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.

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 the following result −

Allocated 16 kb of memory!