Overview of Memory Management

In assembly language, memory management is the foundation of program execution. The operating system provides various system calls to manage memory, among which <span>sys_brk()</span> is an important system call used to dynamically adjust the size of the program’s data segment.

Detailed Explanation of the sys_brk() System Call

Function Description

<span>sys_brk()</span> system call is used to adjust the program’s “break point”, which is the end position of the data segment. By moving the break point, the program can dynamically allocate or free memory.

Calling Convention

In the x86 architecture:

• System call number: 45 (0x2D)
• Parameter: The new break address is stored in the EBX register
• Return value: Returns the new break address on success, or an error code on failure

Code Example: Allocating 16KB of Memory

section .data
    ; Data segment definition
    current_break dd 0    ; Store current break address
    new_break dd 0        ; Store new break address

section .bss
    ; Uninitialized data segment
    allocated_memory resb 16384  ; Reserve 16KB space

section .text
    global _start

_start:
    ; Get current break address
    mov eax, 45          ; sys_brk system call number
    mov ebx, 0           ; Parameter is 0, indicating to get current break
    int 0x80             ; Call kernel

    ; Check for errors
    cmp eax, 0
    jl error_exit

    ; Save current break
    mov [current_break], eax

    ; Calculate new break address (current break + 16KB)
    mov ebx, eax
    add ebx, 16384       ; Increase by 16KB
    mov [new_break], ebx

    ; Set new break
    mov eax, 45          ; sys_brk system call number
    ; ebx already contains new break address
    int 0x80             ; Call kernel

    ; Check for errors
    cmp eax, 0
    jl error_exit

    ; Memory allocation successful, eax contains new break address
    ; Now can use memory from [current_break] to eax

    ; Example: Write data to allocated memory
    mov edi, [current_break]  ; Starting address
    mov ecx, 16384           ; 16KB byte count
    mov al, 0xAA             ; Test data
    rep stosb                ; Fill memory

    ; Exit program normally
    mov eax, 1              ; sys_exit system call
    mov ebx, 0              ; Exit status code
    int 0x80

error_exit:
    ; Error handling
    mov eax, 1              ; sys_exit system call
    mov ebx, 1              ; Exit status code 1 indicates error
    int 0x80

A More Complete Example: Dynamic Memory Allocator

Below is a more complete example demonstrating how to implement a simple dynamic memory allocator:

section .data
    heap_start dd 0     ; Heap start address
    current_brk dd 0    ; Current break

section .bss
    ; Memory block header structure (8 bytes)
    ; +0: Block size (including header)
    ; +4: Usage flag (0=free, 1=allocated)

section .text
    global _start

; Initialize heap
init_heap:
    mov eax, 45         ; sys_brk
    mov ebx, 0
    int 0x80
    mov [heap_start], eax
    mov [current_brk], eax
    ret

; Expand heap size
; Input: ECX = requested byte count
; Output: EAX = allocated memory address, or 0 indicates failure
expand_heap:
    push ebx
    mov eax, 45         ; sys_brk
    mov ebx, [current_brk]
    add ebx, ecx
    int 0x80

    cmp eax, 0
    jl .error

    mov ebx, [current_brk]
    mov [current_brk], eax
    mov eax, ebx        ; Return new memory starting address
    pop ebx
    ret

.error:
    mov eax, 0
    pop ebx
    ret

; Memory allocation function
; Input: ECX = requested byte count
; Output: EAX = allocated memory address, or 0 indicates failure
malloc:
    ; Here you can add logic to find free memory blocks
    ; For simplicity, directly expand heap

    add ecx, 8          ; Add block header size
    call expand_heap
    test eax, eax
    jz .fail

    ; Set block header
    mov [eax], ecx      ; Block size
    mov dword [eax+4], 1 ; Usage flag

    add eax, 8          ; Return address of user available space
    ret

.fail:
    mov eax, 0
    ret

; Memory free function
; Input: EAX = address of memory to free
free:
    sub eax, 8          ; Find block header
    mov dword [eax+4], 0 ; Mark as free
    ; Here you can add logic to merge memory
    ret

_start:
    call init_heap

    ; Allocate 1024 bytes
    mov ecx, 1024
    call malloc
    test eax, eax
    jz error

    ; Use allocated memory...
    mov [eax], dword 0x12345678

    ; Free memory
    call free

    ; Exit normally
    mov eax, 1
    mov ebx, 0
    int 0x80

error:
    mov eax, 1
    mov ebx, 1
    int 0x80

Key Points Explanation

1. Break Point Concept: The break point is the end position of the data segment, and <span>sys_brk()</span> allocates memory by moving this position.

2. Memory Alignment: In practical applications, memory addresses should be aligned to specific boundaries to improve performance.

3. Error Handling: Always check the return value of system calls and handle possible error situations.

4. Memory Management Strategies: A complete dynamic memory allocator needs to implement:

• Free block search
• Memory splitting
• Free block merging
• Fragmentation compaction

System Call Overhead: Frequent calls to <span>sys_brk()</span> can impact performance; it is generally advisable to allocate larger blocks of memory at once and manage them in user space.

By understanding how the <span>sys_brk()</span><span> system call works, one can better grasp the memory management mechanisms in assembly language, laying the foundation for developing more complex system software.</span>