Assembly x86 brk() call use

Question

i am trying to dynamically allocate memory into the heap and then assign values in those memory addresses. I understand how to allocate the memory but how would i assign for

Answer 1

As you have done, call once to retrieve the current bottom of heap, then move the top of heap (which is the brk value). However your code is not correct in using the 64-bit register set r.x. If your Linux is 32-bit (as implied by the use of 45 for the syscall number), then you want the 32-bit register set:

mov eax, 45                            ; brk                                                                                                                                                                                  
mov ebx, 0                             ; arg 1: 0 = fail returning brk value in rax                                                                                                                               
int 0x80                               ; syscall
mov dword ptr [brk_firstLocation], rax ; save result
mov eax, 45                            ; brk                                                                                                                                                                                  
mov ebx, 4                             ; arg 1: allocate 4 bytes for a 32-bit int                                                                                                                                
int 0x80                               ; syscall

mov eax, dword ptr [brk_firstLocation] ; reload rax with allocated memory address.
mov dword ptr [eax], 42 ; use result: store 42 in newly allocated storage

Of course you can re-load the saved value for re-use as many times as needed.