While i was trying do use brk (int 0x80 with 45 in %rax) to implement a simple memory manager program in assembly and print the blocks in order, i kept getting segfault. After a while i could only reproduce the error, but have no idea why is this happening: .section .data helloworld: .ascii "hello world" .section .text .globl _start _start: push %rbp mov %rsp, %rbp movq $45, %rax movq $0, %rbx #brk(0) should just return the current break of the programm int $0x80 #incq %rax #segfault #addq $1, %rax #segfault movq $0, %rax #works fine? #addq $1, %rax #segfault again? movq $helloworld, %rdi call printf movq $1, %rax #exit int $0x80 In the example here, if the commented lines are uncommented, i have a segfault, but some commands (like de movq $0, %rax) work just fine. In my other program, the first couple printf work, but the third crashes... Looking for other questions, i heard that printf sometimes allocates some memory, and that the brk shouldn't be used, because in this case it corrupts the heap or something... I'm very confused, does anyone know something about that? EDIT: I've just found out that for printf to work you need %rax=0.