and 50: The TEST Instruction The TEST instr
Page 51 and 52: L7: ... CMP is often used for compa
Page 53 and 54: jg _exit mov ecx, [num3] _exit: mov
Page 55 and 56: section .bss num resb 1 mov ecx, nu
Page 57 and 58: Such conversions are however, has a
Page 59 and 60: mov ecx,msg ;message to write mov e
Page 61 and 62: LODS - This instruction loads from
Page 63 and 64: mov ecx, len mov esi, s1 mov edi, s
Page 65 and 66: found! Repetition Prefixes The REP
Page 67 and 68: Example: The following example demo
Page 69 and 70: mov ecx, res mov edx, 1 mov ebx, 1
Page 71 and 72: CHAPTER 18 Assembly Recursion Arecu
Page 73 and 74: CHAPTER 19 Assembly Macros Writing
Page 75 and 76: CHAPTER 20 Assembly File Management
Page 77 and 78: Put the buffer size, i.e., the numb
Page 79 and 80: int 0x80 ;call kernel section .data
Page 81: mov eax, 1 xor ebx, ebx int 80h sec

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