Assembly - Memory Management
Memory management is the process of controlling and coordinating a computer's main memory. It ensures that blocks of memory space are properly managed and allocated so the operating system (OS), applications and other running processes have the memory they need to carry out their operations.
As part of this activity, memory management takes into account the capacity limitations of the memory device itself, deallocating memory space when it is no longer needed or extending that space through virtual memory. Memory management strives to optimize memory usage so the CPU can efficiently access the instructions and data it needs to execute the various processes.
In assembly language you can use both static and dynamic memory allocation. By properly managing the process of memory allocation, reallocation and deallocation, large programs can run efficiently even with limited memory.
The assembly language me Sys_brk() system call is provided by the kernel, in order to allocate memory without the need to move it later. This call allocates memory right behind the 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 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.
Exampal
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 Result −
Allocated 16 kb of memory! 
