void enter_user_space()
{
    asm volatile("cli; \
                  mov $0x23, %ax; \
                  mov %ax, %ds; \
                  mov %ax, %es; \
                  mov %ax, %fs; \
                  mov %ax, %gs; \
                  mov %esp, %eax; \
                  push $0x23; \
                  push %eax; \
                  pushf; \
                  pop %eax; \
                  or $0x200, %eax; \
                  push %eax; \
                  push $0x1B; \
                  push $user_code_entry; \
                  iret; \
                  user_code_entry: \
                  call main; \
                  ");
}