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 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 _start         ;must be declared for using gcc
_start: ;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

 mov eax, 4
 mov ebx, 1
 mov ecx, msg
 mov edx, len
 int 80h  ;print a message
exit:
 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!