sys_brk()系统调用由内核提供,用于分配内存而无需稍后移动.此调用在内存中的应用程序映像后面分配内存.此系统函数允许您在数据部分设置最高可用地址.
此系统调用采用一个参数,这是需要设置的最高内存地址.该值存储在EBX寄存器中.
如果发生任何错误,sys_brk()返回-1或返回负错误代码本身.以下示例演示了动态内存分配.
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
当编译并执行上面的代码时,它产生以下结果 :
Allocated 16 kb of memory!