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CHAPTER 21 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 application image in 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 need 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. Example: The following program allocates 16kb of memory using the sys_brk() system call: section .text global main ;must be declared for using gcc main: ;tell linker entry point mov eax, 45 ;sys_brk xor ebx, ebx int 80h add eax, 16384 ;number of bytes to be reserved mov ebx, eax mov eax, 45 ;sys_brk int 80h cmp eax, 0 jl exit ;exit, if error mov edi, eax ;EDI = highest available address sub edi, 4 ;pointing to the last DWORD mov ecx, 4096 ;number of DWORDs allocated xor eax, eax ;clear eax std ;backward rep stosd ;repete for entire allocated area cld ;put DF flag to normal state exit: mov eax, 4 mov ebx, 1 mov ecx, msg mov edx, len int 80h ;print a message TUTORIALS POINT Simply Easy Learning
mov eax, 1 xor ebx, ebx int 80h section .data msg db "Allocated 16 kb of memory!", 10 len equ $ - msg When the above code is compiled and executed, it produces following result: Allocated 16 kb of memory! TUTORIALS POINT Simply Easy Learning
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Assembly Language Tutorial
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ABOUT THE TUTORIAL Assembly Program
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Table of Content Assembly Programmi
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The XOR Instruction ...............
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CHAPTER 1 Assembly Introduction Wha
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Main use of hexadecimal numbers in
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x: memory address When the processo
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Check The netwide assembler (NASM)
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; This program displays a message o
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CHAPTER 4 Assembly Memory Segments
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CHAPTER 5 Assembly Registers Proces
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Control Registers The 32-bit instru
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CHAPTER 6 Assembly System Calls Sys
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Please enter a number: 1234 You hav
Page 29 and 30: Direct Memory Addressing When opera
Page 31 and 32: int 0x80 ;call kernel mov eax,1 ;sy
Page 33 and 34: Processor uses the little-endian by
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: int 0x80 ;call kernel section .data
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